How to check if a variable is set in Bash?

Question

How do I know if a variable is set in Bash?

For example, how do I check if the user gave the first parameter to a function?

function a {
    # if $1 is set ?
}

Answer 1

I always use this one, based on the fact that it seems easy to be understood by anybody who sees the code for the very first time:

if [ "$variable" = "" ]
    then
    echo "Variable X is empty"
fi

And, if wanting to check if not empty;

if [ ! "$variable" = "" ]
    then
    echo "Variable X is not empty"
fi

That's it.

Answer 2

if [[ ${1:+isset} ]]
then echo "It was set and not null." >&2
else echo "It was not set or it was null." >&2
fi

if [[ ${1+isset} ]]
then echo "It was set but might be null." >&2
else echo "It was was not set." >&2
fi

Answer 3

Here's how to test whether a parameter is unset, or empty ("Null") or set with a value:

+--------------------+----------------------+-----------------+-----------------+
|                    |       parameter      |     parameter   |    parameter    |
|                    |   Set and Not Null   |   Set But Null  |      Unset      |
+--------------------+----------------------+-----------------+-----------------+
| ${parameter:-word} | substitute parameter | substitute word | substitute word |
| ${parameter-word}  | substitute parameter | substitute null | substitute word |
| ${parameter:=word} | substitute parameter | assign word     | assign word     |
| ${parameter=word}  | substitute parameter | substitute null | assign word     |
| ${parameter:?word} | substitute parameter | error, exit     | error, exit     |
| ${parameter?word}  | substitute parameter | substitute null | error, exit     |
| ${parameter:+word} | substitute word      | substitute null | substitute null |
| ${parameter+word}  | substitute word      | substitute word | substitute null |
+--------------------+----------------------+-----------------+-----------------+

Source: POSIX: Parameter Expansion:

In all cases shown with "substitute", the expression is replaced with the value shown. In all cases shown with "assign", parameter is assigned that value, which also replaces the expression.

Answer 4

If you wish to test that a variable is bound or unbound, this works well, even after you've turned on the nounset option:

set -o noun set

if printenv variableName >/dev/null; then
    # variable is bound to a value
else
    # variable is unbound
fi

Answer 5

(Usually) The right way

if [ -z ${var+x} ]; then echo "var is unset"; else echo "var is set to '$var'"; fi

where ${var+x} is a parameter expansion which evaluates to nothing if var is unset, and substitutes the string x otherwise.

Quotes Digression

Quotes can be omitted (so we can say ${var+x} instead of "${var+x}") because this syntax & usage guarantees this will only expand to something that does not require quotes (since it either expands to x (which contains no word breaks so it needs no quotes), or to nothing (which results in [ -z ], which conveniently evaluates to the same value (true) that [ -z "" ] does as well)).

However, while quotes can be safely omitted, and it was not immediately obvious to all (it wasn't even apparent to the first author of this quotes explanation who is also a major Bash coder), it would sometimes be better to write the solution with quotes as [ -z "${var+x}" ], at the very small possible cost of an O(1) speed penalty. The first author also added this as a comment next to the code using this solution giving the URL to this answer, which now also includes the explanation for why the quotes can be safely omitted.

(Often) The wrong way

if [ -z "$var" ]; then echo "var is blank"; else echo "var is set to '$var'"; fi

This is often wrong because it doesn't distinguish between a variable that is unset and a variable that is set to the empty string. That is to say, if var='', then the above solution will output "var is blank".

The distinction between unset and "set to the empty string" is essential in situations where the user has to specify an extension, or additional list of properties, and that not specifying them defaults to a non-empty value, whereas specifying the empty string should make the script use an empty extension or list of additional properties.

The distinction may not be essential in every scenario though. In those cases [ -z "$var" ] will be just fine.

Answer 6

if [ "$1" != "" ]; then
  echo \$1 is set
else
  echo \$1 is not set
fi

Although for arguments it is normally best to test $#, which is the number of arguments, in my opinion.

if [ $# -gt 0 ]; then
  echo \$1 is set
else
  echo \$1 is not set
fi