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Man Awk

Manpage of GAWK

GAWK

Section: Utility Commands (1)
Updated: June 26 2005
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NAME

gawk - pattern scanning and processing language

SYNOPSIS

gawk [ POSIX or GNU style options ] -f program-file [ -- ] file ...
gawk [ POSIX or GNU style options ] [ -- ] program-text file ...

pgawk [ POSIX or GNU style options ] -f program-file [ -- ] file ...
pgawk [ POSIX or GNU style options ] [ -- ] program-text file ...

DESCRIPTION

Gawk is the GNU Project's implementation of the AWK programming language. It conforms to the definition of the language in the POSIX 1003.2 Command Language And Utilities Standard. This version in turn is based on the description in The AWK Programming Language, by Aho, Kernighan, and Weinberger, with the additional features found in the System V Release 4 version of UNIX awk. Gawk also provides more recent Bell Laboratories awk extensions, and a number of GNU-specific extensions.

Pgawk is the profiling version of gawk. It is identical in every way to gawk, except that programs run more slowly, and it automatically produces an execution profile in the file awkprof.out when done. See the --profile option, below.

The command line consists of options to gawk itself, the AWK program text (if not supplied via the -f or --file options), and values to be made available in the ARGC and ARGV pre-defined AWK variables.

OPTION FORMAT

Gawk options may be either traditional POSIX one letter options, or GNU style long options. POSIX options start with a single ``-'', while long options start with ``--''. Long options are provided for both GNU-specific features and for POSIX-mandated features.

Following the POSIX standard, gawk-specific options are supplied via arguments to the -W option. Multiple -W options may be supplied Each -W option has a corresponding long option, as detailed below. Arguments to long options are either joined with the option by an = sign, with no intervening spaces, or they may be provided in the next command line argument. Long options may be abbreviated, as long as the abbreviation remains unique.

OPTIONS

Gawk accepts the following options, listed alphabetically.

-F fs

--field-separator fs Use fs for the input field separator (the value of the FS predefined variable).

-v var=val

--assign var=val Assign the value val to the variable var, before execution of the program begins. Such variable values are available to the BEGIN block of an AWK program.

-f program-file

--file program-file Read the AWK program source from the file program-file, instead of from the first command line argument. Multiple -f (or --file) options may be used.

-mf NNN

-mr NNN Set various memory limits to the value NNN. The f flag sets the maximum number of fields, and the r flag sets the maximum record size. These two flags and the -m option are from the Bell Laboratories research version of UNIX awk. They are ignored by gawk, since gawk has no pre-defined limits.

-W compat

-W traditional

--compat

--traditional Run in compatibility mode. In compatibility mode, gawk behaves identically to UNIX awk; none of the GNU-specific extensions are recognized. The use of --traditional is preferred over the other forms of this option. See GNU EXTENSIONS, below, for more information.

-W copyleft

-W copyright

--copyleft

--copyright Print the short version of the GNU copyright information message on the standard output and exit successfully.

-W dump-variables[=file]

--dump-variables[=file] Print a sorted list of global variables, their types and final values to file. If no file is provided, gawk uses a file named awkvars.out in the current directory.

Having a list of all the global variables is a good way to look for typographical errors in your programs. You would also use this option if you have a large program with a lot of functions, and you want to be sure that your functions don't inadvertently use global variables that you meant to be local. (This is a particularly easy mistake to make with simple variable names like i, j, and so on.)

-W exec file

--exec file Similar to -f, however, this is option is the last one processed. This should be used with #! scripts, particularly for CGI applications, to avoid passing in options or source code (!) on the command line from a URL. This option disables command-line variable assignments.

-W gen-po

--gen-po Scan and parse the AWK program, and generate a GNU .po format file on standard output with entries for all localizable strings in the program. The program itself is not executed. See the GNU gettext distribution for more information on .po files.

-W help

-W usage

--help

--usage Print a relatively short summary of the available options on the standard output. (Per the GNU Coding Standards, these options cause an immediate, successful exit.)

-W lint[=value]

--lint[=value] Provide warnings about constructs that are dubious or non-portable to other AWK implementations. With an optional argument of fatal, lint warnings become fatal errors. This may be drastic, but its use will certainly encourage the development of cleaner AWK programs. With an optional argument of invalid, only warnings about things that are actually invalid are issued. (This is not fully implemented yet.)

-W lint-old

--lint-old Provide warnings about constructs that are not portable to the original version of Unix awk.

-W non-decimal-data

--non-decimal-data Recognize octal and hexadecimal values in input data. Use this option with great caution!

-W posix

--posix This turns on compatibility mode, with the following additional restrictions:

*: \x escape sequences are not recognized.
*: Only space and tab act as field separators when FS is set to a single space, newline does not.
*: You cannot continue lines after ? and :.
*: The synonym func for the keyword function is not recognized.
*: The operators ** and **= cannot be used in place of ^ and ^=.
*: The fflush() function is not available.

-W profile[=prof_file]

--profile[=prof_file] Send profiling data to prof_file. The default is awkprof.out. When run with gawk, the profile is just a ``pretty printed'' version of the program. When run with pgawk, the profile contains execution counts of each statement in the program in the left margin and function call counts for each user-defined function.

-W re-interval

--re-interval Enable the use of interval expressions in regular expression matching (see Regular Expressions, below). Interval expressions were not traditionally available in the AWK language. The POSIX standard added them, to make awk and egrep consistent with each other. However, their use is likely to break old AWK programs, so gawk only provides them if they are requested with this option, or when --posix is specified.

-W source program-text

--source program-text Use program-text as AWK program source code. This option allows the easy intermixing of library functions (used via the -f and --file options) with source code entered on the command line. It is intended primarily for medium to large AWK programs used in shell scripts.

-W version

--version Print version information for this particular copy of gawk on the standard output. This is useful mainly for knowing if the current copy of gawk on your system is up to date with respect to whatever the Free Software Foundation is distributing. This is also useful when reporting bugs. (Per the GNU Coding Standards, these options cause an immediate, successful exit.)

-- Signal the end of options. This is useful to allow further arguments to the AWK program itself to start with a ``-''. This is mainly for consistency with the argument parsing convention used by most other POSIX programs.

In compatibility mode, any other options are flagged as invalid, but are otherwise ignored. In normal operation, as long as program text has been supplied, unknown options are passed on to the AWK program in the ARGV array for processing. This is particularly useful for running AWK programs via the ``#!'' executable interpreter mechanism.

AWK PROGRAM EXECUTION

An AWK program consists of a sequence of pattern-action statements and optional function definitions.

: pattern { action statements }

function name(parameter list) { statements }

Gawk first reads the program source from the program-file(s) if specified, from arguments to --source, or from the first non-option argument on the command line. The -f and --source options may be used multiple times on the command line. Gawk reads the program text as if all the program-files and command line source texts had been concatenated together. This is useful for building libraries of AWK functions, without having to include them in each new AWK program that uses them. It also provides the ability to mix library functions with command line programs.

The environment variable AWKPATH specifies a search path to use when finding source files named with the -f option. If this variable does not exist, the default path is ".:/usr/local/share/awk". (The actual directory may vary, depending upon how gawk was built and installed.) If a file name given to the -f option contains a ``/'' character, no path search is performed.

Gawk executes AWK programs in the following order. First, all variable assignments specified via the -v option are performed. Next, gawk compiles the program into an internal form. Then, gawk executes the code in the BEGIN block(s) (if any), and then proceeds to read each file named in the ARGV array. If there are no files named on the command line, gawk reads the standard input.

If a filename on the command line has the form var=val it is treated as a variable assignment. The variable var will be assigned the value val. (This happens after any BEGIN block(s) have been run.) Command line variable assignment is most useful for dynamically assigning values to the variables AWK uses to control how input is broken into fields and records. It is also useful for controlling state if multiple passes are needed over a single data file.

If the value of a particular element of ARGV is empty (""), gawk skips over it.

For each record in the input, gawk tests to see if it matches any pattern in the AWK program. For each pattern that the record matches, the associated action is executed. The patterns are tested in the order they occur in the program.

Finally, after all the input is exhausted, gawk executes the code in the END block(s) (if any).

VARIABLES, RECORDS AND FIELDS

AWK variables are dynamic; they come into existence when they are first used. Their values are either floating-point numbers or strings, or both, depending upon how they are used. AWK also has one dimensional arrays; arrays with multiple dimensions may be simulated. Several pre-defined variables are set as a program runs; these will be described as needed and summarized below.

Records

Normally, records are separated by newline characters. You can control how records are separated by assigning values to the built-in variable RS. If RS is any single character, that character separates records. Otherwise, RS is a regular expression. Text in the input that matches this regular expression separates the record. However, in compatibility mode, only the first character of its string value is used for separating records. If RS is set to the null string, then records are separated by blank lines. When RS is set to the null string, the newline character always acts as a field separator, in addition to whatever value FS may have.

Fields

As each input record is read, gawk splits the record into fields, using the value of the FS variable as the field separator. If FS is a single character, fields are separated by that character. If FS is the null string, then each individual character becomes a separate field. Otherwise, FS is expected to be a full regular expression. In the special case that FS is a single space, fields are separated by runs of spaces and/or tabs and/or newlines. (But see the discussion of --posix, below). NOTE: The value of IGNORECASE (see below) also affects how fields are split when FS is a regular expression, and how records are separated when RS is a regular expression.

If the FIELDWIDTHS variable is set to a space separated list of numbers, each field is expected to have fixed width, and gawk splits up the record using the specified widths. The value of FS is ignored. Assigning a new value to FS overrides the use of FIELDWIDTHS, and restores the default behavior.

Each field in the input record may be referenced by its position, $1, $2, and so on. $0 is the whole record. Fields need not be referenced by constants:

: n = 5
print $n

prints the fifth field in the input record.

The variable NF is set to the total number of fields in the input record.

References to non-existent fields (i.e. fields after $NF) produce the null-string. However, assigning to a non-existent field (e.g., $(NF+2) = 5) increases the value of NF, creates any intervening fields with the null string as their value, and causes the value of $0 to be recomputed, with the fields being separated by the value of OFS. References to negative numbered fields cause a fatal error. Decrementing NF causes the values of fields past the new value to be lost, and the value of $0 to be recomputed, with the fields being separated by the value of OFS.

Assigning a value to an existing field causes the whole record to be rebuilt when $0 is referenced. Similarly, assigning a value to $0 causes the record to be resplit, creating new values for the fields.

Built-in Variables

Gawk's built-in variables are:

ARGC

The number of command line arguments (does not include options to gawk, or the program source).

ARGIND

The index in ARGV of the current file being processed.

ARGV

Array of command line arguments. The array is indexed from 0 to ARGC - 1. Dynamically changing the contents of ARGV can control the files used for data.

BINMODE

On non-POSIX systems, specifies use of ``binary'' mode for all file I/O. Numeric values of 1, 2, or 3, specify that input files, output files, or all files, respectively, should use binary I/O. String values of "r", or "w" specify that input files, or output files, respectively, should use binary I/O. String values of "rw" or "wr" specify that all files should use binary I/O. Any other string value is treated as "rw", but generates a warning message.

CONVFMT

The conversion format for numbers, "%.6g", by default.

ENVIRON

An array containing the values of the current environment. The array is indexed by the environment variables, each element being the value of that variable (e.g., ENVIRON["HOME"] might be /home/arnold). Changing this array does not affect the environment seen by programs which gawk spawns via redirection or the system() function.

ERRNO

If a system error occurs either doing a redirection for getline, during a read for getline, or during a close(), then ERRNO will contain a string describing the error. The value is subject to translation in non-English locales.

FIELDWIDTHS

A white-space separated list of fieldwidths. When set, gawk parses the input into fields of fixed width, instead of using the value of the FS variable as the field separator.

FILENAME

The name of the current input file. If no files are specified on the command line, the value of FILENAME is ``-''. However, FILENAME is undefined inside the BEGIN block (unless set by getline).

FNR

The input record number in the current input file.

FS

The input field separator, a space by default. See Fields, above.

IGNORECASE

Controls the case-sensitivity of all regular expression and string operations. If IGNORECASE has a non-zero value, then string comparisons and pattern matching in rules, field splitting with FS, record separating with RS, regular expression matching with ~ and !~, and the gensub(), gsub(), index(), match(), split(), and sub() built-in functions all ignore case when doing regular expression operations. NOTE: Array subscripting is not affected. However, the asort() and asorti() functions are affected.

Thus, if IGNORECASE is not equal to zero, /aB/ matches all of the strings "ab", "aB", "Ab", and "AB". As with all AWK variables, the initial value of IGNORECASE is zero, so all regular expression and string operations are normally case-sensitive. Under Unix, the full ISO 8859-1 Latin-1 character set is used when ignoring case. As of gawk 3.1.4, the case equivalencies are fully locale-aware, based on the C <ctype.h> facilities such as isalpha(), and tolupper().

LINT

Provides dynamic control of the --lint option from within an AWK program. When true, gawk prints lint warnings. When false, it does not. When assigned the string value "fatal", lint warnings become fatal errors, exactly like --lint=fatal. Any other true value just prints warnings.

NF

The number of fields in the current input record.

NR

The total number of input records seen so far.

OFMT

The output format for numbers, "%.6g", by default.

OFS

The output field separator, a space by default.

ORS

The output record separator, by default a newline.

PROCINFO

The elements of this array provide access to information about the running AWK program. On some systems, there may be elements in the array, "group1" through "groupn" for some n, which is the number of supplementary groups that the process has. Use the in operator to test for these elements. The following elements are guaranteed to be available:

PROCINFO["egid"]: the value of the getegid(2) system call.
PROCINFO["euid"]: the value of the geteuid(2) system call.
PROCINFO["FS"]: "FS" if field splitting with FS is in effect, or "FIELDWIDTHS" if field splitting with FIELDWIDTHS is in effect.
PROCINFO["gid"]: the value of the getgid(2) system call.
PROCINFO["pgrpid"]: the process group ID of the current process.
PROCINFO["pid"]: the process ID of the current process.
PROCINFO["ppid"]: the parent process ID of the current process.
PROCINFO["uid"]: the value of the getuid(2) system call.
PROCINFO["version"]: The version of gawk. This is available from version 3.1.4 and later.

RS

The input record separator, by default a newline.

RT

The record terminator. Gawk sets RT to the input text that matched the character or regular expression specified by RS.

RSTART

The index of the first character matched by match(); 0 if no match. (This implies that character indices start at one.)

RLENGTH

The length of the string matched by match(); -1 if no match.

SUBSEP

The character used to separate multiple subscripts in array elements, by default "\034".

TEXTDOMAIN

The text domain of the AWK program; used to find the localized translations for the program's strings.

Arrays

Arrays are subscripted with an expression between square brackets ([ and ]). If the expression is an expression list (expr, expr ...) then the array subscript is a string consisting of the concatenation of the (string) value of each expression, separated by the value of the SUBSEP variable. This facility is used to simulate multiply dimensioned arrays. For example:

: i = "A"; j = "B"; k = "C"
x[i, j, k] = "hello, world\n"

assigns the string "hello, world\n" to the element of the array x which is indexed by the string "A\034B\034C". All arrays in AWK are associative, i.e. indexed by string values.

The special operator in may be used in an if or while statement to see if an array has an index consisting of a particular value.

if (val in array)
        print array[val]

If the array has multiple subscripts, use (i, j) in array.

The in construct may also be used in a for loop to iterate over all the elements of an array.

An element may be deleted from an array using the delete statement. The delete statement may also be used to delete the entire contents of an array, just by specifying the array name without a subscript.

Variable Typing And Conversion

Variables and fields may be (floating point) numbers, or strings, or both. How the value of a variable is interpreted depends upon its context. If used in a numeric expression, it will be treated as a number, if used as a string it will be treated as a string.

To force a variable to be treated as a number, add 0 to it; to force it to be treated as a string, concatenate it with the null string.

When a string must be converted to a number, the conversion is accomplished using strtod(3). A number is converted to a string by using the value of CONVFMT as a format string for sprintf(3), with the numeric value of the variable as the argument. However, even though all numbers in AWK are floating-point, integral values are always converted as integers. Thus, given

CONVFMT = "%2.2f"
a = 12
b = a ""

the variable b has a string value of "12" and not "12.00".

Gawk performs comparisons as follows: If two variables are numeric, they are compared numerically. If one value is numeric and the other has a string value that is a ``numeric string,'' then comparisons are also done numerically. Otherwise, the numeric value is converted to a string and a string comparison is performed. Two strings are compared, of course, as strings. Note that the POSIX standard applies the concept of ``numeric string'' everywhere, even to string constants. However, this is clearly incorrect, and gawk does not do this. (Fortunately, this is fixed in the next version of the standard.)

Note that string constants, such as "57", are not numeric strings, they are string constants. The idea of ``numeric string'' only applies to fields, getline input, FILENAME, ARGV elements, ENVIRON elements and the elements of an array created by split() that are numeric strings. The basic idea is that user input, and only user input, that looks numeric, should be treated that way.

Uninitialized variables have the numeric value 0 and the string value "" (the null, or empty, string).

Octal and Hexadecimal Constants

Starting with version 3.1 of gawk , you may use C-style octal and hexadecimal constants in your AWK program source code. For example, the octal value 011 is equal to decimal 9, and the hexadecimal value 0x11 is equal to decimal 17.

String Constants

String constants in AWK are sequences of characters enclosed between double quotes ("). Within strings, certain escape sequences are recognized, as in C. These are:

\\: A literal backslash.
\a: The ``alert'' character; usually the ASCII BEL character.
\b: backspace.
\f: form-feed.
\n: newline.
\r: carriage return.
\t: horizontal tab.
\v: vertical tab.
\xhex digits: The character represented by the string of hexadecimal digits following the \x. As in ANSI C, all following hexadecimal digits are considered part of the escape sequence. (This feature should tell us something about language design by committee.) E.g., "\x1B" is the ASCII ESC (escape) character.
\ddd: The character represented by the 1-, 2-, or 3-digit sequence of octal digits. E.g., "\033" is the ASCII ESC (escape) character.
\c: The literal character c.

The escape sequences may also be used inside constant regular expressions (e.g., /[ \t\f\n\r\v]/ matches whitespace characters).

In compatibility mode, the characters represented by octal and hexadecimal escape sequences are treated literally when used in regular expression constants. Thus, /a\52b/ is equivalent to /a\*b/.

PATTERNS AND ACTIONS

AWK is a line-oriented language. The pattern comes first, and then the action. Action statements are enclosed in { and }. Either the pattern may be missing, or the action may be missing, but, of course, not both. If the pattern is missing, the action is executed for every single record of input. A missing action is equivalent to

: { print }

which prints the entire record.

Comments begin with the ``#'' character, and continue until the end of the line. Blank lines may be used to separate statements. Normally, a statement ends with a newline, however, this is not the case for lines ending in a ``,'', {, ?, :, &&, or ||. Lines ending in do or else also have their statements automatically continued on the following line. In other cases, a line can be continued by ending it with a ``\'', in which case the newline will be ignored.

Multiple statements may be put on one line by separating them with a ``;''. This applies to both the statements within the action part of a pattern-action pair (the usual case), and to the pattern-action statements themselves.

Patterns

AWK patterns may be one of the following:

BEGIN
END
/regular expression/
relational expression
pattern && pattern
pattern || pattern
pattern ? pattern : pattern
(pattern)
! pattern
pattern1, pattern2

BEGIN and END are two special kinds of patterns which are not tested against the input. The action parts of all BEGIN patterns are merged as if all the statements had been written in a single BEGIN block. They are executed before any of the input is read. Similarly, all the END blocks are merged, and executed when all the input is exhausted (or when an exit statement is executed). BEGIN and END patterns cannot be combined with other patterns in pattern expressions. BEGIN and END patterns cannot have missing action parts.

For /regular expression/ patterns, the associated statement is executed for each input record that matches the regular expression. Regular expressions are the same as those in egrep(1), and are summarized below.

A relational expression may use any of the operators defined below in the section on actions. These generally test whether certain fields match certain regular expressions.

The &&, ||, and ! operators are logical AND, logical OR, and logical NOT, respectively, as in C. They do short-circuit evaluation, also as in C, and are used for combining more primitive pattern expressions. As in most languages, parentheses may be used to change the order of evaluation.

The ?: operator is like the same operator in C. If the first pattern is true then the pattern used for testing is the second pattern, otherwise it is the third. Only one of the second and third patterns is evaluated.

The pattern1, pattern2 form of an expression is called a range pattern. It matches all input records starting with a record that matches pattern1, and continuing until a record that matches pattern2, inclusive. It does not combine with any other sort of pattern expression.

Regular Expressions

Regular expressions are the extended kind found in egrep. They are composed of characters as follows:

c: matches the non-metacharacter c.
\c: matches the literal character c.
.: matches any character including newline.
^: matches the beginning of a string.
$: matches the end of a string.
[abc...]: character list, matches any of the characters abc....
[^abc...]: negated character list, matches any character except abc....
r1|r2: alternation: matches either r1 or r2.
r1r2: concatenation: matches r1, and then r2.
r+: matches one or more r's.
r*: matches zero or more r's.
r?: matches zero or one r's.
(r): grouping: matches r.
: r{n}
: r{n,}
: r{n,m} One or two numbers inside braces denote an interval expression. If there is one number in the braces, the preceding regular expression r is repeated n times. If there are two numbers separated by a comma, r is repeated n to m times. If there is one number followed by a comma, then r is repeated at least n times.
Interval expressions are only available if either --posix or --re-interval is specified on the command line.
\y: matches the empty string at either the beginning or the end of a word.
\B: matches the empty string within a word.
\<: matches the empty string at the beginning of a word.
\>: matches the empty string at the end of a word.
\w: matches any word-constituent character (letter, digit, or underscore).
\W: matches any character that is not word-constituent.
\`: matches the empty string at the beginning of a buffer (string).
\': matches the empty string at the end of a buffer.

The escape sequences that are valid in string constants (see below) are also valid in regular expressions.

Character classes are a new feature introduced in the POSIX standard. A character class is a special notation for describing lists of characters that have a specific attribute, but where the actual characters themselves can vary from country to country and/or from character set to character set. For example, the notion of what is an alphabetic character differs in the USA and in France.

A character class is only valid in a regular expression inside the brackets of a character list. Character classes consist of [:, a keyword denoting the class, and :]. The character classes defined by the POSIX standard are:

[:alnum:]: Alphanumeric characters.
[:alpha:]: Alphabetic characters.
[:blank:]: Space or tab characters.
[:cntrl:]: Control characters.
[:digit:]: Numeric characters.
[:graph:]: Characters that are both printable and visible. (A space is printable, but not visible, while an a is both.)
[:lower:]: Lower-case alphabetic characters.
[:print:]: Printable characters (characters that are not control characters.)
[:punct:]: Punctuation characters (characters that are not letter, digits, control characters, or space characters).
[:space:]: Space characters (such as space, tab, and formfeed, to name a few).
[:upper:]: Upper-case alphabetic characters.
[:xdigit:]: Characters that are hexadecimal digits.

For example, before the POSIX standard, to match alphanumeric characters, you would have had to write /[A-Za-z0-9]/. If your character set had other alphabetic characters in it, this would not match them, and if your character set collated differently from ASCII, this might not even match the ASCII alphanumeric characters. With the POSIX character classes, you can write /[[:alnum:]]/, and this matches the alphabetic and numeric characters in your character set.

Two additional special sequences can appear in character lists. These apply to non-ASCII character sets, which can have single symbols (called collating elements) that are represented with more than one character, as well as several characters that are equivalent for collating, or sorting, purposes. (E.g., in French, a plain ``e'' and a grave-accented e` are equivalent.)

Collating Symbols: A collating symbol is a multi-character collating element enclosed in [. and .]. For example, if ch is a collating element, then [[.ch.]] is a regular expression that matches this collating element, while [ch] is a regular expression that matches either c or h.
Equivalence Classes: An equivalence class is a locale-specific name for a list of characters that are equivalent. The name is enclosed in [= and =]. For example, the name e might be used to represent all of ``e,'' ``e','' and ``e`.'' In this case, [[=e=]] is a regular expression that matches any of e, e', or e`.

These features are very valuable in non-English speaking locales. The library functions that gawk uses for regular expression matching currently only recognize POSIX character classes; they do not recognize collating symbols or equivalence classes.

The \y, \B, \<, \>, \w, \W, \`, and \' operators are specific to gawk; they are extensions based on facilities in the GNU regular expression libraries.

The various command line options control how gawk interprets characters in regular expressions.

No options: In the default case, gawk provide all the facilities of POSIX regular expressions and the GNU regular expression operators described above. However, interval expressions are not supported.
--posix: Only POSIX regular expressions are supported, the GNU operators are not special. (E.g., \w matches a literal w). Interval expressions are allowed.
--traditional: Traditional Unix awk regular expressions are matched. The GNU operators are not special, interval expressions are not available, and neither are the POSIX character classes ([[:alnum:]] and so on). Characters described by octal and hexadecimal escape sequences are treated literally, even if they represent regular expression metacharacters.
--re-interval: Allow interval expressions in regular expressions, even if --traditional has been provided.

Actions

Action statements are enclosed in braces, { and }. Action statements consist of the usual assignment, conditional, and looping statements found in most languages. The operators, control statements, and input/output statements available are patterned after those in C.

Operators

The operators in AWK, in order of decreasing precedence, are

(...): Grouping
$: Field reference.
++ --: Increment and decrement, both prefix and postfix.
^: Exponentiation (** may also be used, and **= for the assignment operator).
+ - !: Unary plus, unary minus, and logical negation.
* / %: Multiplication, division, and modulus.
+ -: Addition and subtraction.
space: String concatenation.
: < >
: <= >=
: != == The regular relational operators.
~ !~: Regular expression match, negated match. NOTE: Do not use a constant regular expression (/foo/) on the left-hand side of a ~ or !~. Only use one on the right-hand side. The expression /foo/ ~ exp has the same meaning as (($0 ~ /foo/) ~ exp). This is usually not what was intended.
in: Array membership.
&&: Logical AND.
||: Logical OR.
?:: The C conditional expression. This has the form expr1 ? expr2 : expr3. If expr1 is true, the value of the expression is expr2, otherwise it is expr3. Only one of expr2 and expr3 is evaluated.
: = += -=
: *= /= %= ^= Assignment. Both absolute assignment (var = value) and operator-assignment (the other forms) are supported.

Control Statements

The control statements are as follows:

if (condition) statement [ else statement ]
while (condition) statement 
do statement while (condition)
for (expr1; expr2; expr3) statement
for (var in array) statement
break
continue
delete array[index]
delete array
exit [ expression ]
{ statements }

I/O Statements

The input/output statements are as follows:

close(file [, how]): Close file, pipe or co-process. The optional how should only be used when closing one end of a two-way pipe to a co-process. It must be a string value, either "to" or "from".
getline: Set $0 from next input record; set NF, NR, FNR.
getline <file: Set $0 from next record of file; set NF.
getline var: Set var from next input record; set NR, FNR.
getline var <file: Set var from next record of file.
command | getline [var]: Run command piping the output either into $0 or var, as above.
command |& getline [var]: Run command as a co-process piping the output either into $0 or var, as above. Co-processes are a gawk extension.
next: Stop processing the current input record. The next input record is read and processing starts over with the first pattern in the AWK program. If the end of the input data is reached, the END block(s), if any, are executed.
nextfile: Stop processing the current input file. The next input record read comes from the next input file. FILENAME and ARGIND are updated, FNR is reset to 1, and processing starts over with the first pattern in the AWK program. If the end of the input data is reached, the END block(s), if any, are executed.
print: Prints the current record. The output record is terminated with the value of the ORS variable.
print expr-list: Prints expressions. Each expression is separated by the value of the OFS variable. The output record is terminated with the value of the ORS variable.
print expr-list >file: Prints expressions on file. Each expression is separated by the value of the OFS variable. The output record is terminated with the value of the ORS variable.
printf fmt, expr-list: Format and print.
printf fmt, expr-list >file: Format and print on file.
system(cmd-line): Execute the command cmd-line, and return the exit status. (This may not be available on non-POSIX systems.)
fflush([file]): Flush any buffers associated with the open output file or pipe file. If file is missing, then standard output is flushed. If file is the null string, then all open output files and pipes have their buffers flushed.

Additional output redirections are allowed for print and printf.

print ... >> file: appends output to the file.
print ... | command: writes on a pipe.
print ... |& command: sends data to a co-process.

The getline command returns 0 on end of file and -1 on an error. Upon an error, ERRNO contains a string describing the problem.

NOTE: If using a pipe or co-process to getline, or from print or printf within a loop, you must use close() to create new instances of the command. AWK does not automatically close pipes or co-processes when they return EOF.

The printf Statement

The AWK versions of the printf statement and sprintf() function (see below) accept the following conversion specification formats:

%c: An ASCII character. If the argument used for %c is numeric, it is treated as a character and printed. Otherwise, the argument is assumed to be a string, and the only first character of that string is printed.
%d, %i: A decimal number (the integer part).
%e , %E: A floating point number of the form [-]d.dddddde[+-]dd. The %E format uses E instead of e.
%f: A floating point number of the form [-]ddd.dddddd.
%g , %G: Use %e or %f conversion, whichever is shorter, with nonsignificant zeros suppressed. The %G format uses %E instead of %e.
%o: An unsigned octal number (also an integer).
: %u An unsigned decimal number (again, an integer).
%s: A character string.
%x , %X: An unsigned hexadecimal number (an integer). The %X format uses ABCDEF instead of abcdef.
%%: A single % character; no argument is converted.

NOTE: When using the integer format-control letters for values that are outside the range of a C long integer, gawk switches to the %g format specifier. If --lint is provided on the command line gawk warns about this. Other versions of awk may print invalid values or do something else entirely.

Optional, additional parameters may lie between the % and the control letter:

count$: Use the count'th argument at this point in the formatting. This is called a positional specifier and is intended primarily for use in translated versions of format strings, not in the original text of an AWK program. It is a gawk extension.
-: The expression should be left-justified within its field.
space: For numeric conversions, prefix positive values with a space, and negative values with a minus sign.
+: The plus sign, used before the width modifier (see below), says to always supply a sign for numeric conversions, even if the data to be formatted is positive. The + overrides the space modifier.
#: Use an ``alternate form'' for certain control letters. For %o, supply a leading zero. For %x, and %X, supply a leading 0x or 0X for a nonzero result. For %e, %E, and %f, the result always contains a decimal point. For %g, and %G, trailing zeros are not removed from the result.
0: A leading 0 (zero) acts as a flag, that indicates output should be padded with zeroes instead of spaces. This applies even to non-numeric output formats. This flag only has an effect when the field width is wider than the value to be printed.
width: The field should be padded to this width. The field is normally padded with spaces. If the 0 flag has been used, it is padded with zeroes.
.prec: A number that specifies the precision to use when printing. For the %e, %E, and %f formats, this specifies the number of digits you want printed to the right of the decimal point. For the %g, and %G formats, it specifies the maximum number of significant digits. For the %d, %o, %i, %u, %x, and %X formats, it specifies the minimum number of digits to print. For %s, it specifies the maximum number of characters from the string that should be printed.

The dynamic width and prec capabilities of the ANSI C printf() routines are supported. A * in place of either the width or prec specifications causes their values to be taken from the argument list to printf or sprintf(). To use a positional specifier with a dynamic width or precision, supply the count$ after the * in the format string. For example, "%3$*2$.*1$s".

Special File Names

When doing I/O redirection from either print or printf into a file, or via getline from a file, gawk recognizes certain special filenames internally. These filenames allow access to open file descriptors inherited from gawk's parent process (usually the shell). These file names may also be used on the command line to name data files. The filenames are:

/dev/stdin: The standard input.
/dev/stdout: The standard output.
/dev/stderr: The standard error output.
/dev/fd/n: The file associated with the open file descriptor n.

These are particularly useful for error messages. For example:

: print "You blew it!" > "/dev/stderr"

whereas you would otherwise have to use

: print "You blew it!" | "cat 1>&2"

The following special filenames may be used with the |& co-process operator for creating TCP/IP network connections.

/inet/tcp/lport/rhost/rport: File for TCP/IP connection on local port lport to remote host rhost on remote port rport. Use a port of 0 to have the system pick a port.
/inet/udp/lport/rhost/rport: Similar, but use UDP/IP instead of TCP/IP.
/inet/raw/lport/rhost/rport: Reserved for future use.

Other special filenames provide access to information about the running gawk process. These filenames are now obsolete. Use the PROCINFO array to obtain the information they provide. The filenames are:

/dev/pid: Reading this file returns the process ID of the current process, in decimal, terminated with a newline.
/dev/ppid: Reading this file returns the parent process ID of the current process, in decimal, terminated with a newline.
/dev/pgrpid: Reading this file returns the process group ID of the current process, in decimal, terminated with a newline.
/dev/user: Reading this file returns a single record terminated with a newline. The fields are separated with spaces. $1 is the value of the getuid(2) system call, $2 is the value of the geteuid(2) system call, $3 is the value of the getgid(2) system call, and $4 is the value of the getegid(2) system call. If there are any additional fields, they are the group IDs returned by getgroups(2). Multiple groups may not be supported on all systems.

Numeric Functions

AWK has the following built-in arithmetic functions:

atan2(y, x): Returns the arctangent of y/x in radians.
cos(expr): Returns the cosine of expr, which is in radians.
exp(expr): The exponential function.
int(expr): Truncates to integer.
log(expr): The natural logarithm function.
rand(): Returns a random number N, between 0 and 1, such that 0 < N < 1.
sin(expr): Returns the sine of expr, which is in radians.
sqrt(expr): The square root function.
srand([expr]): Uses expr as a new seed for the random number generator. If no expr is provided, the time of day is used. The return value is the previous seed for the random number generator.

String Functions

Gawk has the following built-in string functions:

asort(s [, d]): Returns the number of elements in the source array s. The contents of s are sorted using gawk's normal rules for comparing values, and the indexes of the sorted values of s are replaced with sequential integers starting with 1. If the optional destination array d is specified, then s is first duplicated into d, and then d is sorted, leaving the indexes of the source array s unchanged.
asorti(s [, d]): Returns the number of elements in the source array s. The behavior is the same as that of asort(), except that the array indices are used for sorting, not the array values. When done, the array is indexed numerically, and the values are those of the original indices. The original values are lost; thus provide a second array if you wish to preserve the original.
gensub(r, s, h [, t]): Search the target string t for matches of the regular expression r. If h is a string beginning with g or G, then replace all matches of r with s. Otherwise, h is a number indicating which match of r to replace. If t is not supplied, $0 is used instead. Within the replacement text s, the sequence \n, where n is a digit from 1 to 9, may be used to indicate just the text that matched the n'th parenthesized subexpression. The sequence \0 represents the entire matched text, as does the character &. Unlike sub() and gsub(), the modified string is returned as the result of the function, and the original target string is not changed.
gsub(r, s [, t]): For each substring matching the regular expression r in the string t, substitute the string s, and return the number of substitutions. If t is not supplied, use $0. An & in the replacement text is replaced with the text that was actually matched. Use \& to get a literal &. (This must be typed as "\\&"; see GAWK: Effective AWK Programming for a fuller discussion of the rules for &'s and backslashes in the replacement text of sub(), gsub(), and gensub().)
index(s, t): Returns the index of the string t in the string s, or 0 if t is not present. (This implies that character indices start at one.)
length([s]): Returns the length of the string s, or the length of $0 if s is not supplied. Starting with version 3.1.5, as a non-standard extension, with an array argument, length() returns the number of elements in the array.
match(s, r [, a]): Returns the position in s where the regular expression r occurs, or 0 if r is not present, and sets the values of RSTART and RLENGTH. Note that the argument order is the same as for the ~ operator: str ~ re. If array a is provided, a is cleared and then elements 1 through n are filled with the portions of s that match the corresponding parenthesized subexpression in r. The 0'th element of a contains the portion of s matched by the entire regular expression r. Subscripts a[n, "start"], and a[n, "length"] provide the starting index in the string and length respectively, of each matching substring.
split(s, a [, r]): Splits the string s into the array a on the regular expression r, and returns the number of fields. If r is omitted, FS is used instead. The array a is cleared first. Splitting behaves identically to field splitting, described above.
sprintf(fmt, expr-list): Prints expr-list according to fmt, and returns the resulting string.
strtonum(str): Examines str, and returns its numeric value. If str begins with a leading 0, strtonum() assumes that str is an octal number. If str begins with a leading 0x or 0X, strtonum() assumes that str is a hexadecimal number.
sub(r, s [, t]): Just like gsub(), but only the first matching substring is replaced.
substr(s, i [, n]): Returns the at most n-character substring of s starting at i. If n is omitted, the rest of s is used.
tolower(str): Returns a copy of the string str, with all the upper-case characters in str translated to their corresponding lower-case counterparts. Non-alphabetic characters are left unchanged.
toupper(str): Returns a copy of the string str, with all the lower-case characters in str translated to their corresponding upper-case counterparts. Non-alphabetic characters are left unchanged.

Time Functions

Since one of the primary uses of AWK programs is processing log files that contain time stamp information, gawk provides the following functions for obtaining time stamps and formatting them.

mktime(datespec): Turns datespec into a time stamp of the same form as returned by systime(). The datespec is a string of the form YYYY MM DD HH MM SS[ DST]. The contents of the string are six or seven numbers representing respectively the full year including century, the month from 1 to 12, the day of the month from 1 to 31, the hour of the day from 0 to 23, the minute from 0 to 59, and the second from 0 to 60, and an optional daylight saving flag. The values of these numbers need not be within the ranges specified; for example, an hour of -1 means 1 hour before midnight. The origin-zero Gregorian calendar is assumed, with year 0 preceding year 1 and year -1 preceding year 0. The time is assumed to be in the local timezone. If the daylight saving flag is positive, the time is assumed to be daylight saving time; if zero, the time is assumed to be standard time; and if negative (the default), mktime() attempts to determine whether daylight saving time is in effect for the specified time. If datespec does not contain enough elements or if the resulting time is out of range, mktime() returns -1.
strftime([format [, timestamp]]): Formats timestamp according to the specification in format. The timestamp should be of the same form as returned by systime(). If timestamp is missing, the current time of day is used. If format is missing, a default format equivalent to the output of date(1) is used. See the specification for the strftime() function in ANSI C for the format conversions that are guaranteed to be available. A public-domain version of strftime(3) and a man page for it come with gawk; if that version was used to build gawk, then all of the conversions described in that man page are available to gawk.
systime(): Returns the current time of day as the number of seconds since the Epoch (1970-01-01 00:00:00 UTC on POSIX systems).

Bit Manipulations Functions

Starting with version 3.1 of gawk, the following bit manipulation functions are available. They work by converting double-precision floating point values to unsigned long integers, doing the operation, and then converting the result back to floating point. The functions are:

and(v1, v2): Return the bitwise AND of the values provided by v1 and v2.
compl(val): Return the bitwise complement of val.
lshift(val, count): Return the value of val, shifted left by count bits.
or(v1, v2): Return the bitwise OR of the values provided by v1 and v2.
rshift(val, count): Return the value of val, shifted right by count bits.
xor(v1, v2): Return the bitwise XOR of the values provided by v1 and v2.

Internationalization Functions

Starting with version 3.1 of gawk, the following functions may be used from within your AWK program for translating strings at run-time. For full details, see GAWK: Effective AWK Programming.

bindtextdomain(directory [, domain]): Specifies the directory where gawk looks for the .mo files, in case they will not or cannot be placed in the ``standard'' locations (e.g., during testing). It returns the directory where domain is ``bound.''
The default domain is the value of TEXTDOMAIN. If directory is the null string (""), then bindtextdomain() returns the current binding for the given domain.
dcgettext(string [, domain [, category]]): Returns the translation of string in text domain domain for locale category category. The default value for domain is the current value of TEXTDOMAIN. The default value for category is "LC_MESSAGES".
If you supply a value for category, it must be a string equal to one of the known locale categories described in GAWK: Effective AWK Programming. You must also supply a text domain. Use TEXTDOMAIN if you want to use the current domain.
dcngettext(string1 , string2 , number [, domain [, category]]): Returns the plural form used for number of the translation of string1 and string2 in text domain domain for locale category category. The default value for domain is the current value of TEXTDOMAIN. The default value for category is "LC_MESSAGES".
If you supply a value for category, it must be a string equal to one of the known locale categories described in GAWK: Effective AWK Programming. You must also supply a text domain. Use TEXTDOMAIN if you want to use the current domain.

USER-DEFINED FUNCTIONS

Functions in AWK are defined as follows:

: function name(parameter list) { statements }

Functions are executed when they are called from within expressions in either patterns or actions. Actual parameters supplied in the function call are used to instantiate the formal parameters declared in the function. Arrays are passed by reference, other variables are passed by value.

Since functions were not originally part of the AWK language, the provision for local variables is rather clumsy: They are declared as extra parameters in the parameter list. The convention is to separate local variables from real parameters by extra spaces in the parameter list. For example:

function  f(p, q,     a, b)     # a and b are local
{
        ...
}

/abc/   { ... ; f(1, 2) ; ... }

The left parenthesis in a function call is required to immediately follow the function name, without any intervening white space. This is to avoid a syntactic ambiguity with the concatenation operator. This restriction does not apply to the built-in functions listed above.

Functions may call each other and may be recursive. Function parameters used as local variables are initialized to the null string and the number zero upon function invocation.

Use return expr to return a value from a function. The return value is undefined if no value is provided, or if the function returns by ``falling off'' the end.

If --lint has been provided, gawk warns about calls to undefined functions at parse time, instead of at run time. Calling an undefined function at run time is a fatal error.

The word func may be used in place of function.

DYNAMICALLY LOADING NEW FUNCTIONS

Beginning with version 3.1 of gawk, you can dynamically add new built-in functions to the running gawk interpreter. The full details are beyond the scope of this manual page; see GAWK: Effective AWK Programming for the details.

extension(object, function): Dynamically link the shared object file named by object, and invoke function in that object, to perform initialization. These should both be provided as strings. Returns the value returned by function.

This function is provided and documented in GAWK: Effective AWK Programming, but everything about this feature is likely to change in the next release. We STRONGLY recommend that you do not use this feature for anything that you aren't willing to redo.

SIGNALS

pgawk accepts two signals. SIGUSR1 causes it to dump a profile and function call stack to the profile file, which is either awkprof.out, or whatever file was named with the --profile option. It then continues to run. SIGHUP causes it to dump the profile and function call stack and then exit.

EXAMPLES

Print and sort the login names of all users:

        BEGIN   { FS = ":" }
                { print $1 | "sort" }

Count lines in a file:

                { nlines++ }
        END     { print nlines }

Precede each line by its number in the file:

        { print FNR, $0 }

Concatenate and line number (a variation on a theme):

        { print NR, $0 }
Run an external command for particular lines of data:

        tail -f access_log |
        awk '/myhome.html/ { system("nmap " $1 ">> logdir/myhome.html") }'

INTERNATIONALIZATION

String constants are sequences of characters enclosed in double quotes. In non-English speaking environments, it is possible to mark strings in the AWK program as requiring translation to the native natural language. Such strings are marked in the AWK program with a leading underscore (``_''). For example,

: gawk 'BEGIN { print "hello, world" }'

always prints hello, world. But,

: gawk 'BEGIN { print _"hello, world" }'

might print bonjour, monde in France.

There are several steps involved in producing and running a localizable AWK program.

1.

Add a BEGIN action to assign a value to the TEXTDOMAIN variable to set the text domain to a name associated with your program.

BEGIN { TEXTDOMAIN = "myprog" }

This allows gawk to find the .mo file associated with your program. Without this step, gawk uses the messages text domain, which likely does not contain translations for your program.

2.

Mark all strings that should be translated with leading underscores.

3.

If necessary, use the dcgettext() and/or bindtextdomain() functions in your program, as appropriate.

4.

Run gawk --gen-po -f myprog.awk > myprog.po to generate a .po file for your program.

5.

Provide appropriate translations, and build and install a corresponding .mo file.

The internationalization features are described in full detail in GAWK: Effective AWK Programming.

POSIX COMPATIBILITY

A primary goal for gawk is compatibility with the POSIX standard, as well as with the latest version of UNIX awk. To this end, gawk incorporates the following user visible features which are not described in the AWK book, but are part of the Bell Laboratories version of awk, and are in the POSIX standard.

The book indicates that command line variable assignment happens when awk would otherwise open the argument as a file, which is after the BEGIN block is executed. However, in earlier implementations, when such an assignment appeared before any file names, the assignment would happen before the BEGIN block was run. Applications came to depend on this ``feature.'' When awk was changed to match its documentation, the -v option for assigning variables before program execution was added to accommodate applications that depended upon the old behavior. (This feature was agreed upon by both the Bell Laboratories and the GNU developers.)

The -W option for implementation specific features is from the POSIX standard.

When processing arguments, gawk uses the special option ``--'' to signal the end of arguments. In compatibility mode, it warns about but otherwise ignores undefined options. In normal operation, such arguments are passed on to the AWK program for it to process.

The AWK book does not define the return value of srand(). The POSIX standard has it return the seed it was using, to allow keeping track of random number sequences. Therefore srand() in gawk also returns its current seed.

Other new features are: The use of multiple -f options (from MKS awk); the ENVIRON array; the \a, and \v escape sequences (done originally in gawk and fed back into the Bell Laboratories version); the tolower() and toupper() built-in functions (from the Bell Laboratories version); and the ANSI C conversion specifications in printf (done first in the Bell Laboratories version).

HISTORICAL FEATURES

There are two features of historical AWK implementations that gawk supports. First, it is possible to call the length() built-in function not only with no argument, but even without parentheses! Thus,

: a = length # Holy Algol 60, Batman!

is the same as either of

: a = length()
a = length($0)

This feature is marked as ``deprecated'' in the POSIX standard, and gawk issues a warning about its use if --lint is specified on the command line.

The other feature is the use of either the continue or the break statements outside the body of a while, for, or do loop. Traditional AWK implementations have treated such usage as equivalent to the next statement. Gawk supports this usage if --traditional has been specified.

GNU EXTENSIONS

Gawk has a number of extensions to POSIX awk. They are described in this section. All the extensions described here can be disabled by invoking gawk with the --traditional option.

The following features of gawk are not available in POSIX awk.

*: No path search is performed for files named via the -f option. Therefore the AWKPATH environment variable is not special.
*: The \x escape sequence. (Disabled with --posix.)
*: The fflush() function. (Disabled with --posix.)
*: The ability to continue lines after ? and :. (Disabled with --posix.)
*: Octal and hexadecimal constants in AWK programs.
*: The ARGIND, BINMODE, ERRNO, LINT, RT and TEXTDOMAIN variables are not special.
*: The IGNORECASE variable and its side-effects are not available.
*: The FIELDWIDTHS variable and fixed-width field splitting.
*: The PROCINFO array is not available.
*: The use of RS as a regular expression.
*: The special file names available for I/O redirection are not recognized.
*: The |& operator for creating co-processes.
*: The ability to split out individual characters using the null string as the value of FS, and as the third argument to split().
*: The optional second argument to the close() function.
*: The optional third argument to the match() function.
*: The ability to use positional specifiers with printf and sprintf().
*: The use of delete array to delete the entire contents of an array.
*: The use of nextfile to abandon processing of the current input file.
*: The and(), asort(), asorti(), bindtextdomain(), compl(), dcgettext(), dcngettext(), gensub(), lshift(), mktime(), or(), rshift(), strftime(), strtonum(), systime() and xor() functions.
*: Localizable strings.
*: Adding new built-in functions dynamically with the extension() function.

The AWK book does not define the return value of the close() function. Gawk's close() returns the value from fclose(3), or pclose(3), when closing an output file or pipe, respectively. It returns the process's exit status when closing an input pipe. The return value is -1 if the named file, pipe or co-process was not opened with a redirection.

When gawk is invoked with the --traditional option, if the fs argument to the -F option is ``t'', then FS is set to the tab character. Note that typing gawk -F\t ... simply causes the shell to quote the ``t,'', and does not pass ``\t'' to the -F option. Since this is a rather ugly special case, it is not the default behavior. This behavior also does not occur if --posix has been specified. To really get a tab character as the field separator, it is best to use single quotes: gawk -F'\t' ....

If gawk is configured with the --enable-switch option to the configure command, then it accepts an additional control-flow statement:

switch (expression) {
case value|regex : statement
...
[ default: statement ]
}

ENVIRONMENT VARIABLES

The AWKPATH environment variable can be used to provide a list of directories that gawk searches when looking for files named via the -f and --file options.

If POSIXLY_CORRECT exists in the environment, then gawk behaves exactly as if --posix had been specified on the command line. If --lint has been specified, gawk issues a warning message to this effect.

BUGS

The -F option is not necessary given the command line variable assignment feature; it remains only for backwards compatibility.

Syntactically invalid single character programs tend to overflow the parse stack, generating a rather unhelpful message. Such programs are surprisingly difficult to diagnose in the completely general case, and the effort to do so really is not worth it.

AUTHORS

The original version of UNIX awk was designed and implemented by Alfred Aho, Peter Weinberger, and Brian Kernighan of Bell Laboratories. Brian Kernighan continues to maintain and enhance it.

Paul Rubin and Jay Fenlason, of the Free Software Foundation, wrote gawk, to be compatible with the original version of awk distributed in Seventh Edition UNIX. John Woods contributed a number of bug fixes. David Trueman, with contributions from Arnold Robbins, made gawk compatible with the new version of UNIX awk. Arnold Robbins is the current maintainer.

The initial DOS port was done by Conrad Kwok and Scott Garfinkle. Scott Deifik is the current DOS maintainer. Pat Rankin did the port to VMS, and Michal Jaegermann did the port to the Atari ST. The port to OS/2 was done by Kai Uwe Rommel, with contributions and help from Darrel Hankerson. Fred Fish supplied support for the Amiga, Stephen Davies provided the Tandem port, and Martin Brown provided the BeOS port.

VERSION INFORMATION

This man page documents gawk, version 3.1.5.

BUG REPORTS

If you find a bug in gawk, please send electronic mail to bug-gawk@gnu.org. Please include your operating system and its revision, the version of gawk (from gawk --version), what C compiler you used to compile it, and a test program and data that are as small as possible for reproducing the problem.

Before sending a bug report, please do two things. First, verify that you have the latest version of gawk. Many bugs (usually subtle ones) are fixed at each release, and if yours is out of date, the problem may already have been solved. Second, please read this man page and the reference manual carefully to be sure that what you think is a bug really is, instead of just a quirk in the language.

Whatever you do, do NOT post a bug report in comp.lang.awk. While the gawk developers occasionally read this newsgroup, posting bug reports there is an unreliable way to report bugs. Instead, please use the electronic mail addresses given above.

If you're using a GNU/Linux system or BSD-based system, you may wish to submit a bug report to the vendor of your distribution. That's fine, but please send a copy to the official email address as well, since there's no guarantee that the bug will be forwarded to the gawk maintainer.

ACKNOWLEDGEMENTS

Brian Kernighan of Bell Laboratories provided valuable assistance during testing and debugging. We thank him.

COPYING PERMISSIONS

Permission is granted to make and distribute verbatim copies of this manual page provided the copyright notice and this permission notice are preserved on all copies.

Permission is granted to copy and distribute modified versions of this manual page under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one.

Permission is granted to copy and distribute translations of this manual page into another language, under the above conditions for modified versions, except that this permission notice may be stated in a translation approved by the Foundation.

Index

AWK PROGRAM EXECUTION

VARIABLES, RECORDS AND FIELDS

Records
Fields
Built-in Variables
Arrays
Variable Typing And Conversion
Octal and Hexadecimal Constants
String Constants

PATTERNS AND ACTIONS

Patterns
Regular Expressions
Actions
Operators
Control Statements
I/O Statements
The printf Statement
Special File Names
Numeric Functions
String Functions
Time Functions
Bit Manipulations Functions
Internationalization Functions

USER-DEFINED FUNCTIONS

DYNAMICALLY LOADING NEW FUNCTIONS

SIGNALS

EXAMPLES

INTERNATIONALIZATION

POSIX COMPATIBILITY

HISTORICAL FEATURES

GNU EXTENSIONS

ENVIRONMENT VARIABLES

SEE ALSO

BUGS

AUTHORS

VERSION INFORMATION

BUG REPORTS

ACKNOWLEDGEMENTS

COPYING PERMISSIONS

This document was created by man2html, using the manual pages.
Time: 01:58:52 GMT, May 26, 2008

Man Bash

The Bash Man Page

Manpage of BASH

BASH

Section: User Commands (1)
Updated: 2005 Dec 28
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NAME

bash - GNU Bourne-Again SHell

SYNOPSIS

bash [options] [file]

COPYRIGHT

DESCRIPTION

Bash is an sh-compatible command language interpreter that executes commands read from the standard input or from a file. Bash also incorporates useful features from the Korn and C shells (ksh and csh).

Bash is intended to be a conformant implementation of the IEEE POSIX Shell and Tools specification (IEEE Working Group 1003.2). Bash can be configured to be POSIX-conformant by default.

OPTIONS

In addition to the single-character shell options documented in the description of the set builtin command, bash interprets the following options when it is invoked:

-c string: If the -c option is present, then commands are read from string. If there are arguments after the string, they are assigned to the positional parameters, starting with $0.
-i: If the -i option is present, the shell is interactive.
-l: Make bash act as if it had been invoked as a login shell (see INVOCATION below).
-r: If the -r option is present, the shell becomes restricted (see RESTRICTED SHELL below).
-s: If the -s option is present, or if no arguments remain after option processing, then commands are read from the standard input. This option allows the positional parameters to be set when invoking an interactive shell.
-D: A list of all double-quoted strings preceded by $ is printed on the standard output. These are the strings that are subject to language translation when the current locale is not C or POSIX. This implies the -n option; no commands will be executed.
[-+]O [shopt_option]: shopt_option is one of the shell options accepted by the shopt builtin (see SHELL BUILTIN COMMANDS below). If shopt_option is present, -O sets the value of that option; +O unsets it. If shopt_option is not supplied, the names and values of the shell options accepted by shopt are printed on the standard output. If the invocation option is +O, the output is displayed in a format that may be reused as input.
--: A -- signals the end of options and disables further option processing. Any arguments after the -- are treated as filenames and arguments. An argument of - is equivalent to --.

Bash also interprets a number of multi-character options. These options must appear on the command line before the single-character options to be recognized.

--debugger: Arrange for the debugger profile to be executed before the shell starts. Turns on extended debugging mode (see the description of the extdebug option to the shopt builtin below) and shell function tracing (see the description of the -o functrace option to the set builtin below).
--dump-po-strings: Equivalent to -D, but the output is in the GNU gettext po (portable object) file format.
--dump-strings: Equivalent to -D.
--help: Display a usage message on standard output and exit successfully.
--init-file file
--rcfile file: Execute commands from file instead of the standard personal initialization file ~/.bashrc if the shell is interactive (see INVOCATION below).
--login: Equivalent to -l.
--noediting: Do not use the GNU readline library to read command lines when the shell is interactive.
--noprofile: Do not read either the system-wide startup file /etc/profile or any of the personal initialization files ~/.bash_profile, ~/.bash_login, or ~/.profile. By default, bash reads these files when it is invoked as a login shell (see INVOCATION below).
--norc: Do not read and execute the personal initialization file ~/.bashrc if the shell is interactive. This option is on by default if the shell is invoked as sh.
--posix: Change the behavior of bash where the default operation differs from the POSIX 1003.2 standard to match the standard (posix mode).
--restricted: The shell becomes restricted (see RESTRICTED SHELL below).
--rpm-requires: Produce the list of files that are required for the shell script to run. This implies '-n' and is subject to the same limitations as compile time error checking checking; Backticks, [] tests, and evals are not parsed so some dependencies may be missed.
--verbose: Equivalent to -v.
--version: Show version information for this instance of bash on the standard output and exit successfully.

ARGUMENTS

If arguments remain after option processing, and neither the -c nor the -s option has been supplied, the first argument is assumed to be the name of a file containing shell commands. If bash is invoked in this fashion, $0 is set to the name of the file, and the positional parameters are set to the remaining arguments. Bash reads and executes commands from this file, then exits. Bash's exit status is the exit status of the last command executed in the script. If no commands are executed, the exit status is 0. An attempt is first made to open the file in the current directory, and, if no file is found, then the shell searches the directories in PATH for the script.

INVOCATION

A login shell is one whose first character of argument zero is a -, or one started with the --login option.

An interactive shell is one started without non-option arguments and without the -c option whose standard input and error are both connected to terminals (as determined by isatty(3)), or one started with the -i option. PS1 is set and $- includes i if bash is interactive, allowing a shell script or a startup file to test this state.

The following paragraphs describe how bash executes its startup files. If any of the files exist but cannot be read, bash reports an error. Tildes are expanded in file names as described below under Tilde Expansion in the EXPANSION section.

When bash is invoked as an interactive login shell, or as a non-interactive shell with the --login option, it first reads and executes commands from the file /etc/profile, if that file exists. After reading that file, it looks for ~/.bash_profile, ~/.bash_login, and ~/.profile, in that order, and reads and executes commands from the first one that exists and is readable. The --noprofile option may be used when the shell is started to inhibit this behavior.

When a login shell exits, bash reads and executes commands from the file ~/.bash_logout, if it exists.

When an interactive shell that is not a login shell is started, bash reads and executes commands from ~/.bashrc, if that file exists. This may be inhibited by using the --norc option. The --rcfile file option will force bash to read and execute commands from file instead of ~/.bashrc.

When bash is started non-interactively, to run a shell script, for example, it looks for the variable BASH_ENV in the environment, expands its value if it appears there, and uses the expanded value as the name of a file to read and execute. Bash behaves as if the following command were executed:

: if [ -n "$BASH_ENV" ]; then . "$BASH_ENV"; fi

but the value of the PATH variable is not used to search for the file name.

If bash is invoked with the name sh, it tries to mimic the startup behavior of historical versions of sh as closely as possible, while conforming to the POSIX standard as well. When invoked as an interactive login shell, or a non-interactive shell with the --login option, it first attempts to read and execute commands from /etc/profile and ~/.profile, in that order. The --noprofile option may be used to inhibit this behavior. When invoked as an interactive shell with the name sh, bash looks for the variable ENV, expands its value if it is defined, and uses the expanded value as the name of a file to read and execute. Since a shell invoked as sh does not attempt to read and execute commands from any other startup files, the --rcfile option has no effect. A non-interactive shell invoked with the name sh does not attempt to read any other startup files. When invoked as sh, bash enters posix mode after the startup files are read.

When bash is started in posix mode, as with the --posix command line option, it follows the POSIX standard for startup files. In this mode, interactive shells expand the ENV variable and commands are read and executed from the file whose name is the expanded value. No other startup files are read.

Bash attempts to determine when it is being run by the remote shell daemon, usually rshd. If bash determines it is being run by rshd, it reads and executes commands from ~/.bashrc, if that file exists and is readable. It will not do this if invoked as sh. The --norc option may be used to inhibit this behavior, and the --rcfile option may be used to force another file to be read, but rshd does not generally invoke the shell with those options or allow them to be specified.

If the shell is started with the effective user (group) id not equal to the real user (group) id, and the -p option is not supplied, no startup files are read, shell functions are not inherited from the environment, the SHELLOPTS variable, if it appears in the environment, is ignored, and the effective user id is set to the real user id. If the -p option is supplied at invocation, the startup behavior is the same, but the effective user id is not reset.

DEFINITIONS

The following definitions are used throughout the rest of this document.

blank

A space or tab.

word

A sequence of characters considered as a single unit by the shell. Also known as a token.

name

A word consisting only of alphanumeric characters and underscores, and beginning with an alphabetic character or an underscore. Also referred to as an identifier.

metacharacter

A character that, when unquoted, separates words. One of the following:

: | & ; ( ) < > space tab

control operator

A token that performs a control function. It is one of the following symbols:

: || & && ; ;; ( ) | <newline>

RESERVED WORDS

Reserved words are words that have a special meaning to the shell. The following words are recognized as reserved when unquoted and either the first word of a simple command (see SHELL GRAMMAR below) or the third word of a case or for command:

! case do done elif else esac fi for function if in select then until while { } time [[ ]]

SHELL GRAMMAR

Simple Commands

A simple command is a sequence of optional variable assignments followed by blank-separated words and redirections, and terminated by a control operator. The first word specifies the command to be executed, and is passed as argument zero. The remaining words are passed as arguments to the invoked command.

The return value of a simple command is its exit status, or 128+n if the command is terminated by signal n.

Pipelines

A pipeline is a sequence of one or more commands separated by the character |. The format for a pipeline is:

: [time [-p]] [ ! ] command [ | command2 ... ]

The standard output of command is connected via a pipe to the standard input of command2. This connection is performed before any redirections specified by the command (see REDIRECTION below).

The return status of a pipeline is the exit status of the last command, unless the pipefail option is enabled. If pipefail is enabled, the pipeline's return status is the value of the last (rightmost) command to exit with a non-zero status, or zero if all commands exit successfully. If the reserved word ! precedes a pipeline, the exit status of that pipeline is the logical negation of the exit status as described above. The shell waits for all commands in the pipeline to terminate before returning a value.

If the time reserved word precedes a pipeline, the elapsed as well as user and system time consumed by its execution are reported when the pipeline terminates. The -p option changes the output format to that specified by POSIX. The TIMEFORMAT variable may be set to a format string that specifies how the timing information should be displayed; see the description of TIMEFORMAT under Shell Variables below.

Each command in a pipeline is executed as a separate process (i.e., in a subshell).

Lists

A list is a sequence of one or more pipelines separated by one of the operators ;, &, &&, or ||, and optionally terminated by one of ;, &, or <newline>.

Of these list operators, && and || have equal precedence, followed by ; and &, which have equal precedence.

A sequence of one or more newlines may appear in a list instead of a semicolon to delimit commands.

If a command is terminated by the control operator &, the shell executes the command in the background in a subshell. The shell does not wait for the command to finish, and the return status is 0. Commands separated by a ; are executed sequentially; the shell waits for each command to terminate in turn. The return status is the exit status of the last command executed.

The control operators && and || denote AND lists and OR lists, respectively. An AND list has the form

: command1 && command2

command2 is executed if, and only if, command1 returns an exit status of zero.

An OR list has the form

command1 || command2

command2 is executed if and only if command1 returns a non-zero exit status. The return status of AND and OR lists is the exit status of the last command executed in the list.

Compound Commands

A compound command is one of the following:

(list)

list is executed in a subshell environment (see COMMAND EXECUTION ENVIRONMENT below). Variable assignments and builtin commands that affect the shell's environment do not remain in effect after the command completes. The return status is the exit status of list.

{ list; }

list is simply executed in the current shell environment. list must be terminated with a newline or semicolon. This is known as a group command. The return status is the exit status of list. Note that unlike the metacharacters ( and ), { and } are reserved words and must occur where a reserved word is permitted to be recognized. Since they do not cause a word break, they must be separated from list by whitespace.

((expression))

The expression is evaluated according to the rules described below under ARITHMETICEVALUATION. If the value of the expression is non-zero, the return status is 0; otherwise the return status is 1. This is exactly equivalent to let "expression".

[[ expression ]]

Return a status of 0 or 1 depending on the evaluation of the conditional expression expression. Expressions are composed of the primaries described below under CONDITIONALEXPRESSIONS. Word splitting and pathname expansion are not performed on the words between the [[ and ]]; tilde expansion, parameter and variable expansion, arithmetic expansion, command substitution, process substitution, and quote removal are performed. Conditional operators such as -f must be unquoted to be recognized as primaries.

When the == and != operators are used, the string to the right of the operator is considered a pattern and matched according to the rules described below under Pattern Matching. If the shell option nocasematch is enabled, the match is performed without regard to the case of alphabetic characters. The return value is 0 if the string matches (==) or does not match (!=) the pattern, and 1 otherwise. Any part of the pattern may be quoted to force it to be matched as a string.

An additional binary operator, =~, is available, with the same precedence as == and !=. When it is used, the string to the right of the operator is considered an extended regular expression and matched accordingly (as in regex(3)). The return value is 0 if the string matches the pattern, and 1 otherwise. If the regular expression is syntactically incorrect, the conditional expression's return value is 2. If the shell option nocasematch is enabled, the match is performed without regard to the case of alphabetic characters. Substrings matched by parenthesized subexpressions within the regular expression are saved in the array variable BASH_REMATCH. The element of BASH_REMATCH with index 0 is the portion of the string matching the entire regular expression. The element of BASH_REMATCH with index n is the portion of the string matching the nth parenthesized subexpression.

Expressions may be combined using the following operators, listed in decreasing order of precedence:

( expression ): Returns the value of expression. This may be used to override the normal precedence of operators.
! expression: True if expression is false.
expression1 && expression2: True if both expression1 and expression2 are true.
: expression1 || expression2 True if either expression1 or expression2 is true.

The && and || operators do not evaluate expression2 if the value of expression1 is sufficient to determine the return value of the entire conditional expression.

for name [ in word ] ; do list ; done

The list of words following in is expanded, generating a list of items. The variable name is set to each element of this list in turn, and list is executed each time. If the in word is omitted, the for command executes list once for each positional parameter that is set (see PARAMETERS below). The return status is the exit status of the last command that executes. If the expansion of the items following in results in an empty list, no commands are executed, and the return status is 0.

for (( expr1 ; expr2 ; expr3 )) ; do list ; done

First, the arithmetic expression expr1 is evaluated according to the rules described below under ARITHMETICEVALUATION. The arithmetic expression expr2 is then evaluated repeatedly until it evaluates to zero. Each time expr2 evaluates to a non-zero value, list is executed and the arithmetic expression expr3 is evaluated. If any expression is omitted, it behaves as if it evaluates to 1. The return value is the exit status of the last command in list that is executed, or false if any of the expressions is invalid.

select name [ in word ] ; do list ; done

The list of words following in is expanded, generating a list of items. The set of expanded words is printed on the standard error, each preceded by a number. If the in word is omitted, the positional parameters are printed (see PARAMETERS below). The PS3 prompt is then displayed and a line read from the standard input. If the line consists of a number corresponding to one of the displayed words, then the value of name is set to that word. If the line is empty, the words and prompt are displayed again. If EOF is read, the command completes. Any other value read causes name to be set to null. The line read is saved in the variable REPLY. The list is executed after each selection until a break command is executed. The exit status of select is the exit status of the last command executed in list, or zero if no commands were executed.

case word in [ [(] pattern [ | pattern ]

A case command first expands word, and tries to match it against each pattern in turn, using the same matching rules as for pathname expansion (see Pathname Expansion below). The word is expanded using tilde expansion, parameter and variable expansion, arithmetic substituion, command substitution, process substitution and quote removal. Each pattern examined is expanded using tilde expansion, parameter and variable expansion, arithmetic substituion, command substitution, and process substitution. If the shell option nocasematch is enabled, the match is performed without regard to the case of alphabetic characters. When a match is found, the corresponding list is executed. After the first match, no subsequent matches are attempted. The exit status is zero if no pattern matches. Otherwise, it is the exit status of the last command executed in list.

if list; then list; [ elif list; then list; ] ... [ else list; ] fi

The if list is executed. If its exit status is zero, the then list is executed. Otherwise, each elif list is executed in turn, and if its exit status is zero, the corresponding then list is executed and the command completes. Otherwise, the else list is executed, if present. The exit status is the exit status of the last command executed, or zero if no condition tested true.

while list; do list; done

until list; do list; done

The while command continuously executes the do list as long as the last command in list returns an exit status of zero. The until command is identical to the while command, except that the test is negated; the do list is executed as long as the last command in list returns a non-zero exit status. The exit status of the while and until commands is the exit status of the last do list command executed, or zero if none was executed.

Shell Function Definitions

A shell function is an object that is called like a simple command and executes a compound command with a new set of positional parameters. Shell functions are declared as follows:

[ function ] name () compound-command [redirection]: This defines a function named name. The reserved word function is optional. If the function reserved word is supplied, the parentheses are optional. The body of the function is the compound command compound-command (see Compound Commands above). That command is usually a list of commands between { and }, but may be any command listed under Compound Commands above. compound-command is executed whenever name is specified as the name of a simple command. Any redirections (see REDIRECTION below) specified when a function is defined are performed when the function is executed. The exit status of a function definition is zero unless a syntax error occurs or a readonly function with the same name already exists. When executed, the exit status of a function is the exit status of the last command executed in the body. (See FUNCTIONS below.)

COMMENTS

In a non-interactive shell, or an interactive shell in which the interactive_comments option to the shopt builtin is enabled (see SHELL BUILTIN COMMANDS below), a word beginning with # causes that word and all remaining characters on that line to be ignored. An interactive shell without the interactive_comments option enabled does not allow comments. The interactive_comments option is on by default in interactive shells.

QUOTING

Quoting is used to remove the special meaning of certain characters or words to the shell. Quoting can be used to disable special treatment for special characters, to prevent reserved words from being recognized as such, and to prevent parameter expansion.

Each of the metacharacters listed above under DEFINITIONS has special meaning to the shell and must be quoted if it is to represent itself.

When the command history expansion facilities are being used (see HISTORY EXPANSION below), the history expansion character, usually !, must be quoted to prevent history expansion.

There are three quoting mechanisms: the escape character, single quotes, and double quotes.

A non-quoted backslash (\) is the escape character. It preserves the literal value of the next character that follows, with the exception of <newline>. If a \<newline> pair appears, and the backslash is not itself quoted, the \<newline> is treated as a line continuation (that is, it is removed from the input stream and effectively ignored).

Enclosing characters in single quotes preserves the literal value of each character within the quotes. A single quote may not occur between single quotes, even when preceded by a backslash.

Enclosing characters in double quotes preserves the literal value of all characters within the quotes, with the exception of $, `, \, and, when history expansion is enabled, !. The characters $ and ` retain their special meaning within double quotes. The backslash retains its special meaning only when followed by one of the following characters: $, `, ", \, or <newline>. A double quote may be quoted within double quotes by preceding it with a backslash. If enabled, history expansion will be performed unless an ! appearing in double quotes is escaped using a backslash. The backslash preceding the ! is not removed.

The special parameters * and @ have special meaning when in double quotes (see PARAMETERS below).

Words of the form $aqstringaq are treated specially. The word expands to string, with backslash-escaped characters replaced as specified by the ANSI C standard. Backslash escape sequences, if present, are decoded as follows:

\a: alert (bell)
\b: backspace
\e: an escape character
\f: form feed
\n: new line
\r: carriage return
\t: horizontal tab
\v: vertical tab
\\: backslash
\aq: single quote
\nnn: the eight-bit character whose value is the octal value nnn (one to three digits)
\xHH: the eight-bit character whose value is the hexadecimal value HH (one or two hex digits)
\cx: a control-x character

The expanded result is single-quoted, as if the dollar sign had not been present.

A double-quoted string preceded by a dollar sign ($) will cause the string to be translated according to the current locale. If the current locale is C or POSIX, the dollar sign is ignored. If the string is translated and replaced, the replacement is double-quoted.

PARAMETERS

A parameter is an entity that stores values. It can be a name, a number, or one of the special characters listed below under Special Parameters. A variable is a parameter denoted by a name. A variable has a value and zero or more attributes. Attributes are assigned using the declare builtin command (see declare below in SHELLBUILTINCOMMANDS).

A parameter is set if it has been assigned a value. The null string is a valid value. Once a variable is set, it may be unset only by using the unset builtin command (see SHELL BUILTIN COMMANDS below).

A variable may be assigned to by a statement of the form

: name=[value]

If value is not given, the variable is assigned the null string. All values undergo tilde expansion, parameter and variable expansion, command substitution, arithmetic expansion, and quote removal (see EXPANSION below). If the variable has its integer attribute set, then value is evaluated as an arithmetic expression even if the $((...)) expansion is not used (see Arithmetic Expansion below). Word splitting is not performed, with the exception of "$@" as explained below under Special Parameters. Pathname expansion is not performed. Assignment statements may also appear as arguments to the alias, declare, typeset, export, readonly, and local builtin commands.

In the context where an assignment statement is assigning a value to a shell variable or array index, the += operator can be used to append to or add to the variable's previous value. When += is applied to a variable for which the integer attribute has been set, value is evaluated as an arithmetic expression and added to the variable's current value, which is also evaluated. When += is applied to an array variable using compound assignment (see Arrays below), the variable's value is not unset (as it is when using =), and new values are appended to the array beginning at one greater than the array's maximum index. When applied to a string-valued variable, value is expanded and appended to the variable's value.

Positional Parameters

A positional parameter is a parameter denoted by one or more digits, other than the single digit 0. Positional parameters are assigned from the shell's arguments when it is invoked, and may be reassigned using the set builtin command. Positional parameters may not be assigned to with assignment statements. The positional parameters are temporarily replaced when a shell function is executed (see FUNCTIONS below).

When a positional parameter consisting of more than a single digit is expanded, it must be enclosed in braces (see EXPANSION below).

Special Parameters

The shell treats several parameters specially. These parameters may only be referenced; assignment to them is not allowed.

*: Expands to the positional parameters, starting from one. When the expansion occurs within double quotes, it expands to a single word with the value of each parameter separated by the first character of the IFS special variable. That is, "$*" is equivalent to "$1c$2c...", where c is the first character of the value of the IFS variable. If IFS is unset, the parameters are separated by spaces. If IFS is null, the parameters are joined without intervening separators.
@: Expands to the positional parameters, starting from one. When the expansion occurs within double quotes, each parameter expands to a separate word. That is, "$@" is equivalent to "$1" "$2" ... If the double-quoted expansion occurs within a word, the expansion of the first parameter is joined with the beginning part of the original word, and the expansion of the last parameter is joined with the last part of the original word. When there are no positional parameters, "$@" and $@ expand to nothing (i.e., they are removed).
#: Expands to the number of positional parameters in decimal.
?: Expands to the status of the most recently executed foreground pipeline.
-: Expands to the current option flags as specified upon invocation, by the set builtin command, or those set by the shell itself (such as the -i option).
$: Expands to the process ID of the shell. In a () subshell, it expands to the process ID of the current shell, not the subshell.
!: Expands to the process ID of the most recently executed background (asynchronous) command.
0: Expands to the name of the shell or shell script. This is set at shell initialization. If bash is invoked with a file of commands, $0 is set to the name of that file. If bash is started with the -c option, then $0 is set to the first argument after the string to be executed, if one is present. Otherwise, it is set to the file name used to invoke bash, as given by argument zero.
_: At shell startup, set to the absolute pathname used to invoke the shell or shell script being executed as passed in the environment or argument list. Subsequently, expands to the last argument to the previous command, after expansion. Also set to the full pathname used to invoke each command executed and placed in the environment exported to that command. When checking mail, this parameter holds the name of the mail file currently being checked.

Shell Variables

The following variables are set by the shell:

BASH

Expands to the full file name used to invoke this instance of bash.

BASH_ARGC

An array variable whose values are the number of parameters in each frame of the current bash execution call stack. The number of parameters to the current subroutine (shell function or script executed with . or source) is at the top of the stack. When a subroutine is executed, the number of parameters passed is pushed onto BASH_ARGC. The shell sets BASH_ARGC only when in extended debugging mode (see the description of the extdebug option to the shopt builtin below)

BASH_ARGV

An array variable containing all of the parameters in the current bash execution call stack. The final parameter of the last subroutine call is at the top of the stack; the first parameter of the initial call is at the bottom. When a subroutine is executed, the parameters supplied are pushed onto BASH_ARGV. The shell sets BASH_ARGV only when in extended debugging mode (see the description of the extdebug option to the shopt builtin below)

BASH_COMMAND

The command currently being executed or about to be executed, unless the shell is executing a command as the result of a trap, in which case it is the command executing at the time of the trap.

BASH_EXECUTION_STRING

The command argument to the -c invocation option.

BASH_LINENO

An array variable whose members are the line numbers in source files corresponding to each member of FUNCNAME. ${BASH_LINENO[$i]} is the line number in the source file where ${FUNCNAME[$ifP]} was called. The corresponding source file name is ${BASH_SOURCE[$i]}. Use LINENO to obtain the current line number.

BASH_REMATCH

An array variable whose members are assigned by the =~ binary operator to the [[ conditional command. The element with index 0 is the portion of the string matching the entire regular expression. The element with index n is the portion of the string matching the nth parenthesized subexpression. This variable is read-only.

BASH_SOURCE

An array variable whose members are the source filenames corresponding to the elements in the FUNCNAME array variable.

BASH_SUBSHELL

Incremented by one each time a subshell or subshell environment is spawned. The initial value is 0.

BASH_VERSINFO

A readonly array variable whose members hold version information for this instance of bash. The values assigned to the array members are as follows:

BASH_VERSINFO[0]: The major version number (the release).
BASH_VERSINFO[1]: The minor version number (the version).
BASH_VERSINFO[2]: The patch level.
BASH_VERSINFO[3]: The build version.
BASH_VERSINFO[4]: The release status (e.g., beta1).
BASH_VERSINFO[5]: The value of MACHTYPE.

BASH_VERSION

Expa