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

Monday, 17 December 2018

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

Very close to what you posted, actually:

FOO=${VARIABLE:-default}  # If variable not set or null, use default.

Or, which will assign default to VARIABLE as well:

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

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