Assigning default values to shell variables with a single command in bash

Question

I have a whole bunch of tests on variables in a bash (3.00) shell script where if the variable is not set, then it assigns a default, e.g.:

if [ -z "${VARIABLE}" ]; then      FOO='default' else      FOO=${VARIABLE} fi 

I seem to recall there's some syntax to doing this in one line, something resembling a ternary operator, e.g.:

FOO=${ ${VARIABLE} : 'default' } 

(though I know that won't work...)

Am I crazy, or does something like that exist?

Answer 1

Very close to what you posted, actually. You can use something called Bash parameter expansion to accomplish this.

To get the assigned value, or default if it's missing:

FOO="${VARIABLE:-default}"  # If variable not set or null, use default. # If VARIABLE was unset or null, it still is after this (no assignment done). 

Or to assign default to VARIABLE at the same time:

FOO="${VARIABLE:=default}"  # If variable not set or null, set it to default.