How to find whether or not a variable is empty in Bash

How can I check if a variable is empty in Bash?

10 Answers

In Bash at least the following command tests if $var is empty:

if [[ -z "$var" ]]; then # Do what you want fi 

The command man test is your friend.

Presuming Bash:

var="" if [ -n "$var" ]; then echo "not empty" else echo "empty" fi 

I have also seen

if [ "x$variable" = "x" ]; then ... 

which is obviously very robust and shell independent.

Also, there is a difference between "empty" and "unset". See How to tell if a string is not defined in a Bash shell script.

if [ ${foo:+1} ] then echo "yes" fi 

prints yes if the variable is set. ${foo:+1} will return 1 when the variable is set, otherwise it will return empty string.

if [[ "$variable" == "" ]] ... 

[ "$variable" ] || echo empty : ${variable="value_to_set_if_unset"} 

This will return true if a variable is unset or set to the empty string ("").

if [ -z "$MyVar" ] then echo "The variable MyVar has nothing in it." elif ! [ -z "$MyVar" ] then echo "The variable MyVar has something in it." fi 

The question asks how to check if a variable is an empty string and the best answers are already given for that.

But I landed here after a period passed programming in PHP, and I was actually searching for a check like the empty function in PHP working in a Bash shell.

After reading the answers I realized I was not thinking properly in Bash, but anyhow in that moment a function like empty in PHP would have been soooo handy in my Bash code.

As I think this can happen to others, I decided to convert the PHP empty function in Bash.

According to the PHP manual:

a variable is considered empty if it doesn't exist or if its value is one of the following:

• "" (an empty string)
• 0 (0 as an integer)
• 0.0 (0 as a float)
• "0" (0 as a string)
• an empty array
• a variable declared, but without a value

Of course the null and false cases cannot be converted in bash, so they are omitted.

function empty { local var="$1" # Return true if: # 1. var is a null string ("" as empty string) # 2. a non set variable is passed # 3. a declared variable or array but without a value is passed # 4. an empty array is passed if test -z "$var" then [[ $( echo "1" ) ]] return # Return true if var is zero (0 as an integer or "0" as a string) elif [ "$var" == 0 2> /dev/null ] then [[ $( echo "1" ) ]] return # Return true if var is 0.0 (0 as a float) elif [ "$var" == 0.0 2> /dev/null ] then [[ $( echo "1" ) ]] return fi [[ $( echo "" ) ]] } 

Example of usage:

if empty "${var}" then echo "empty" else echo "not empty" fi 

Demo:
The following snippet:

#!/bin/bash vars=( "" 0 0.0 "0" 1 "string" " " ) for (( i=0; i<${#vars[@]}; i++ )) do var="${vars[$i]}" if empty "${var}" then what="empty" else what="not empty" fi echo "VAR \"$var\" is $what" done exit 

outputs:

VAR "" is empty VAR "0" is empty VAR "0.0" is empty VAR "0" is empty VAR "1" is not empty VAR "string" is not empty VAR " " is not empty 

Having said that in a Bash logic the checks on zero in this function can cause side problems imho, anyone using this function should evaluate this risk and maybe decide to cut those checks off leaving only the first one.

You may want to distinguish between unset variables and variables that are set and empty:

is_empty() { local var_name="$1" local var_value="${!var_name}" if [[ -v "$var_name" ]]; then if [[ -n "$var_value" ]]; then echo "set and non-empty" else echo "set and empty" fi else echo "unset" fi } str="foo" empty="" is_empty str is_empty empty is_empty none 

Result:

set and non-empty set and empty unset 

BTW, I recommend using set -u which will cause an error when reading unset variables, this can save you from disasters such as

rm -rf $dir 

You can read about this and other best practices for a "strict mode" here.

To check if variable v is not set

if [ "$v" == "" ]; then echo "v not set" fi