I am writing a shell script. In this shell script, I am have a variable that either takes a default value, or the value of an environment variable. However, the environment variable doesn't have to be present.
For instance, assume, before running the script, I perform the following operation:
export DEPLOY_ENV=dev How do I tell the script to search for this environment variable, and store its value in a variable inside the script. Moreover, how do I tell the script that if this environment variable does not exist, store a default variable?
35 Answers
[ -z "${DEPLOY_ENV}" ] checks whether DEPLOY_ENV has length equal to zero. So you could run:
if [[ -z "${DEPLOY_ENV}" ]]; then MY_SCRIPT_VARIABLE="Some default value because DEPLOY_ENV is undefined" else MY_SCRIPT_VARIABLE="${DEPLOY_ENV}" fi # or using a short-hand version [[ -z "${DEPLOY_ENV}" ]] && MyVar='default' || MyVar="${DEPLOY_ENV}" # or even shorter use MyVar="${DEPLOY_ENV:-default_value}" You could just use parameter expansion:
${parameter:-word}
If parameter is unset or null, the expansion of word is substituted. Otherwise, the value of parameter is substituted.
So try this:
var=${DEPLOY_ENV:-default_value} There's also the ${parameter-word} form, which substitutes the default value only when parameter is unset (but not when it's null).
To demonstrate the difference between the two:
$ unset DEPLOY_ENV $ echo "'${DEPLOY_ENV:-default_value}' '${DEPLOY_ENV-default_value}'" 'default_value' 'default_value' $ DEPLOY_ENV= $ echo "'${DEPLOY_ENV:-default_value}' '${DEPLOY_ENV-default_value}'" 'default_value' '' If you don't care about the difference between an unset variable or a variable with an empty value, you can use the default-value parameter expansion:
foo=${DEPLOY_ENV:-default} If you do care about the difference, drop the colon
foo=${DEPLOY_ENV-default} You can also use the -v operator to explicitly test if a parameter is set.
if [[ ! -v DEPLOY_ENV ]]; then echo "DEPLOY_ENV is not set" elif [[ -z "$DEPLOY_ENV" ]]; then echo "DEPLOY_ENV is set to the empty string" else echo "DEPLOY_ENV has the value: $DEPLOY_ENV" fi There is no difference between environment variables and variables in a script. Environment variables are just defined earlier, outside the script, before the script is called. From the script's point of view, a variable is a variable.
You can check if a variable is defined:
if [ -z "$a" ] then echo "not defined" else echo "defined" fi and then set a default value for undefined variables or do something else.
The -z checks for a zero-length (i.e. empty) string. See man bash and look for the CONDITIONAL EXPRESSIONS section.
You can also use set -u at the beginning of your script to make it fail once it encounters an undefined variable, if you want to avoid having an undefined variable breaking things in creative ways.
NEW_VAR="" if [[ ${ENV_VAR} && ${ENV_VAR-x} ]]; then NEW_VAR=${ENV_VAR} else NEW_VAR="new value" fi 
