How to check if a variable is set in Bash?

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

function a {
    # if $1 is set ?
}

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.

How to check if a variable is set in Bash?

to check for non-null/non-zero string variable, i.e. if set, use

if [ -n "$1" ]

It’s the opposite of -z. I find myself using -n more than -z.

You would use it like:

if [ -n "$1" ]; then
  echo "You supplied the first parameter!"
else
  echo "First parameter not supplied."
fi

Answer #3:

While most of the techniques stated here are correct, Bash 4.2 supports an actual test for the presence of a variable (man bash), rather than testing the value of the variable.

[[ -v foo ]]; echo $?
# 1

foo=bar
[[ -v foo ]]; echo $?
# 0

foo=""
[[ -v foo ]]; echo $?
# 0

Notably, this approach will not cause an error when used to check for an unset variable in set -u / set -o nounset mode, unlike many other approaches, such as using [ -z.

Answer #4:

There are many ways to do this with the following being one of them:

if [ -z "$1" ]

This succeeds if $1 is null or unset.

Answer #5:

I always find the POSIX table in the other answer slow to grok, so here’s my take on it:

parameter expansionVARIABLE setVARIABLE emptyVARIABLE unset
${VARIABLE-default}$VARIABLE"""default"
${VARIABLE=default}$VARIABLE""$(VARIABLE="default")
${VARIABLE?default}$VARIABLE""exit 127
${VARIABLE+default}"default""default"""
${VARIABLE:-default}$VARIABLE"default""default"
${VARIABLE:=default}$VARIABLE$(VARIABLE="default")$(VARIABLE="default")
${VARIABLE:?default}$VARIABLEexit 127exit 127
${VARIABLE:+default}"default"""""

Note that each group (with and without preceding colon) has the same set and unset cases, so the only thing that differs is how the empty cases are handled.

With the preceding colon, the empty and unset cases are identical, so I would use those where possible (i.e. use :=, not just =, because the empty case is inconsistent).

Headings:

  • set means VARIABLE is non-empty (VARIABLE="something")
  • empty means VARIABLE is empty/null (VARIABLE="")
  • unset means VARIABLE does not exist (unset VARIABLE)

Values:

  • $VARIABLE means the result is the original value of the variable.
  • "default" means the result was the replacement string provided.
  • "" means the result is null (an empty string).
  • exit 127 means the script stops executing with exit code 127.
  • $(VARIABLE="default") means the result is "default" and that VARIABLE (previously empty or unset) will also be set equal to "default".

How to check if a variable is set in Bash?

To see if a variable is nonempty, I use

if [[ $var ]]; then ...       # `$var' expands to a nonempty string

The opposite tests if a variable is either unset or empty:

if [[ ! $var ]]; then ...     # `$var' expands to the empty string (set or not)

To see if a variable is set (empty or nonempty), I use

if [[ ${var+x} ]]; then ...   # `var' exists (empty or nonempty)
if [[ ${1+x} ]]; then ...     # Parameter 1 exists (empty or nonempty)

The opposite tests if a variable is unset:

if [[ ! ${var+x} ]]; then ... # `var' is not set at all
if [[ ! ${1+x} ]]; then ...   # We were called with no arguments

Hope you learned something from this post.

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Linux and Python enthusiast, in love with open source since 2014, Writer at programming-articles.com, India.

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