Bash
Test if a variable is set without expanding it
Test whether a variable is set, even to an empty string, with [[ -v name ]] in Bash 4.2 or later. It tells unset apart from empty without expanding.
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A variable can be empty or it can be unset, and [[ -z $x ]] cannot tell the two apart. The -v test can.
a=""
unset b
[[ -v a ]] && echo "a is set"
[[ -v b ]] || echo "b is unset"
[[ -n $a ]] || echo "yet [[ -n \$a ]] is false for a"
set -u
[[ -v b ]] || echo "no error under set -u"a is set
b is unset
yet [[ -n $a ]] is false for a
no error under set -u[[ -v name ]] is true when the variable has been assigned a value, including the empty string, and it takes the name rather than the expansion, so set -u does not abort the script on an unset variable. The test is listed under Bash conditional expressions.
It needs Bash 4.2 or later, and it is not in POSIX sh. In a portable script use ${name+x}, which expands to x only when the variable is set. On an array element, [[ -v arr[2] ]] works from Bash 4.3.
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