HACKTUX notes from the trenches

Bash is the default command line interface for many Linux distributions and a powerful scripting language. Here are some suggestions that will keep your Bash scripts efficient and lean. Feel free to comment with your own suggestions.

1. Avoid Full Paths to Bash Builtins

   Bash has many builtins that can be used instead of calling external commands. You should leverage the builtin commands whenever possible since it avoids calling a subcommand from the system.

   Since Bash has builtins for some commands found in /bin and /usr/bin (such as echo), avoid using the full path for these commands and the builtin will be used.

   # avoid this
/bin/echo "hello"

   Use the Bash builtin instead:

   echo "hello"

   Other bash builtins include: test, read, declare, eval, let pushd and popd. See the Bash man page for a full listing of builtins.

2. Avoid External Commands for Integer Math

   Bash also provides builtins that can be used for integer arithmetic. Only use /usr/bin/bc if you need to do floating point arithmetic. Integer calculations can be made with these Bash builtins:

   four=$(( 2 + 2 ))
four=$[ 2 + 2 ]
let four="2 + 2"

3. Avoid using Cat

   Tools like Grep, Awk and Sed will take files as arguments. There is rarely a need to use /bin/cat. For instance, the following is unnecessary:

   # avoid this
cat /etc/hosts | grep localhost

   Instead, use Grep's native ability to read files:

   grep localhost /etc/hosts

4. Avoid Piping Grep to Awk

   If using Awk, you can often eliminate the need for grep. Try not to pipe Grep to Awk:

   # avoid this
grep error /var/log/messages | awk '{ print $4 }'

   Use Awk's native ability to parse text and save yourself a command.

   awk '/error/ { print $4 }' /var/log/messages

5. Avoid Piping Sed to Sed

   Sed can take more than one command in a single execution. Avoid piping sed to sed.

   # avoid this
sed 's/hello/goodbye/g' filename | sed 's/monday/friday/g'

   Instead, use sed -e or delimit the sed expressions with a semicolon (;)

   sed -e 's/hello/goodbye/g' -e 's/monday/friday/g' filename
sed -e 's/hello/goodbye/g; s/monday/friday/g' filename

6. Use Double Brackets for Compound and Regex Tests

   The [ or test builtins can be used to test expressions, but the [[ builtin operator additionally provides compound commands and regular expression matching.

   if [[ expression1 || expression2 ]]; then do_something; fi
if [[ string =~ regex ]]; then do_something; fi

7. Use Functions for Repetitive Tasks

   Break your script up into pieces and use functions to conduct repetitive tasks. Functions can be declared like so:

   function_name() {
  do_something
  return $?
}

   Make your functions usable by more than one shell script by sourcing a functions file from the various scripts. You can source another file in Bash using the . builtin.

   #!/bin/bash
. /path/to/shared_functions

   See the Bash man page, or my article on Bash functions for more information.

8. Use Arrays Instead of Multiple Variables

   Bash arrays are very powerful. Avoid using unnecessary variables:

   # avoid this
color1='Blue'
color2='Red'
echo $color1
echo $color2

   Instead, use Bash arrays.

   colors=('Blue' 'Red')
echo ${colors[0]}
echo ${colors[1]}

   Check out my article on Bash arrays for more details.

9. Use /bin/mktemp to Create Temp Files

   Need a temporary file? Use /bin/mktemp to create temporary files or folders.

   tempfile=$(/bin/mktemp)
tempdir=$(/bin/mktemp -d)

10. Use /bin/egrep or /bin/sed for Regex Pattern Matching

    Think you need Perl? Check out Sed or Egrep (grep -e) for regex pattern matching.

    # grep for localhost or 127.0.0.1 in /etc/hosts
egrep 'localhost|127\.0\.0\.1' /etc/hosts
 
# print pattern localhost.* in /etc/hosts
sed -n 's/localhost.*/&/p' /etc/hosts