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

user_87408 0
Viewed 594851

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?

shell bash
10 Answers

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.
user_2417740

For command line arguments:

VARIABLE="${1:-$DEFAULTVALUE}"

which assigns to VARIABLE the value of the 1st argument passed to the script or the value of DEFAULTVALUE if no such argument was passed. Quoting prevents globbing and word splitting.

user_893910

see here under 3.5.3(shell parameter expansion)

so in your case

${VARIABLE:-default}
user_1315270

Then there's the way of expressing your 'if' construct more tersely:

FOO='default'
[ -n "${VARIABLE}" ] && FOO=${VARIABLE}
Rob_Wells_29740

It is possible to chain default values like so:

DOCKER_LABEL=${GIT_TAG:-${GIT_COMMIT_AND_DATE:-latest}}

i.e. if $GIT_TAG doesnt exist, take $GIT_COMMIT_AND_DATE - if this doesnt exist, take "latest"

user_31439530
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