10 Tips for Writing Efficient Bash Scripts

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.

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.
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"
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
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
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
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
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.
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.
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)
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

Bash

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Sometimes you can do without

Sometimes you can do without xargs:

find ~/Desktop -type f -print0 | xargs -0 tar -zcf files.tar.gz

find ~/Desktop -type f -print0 | tar --null --no-recursion -zcf files.tar.gz --files-from -

http://codesnippets.joyent.com/posts/show/1962

Fri, 02/06/2009 - 4:14am — Stephan Beal (not verified)

Stop using "wc -l" in

Stop using "wc -l" in conjunction with grep, e.g.: grep ... | wc -l

Instead use: grep -c ...

Wed, 02/04/2009 - 10:42pm — Anonymous (not verified)

in #10, the substitute in

in #10, the substitute in the sed command is inefficient. try:
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Mon, 02/02/2009 - 5:24pm — Rocket Ray (not verified)

Wow. As a non-techie who's

Wow. As a non-techie who's been piddling around with shell scripts for a few years, this is just about the best freaking collection of tips I ever saw!

Mon, 02/02/2009 - 4:16pm — Anonymous (not verified)

Thats good stuff for someone

Thats good stuff for someone who is starting learn bash scripts. I could imagine that is would be a good idea to show this page to my prof at the uni ;-)....

Mon, 02/02/2009 - 2:05pm — flint (not verified)

Very nice collection. But it

Very nice collection. But it looks like tip #9 violates tip #1. :p

Mon, 02/02/2009 - 3:36pm — Wil Moore III (not verified)

@flint >>...But it looks

@flint

>>...But it looks like tip #9 violates tip #1. :p

Actually mktemp is not a bash builtin so the post is accurate. You can verify this by typing "builtins".

-Wil Moore III

Mon, 02/02/2009 - 10:32am — Pádraig Brady (not verified)

Good tips thanks. For

Good tips thanks. For general scripting tips along the same lines but not bash specific, see: common shell script mistakes

Mon, 02/02/2009 - 10:04am — Anonymous (not verified)

While you should always

While you should always avoid changing your working directory in a shell script, if you must do so, use pushd and popd. That way, you don't have to explicitly save the current path to a variable to return to it later. (Handy on the command line too.)

Tue, 02/10/2009 - 1:21pm — Anonymous (not verified)

Parenthesis.... Actually, it

Parenthesis....

Actually, it is much easier to use parenthesis.

That is,

pwd
( cd /tmp/; pwd )
pwd # the directory is back to the original location

Mon, 02/02/2009 - 7:13am — Cyril Chaboisseau (not verified)

Along the same line...

Along the same line... (find)

Some other optimization that saves a lot of extra shell is the use of the handy "xargs" command (especially in find) :

For instance, instead of doing a
find dir -type f -exec grep pattern "{}" \;
there is a much more optimized way of regrouping all the individual grep into one :
find dir -type f | xargs grep pattern

In terms of performance, that means running a few grep in a subshell (depending of the number of files) instead of 1 subshell +1 grep per file (the benefit increases as the number of files gets bigger)

The other nice "side effect" of the latter is that you also have filenames prepended before each matching which could also be accomplished in the 1st form by adding a second filename for each grep :
find dir -type f -exec grep pattern "{}" /dev/null \;

Mon, 02/02/2009 - 3:57pm — porges (not verified)

Check the find man page, it

Check the find man page, it provides the "+" -exec option in addition to the ";" one :)

You can also tell grep to print filenames with the -H option (again, check the man page).

Mon, 02/02/2009 - 6:46am — Cyril Chaboisseau (not verified)

No more YAUUOC please :-) #3

No more YAUUOC please :-)
#3 shows how to avoid YAUUOC's (yet another useless use of cat) but that is not restricted to commands like grep which handle filenames as a parameter :

for instance, tr which work from it's standard input can be used as followed :
tr pattern < file
instead of the "classical"
cat file | tr pattern

So, one thing to remember : cat is *NOT* a tool to print files onto the standard output (or pipe) but as it's name says : it is a way to concatenate files

Mon, 02/02/2009 - 5:53am — bpp (not verified)

> function_name() { >

> function_name() {
> do_something
> return $?
> }

In some cases, there is no need to return $?, because bash functions return exit status of the last executed command.

P.S. Your captcha is awful

Mon, 02/02/2009 - 3:17am — Anonymous (not verified)

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