Shell Scripting Cookbook

Quit or Exit the Script if a Failure Occurs

Set the shell's e option.

#!/bin/sh
set -e
true
false

Alternative way to set the e option.

#!/bin/sh -e
true
false

To disable the option use +.

#!/bin/sh
set -e
true
set +e
false

Print Command Before Executing

Set the shell's x option.

#!/bin/sh
set -x
true
false

Alternative way to set the e option.

#!/bin/sh -x
true
false

There's a difference between x and v. In v a variable is printed as a variable and in x the variable's value is printed.

#!/bin/sh
a='Hello World!'
set -v
printf "${a}"
set +v
set -x
printf "${a}"
set +x

To disable the option use + as shown in the above example.

Compare Version Numbers

Comparing two version numbers in dot separated format (e.g. 1.2.3) can be done in two ways.

Using BSD or GNU sort to find older version.

#!/bin/sh
a=1.2.0
b=1.2.1
printf "${a}\n${b}\n" | sort -t. -k 1,1n -k 2,2n -k 3,3n | head -n1

Using BSD or GNU sort to find newer version.

#!/bin/sh
a=1.2.0
b=1.2.1
printf "${a}\n${b}\n" | sort -t. -k 1,1nr -k 2,2nr -k 3,3nr | head -n1

Using GNU sort to find older version.

#!/bin/sh
a=1.2.0
b=1.2.1
printf "${a}\n${b}\n" | sort -V | head -n1

Using GNU sort to find newer version.

#!/bin/sh
a=1.2.0
b=1.2.1
printf "${a}\n${b}\n" | sort -Vr | head -n1

Assert

The test command is a rough equivalent to assert in other languages. Read its man page (man test) for more information.

Recover from Error Return Code

For commands that can return a non-zero exit code and possibly stop the script, use OR (||) operator with an alternative command that is sure to succeed. The true boolean value is one such command.

#!/bin/sh -e
git clone https://example.com/example.git /path/already/exists || true

Another example is to create and checkout a git branch that already exists.

#!/bin/sh -e
git checkout -b mybranch origin/mybranch || git checkout mybranch

The shell builtin : (used without arguments here) returns a zero exit code. In this case true and : are functionally equivalent.

#!/bin/sh -e
git clone https://example.com/example.git /path/already/exists || :

Source a File

To source a file means to execute it as if it was entered by a user manually. It is not run in a separate shell. So e.g. any export-ed variables become available in the current script as environment variables.

In Bourne shell (/bin/sh) command . is used to source a file. Command source does not work in /bin/sh.

#!/bin/sh
. file_to_source.sh