Test if a variable is set in bash when using "set -o nounset"

Question

The following code exits with a unbound variable error. How to fix this, while still using the set -o nounset option?

#!/bin/bash  set -o nounset  if [ ! -z ${WHATEVER} ];  then echo "yo" fi  echo "whatever" 

Answer 1

#!/bin/bash  set -o nounset   VALUE=${WHATEVER:-}  if [ ! -z ${VALUE} ];  then echo "yo" fi  echo "whatever" 

In this case, VALUE ends up being an empty string if WHATEVER is not set. We're using the {parameter:-word} expansion, which you can look up in man bash under "Parameter Expansion".

Answer 2

You need to quote the variables if you want to get the result you expect:

check() {     if [ -n "${WHATEVER-}" ]     then         echo 'not empty'     elif [ "${WHATEVER+defined}" = defined ]     then         echo 'empty but defined'     else         echo 'unset'     fi } 

Test:

$ unset WHATEVER $ check unset $ WHATEVER= $ check empty but defined $ WHATEVER='   ' $ check not empty