In Bash, how do I test if a variable is defined in “-u” mode

Question

I just discovered set -u in bash and it helped me find several previously unseen bugs. But I also have a scenario where I need to test if a variable is defined

Answer 1

What Doesn't Work: Test for Zero-Length Strings

You can test for undefined strings in a few ways. Using the standard test conditional looks like this:

# Test for zero-length string.
[ -z "$variable" ] || variable='foo'

This will not work with set -u, however.

What Works: Conditional Assignment

Alternatively, you can use conditional assignment, which is a more Bash-like way to do this. For example:

# Assign value if variable is unset or null.
: "${variable:=foo}"

Because of the way Bash handles expansion of this expression, you can safely use this with set -u without getting a "bash: variable: unbound variable" error.

Answer 2

In bash 4.2 and newer there is an explicit way to check whether a variable is set, which is to use -v. The example from the question could then be implemented like this:

if [[ ! -v variable ]]; then
   variable="$(...)"
fi

See http://www.gnu.org/software/bash/manual/bashref.html#Bash-Conditional-Expressions

If you only want to set the variable, if it is not already set you are probably better of doing something along these lines:

variable="${variable-$(...)}"

Note that this does not deal with a defined but empty variable.

Answer 3

This is what I've found works best for me, taking inspiration from the other answers:

if [ -z "${varname-}" ]; then
  ...
  varname=$(...)
fi

Answer 4

In the beginning of your script, you could define your variables with an empty value

variable_undefined=""

Then

if [ "${variable_undefined}" == "" ]; then
    variable="$(...)"
fi

Answer 5

if [ "${var+SET}" = "SET" ] ; then
    echo "\$var = ${var}"
fi

I don't know how far back ${var+value} is supported, but it works at least as far back as 4.1.2. Older versions didn't have ${var+value}, they only had ${var:+value}. The difference is that ${var:+value} will only evaluate to "value" if $var is set to a nonempty string, while ${var+value} will also evaluate to "value" if $var is set to the empty string.

Without [[ -v var ]] or ${var+value} I think you'd have to use another method. Probably a subshell test as was described in a previous answer:

if ( set -u; echo "$var" ) &> /dev/null; then
    echo "\$var = ${var}
fi

If your shell process has "set -u" active already it'll be active in the subshell as well without the need for "set -u" again, but including it in the subshell command allows the solution to also work if the parent process hasn't got "set -u" enabled.

(You could also use another process like "printenv" or "env" to test for the presence of the variable, but then it'd only work if the variable is exported.)

Answer 6

Unfortunatly [[ -v variable ]] is not supported in older versions of bash (at least not in version 4.1.5 I have on Debian Squeeze)

You could instead use a sub shell as in this :

if (true $variable)&>/dev/null; then
    variable="$(...)"
fi