Bash: validating variables and parameters

This is a wiki page. Be bold and improve it!

If you have any questions about the content on this page, don't hesitate to open a new ticket and we'll do our best to assist you.

Table of Contents
  1. Variables
  2. Strings
  3. Files
  4. See also
  5. More

Help us complete this page with more examples of validation.

Variables

The simplest validation is as follows:

if [[ "$variable" ]]; then
        echo "set"   
else
        echo "unset"
fi

It checks if a variable is set.

Beware, if you want to validate the value of the variable, you'll have to be more explicit:

variable=0
if [[ $variable ]]; then
        echo "This gets printed because the variable is set."   
elif [[ $variable == 1 ]]; then
        echo "This does not get printed because the variable equals 0."
fi

Strings

You can use the equal-tilde operator (=~) to test strings against a regular expression:

#!/bin/bash

if ! [[ "$1" =~ [^a-zA-Z0-9] ]]; then
  echo "$1 is NOT alphanumeric."
else
  echo "$1 is alphanumeric"
fi

Files

if [[ -e "$1" ]]
then
        echo "The file exists."
else
        echo "The file does not exist."
fi

See also

More

See section "6.4 Bash Conditional Expressions" of the official documentation:
http://www.gnu.org/software/bash/manual/bash.html#Bash-Conditional-Expre...