Check if a variable is set in Bash

Checking if a variable is set is easy in Bash; however, there are two common scenarios:

1. Checking if a variable is empty or unset
2. Checking if a variable is unset

Checking if a variable is empty or unset

Firstly, a simple test may be used to determine if a variable’s value is empty or if it’s unset.

1if [[ -z "$var" ]]; then
2    echo "it's empty or unset"
3fi

OR

1var=""
2if [[ -z "$var" ]]; then
3    echo "it's empty or unset"
4fi

Output:

1it's empty or unset

The -z tests if the string length is zero.

Checking if a variable is unset

Checking if a variable is unset requires a different implementation.

1if [[ -z "${var+set}" ]]; then
2    echo "it's unset"
3fi

Output:

1it's unset

However, if the var value is set to an empty value:

1var=
2if [[ -z "${var+set}" ]]; then
3    echo "it's unset"
4fi

OR

1var=""
2if [[ -z "${var+set}" ]]; then
3    echo "it's unset"
4fi

the output will not print anything.

The ${var+set} works because of parameter expansion:

If parameter is unset or null, null shall be substituted; otherwise, the expansion of word (or an empty string if word is omitted) shall be substituted.