How to Bash Shell: Find Out If a Variable Is Set or Not

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In Bash shell, you can use the ${var+x} construct to check if a variable is set or not. If the variable is set, the construct will return the value of the variable, otherwise it will return the string “x”.

Here is an example of how to use this construct to check if a variable named “var” is set:

if [ -n "${var+x}" ]; then
echo "var is set"
else
echo "var is not set"
fi

You can also use the ${var:+x} construct which also returns the value of the variable if it is set, or the string “x” otherwise.

if [ -n "${var:+x}" ]; then
echo "var is set"
else
echo "var is not set"
fi

Alternatively, you can use the ${var:=x} construct which assigns the default value “x” to the variable if it is not set.

echo ${var:=default_value}

You can also use the declare -p var command which returns the type and value of the variable, if it is set, or returns nothing if the variable is not set.

declare -p var

You can also use the [[ -v var ]] and [[ -z ${var+x} ]] to check if a variable is set or not:

if [[ -v var ]]; then
echo "var is set"
else
echo "var is not set"
fi
if [[ -z ${var+x} ]]; then
echo "var is not set"
else
echo "var is set"
fi

Please note that the -n and -z options of the if statement check if the string is non-empty or empty respectively.

In Bash, if a variable is not set, it is considered as empty, and the -z option will return true.

It is also recommended to check the Bash documentation for more information about the available constructs and options.

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