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

Question

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

Answer 1

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

Answer 2

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.

Answer 3

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

Answer 4

To check if variable v is not set

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