How to check if an environment variable exists and get its value?

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?

[ -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