if [ -z "$STATE" ]; then
    echo "Need to set STATE"
    exit 1
fi

if [ -z "$DEST" ]; then
    echo "Need to set DEST"
    exit 1
fi

: ${STATE?"Need to set STATE"}
: ${DEST:?"Need to set DEST non-empty"}

: "${STATE?Need to set STATE}"
: "${DEST:?Need to set DEST non-empty}"

${parameter:?word}

In script.sh line 13:
: ${FOO:?"The environment variable 'FOO' must be set and non-empty"}
  ^-- SC2086: Double quote to prevent globbing and word splitting.

: "${STATE?Need to set STATE}"
: "${DEST:?Need to set DEST non-empty}"

$ mkdir junk
$ cd junk
$ > abc
$ > def
$ > ghi
$
$ x="*"
$ printf "%sn" ${x:?You must set x}    # Careless; not recommended
abc
def
ghi
$ unset x
$ printf "%sn" ${x:?You must set x}    # Careless; not recommended
bash: x: You must set x
$ printf "%sn" "${x:?You must set x}"  # Careful: should be used
bash: x: You must set x
$ x="*"
$ printf "%sn" "${x:?You must set x}"  # Careful: should be used
*
$ printf "%sn" ${x:?"You must set x"}  # Not quite careful enough
abc
def
ghi
$ x=
$ printf "%sn" ${x:?"You must set x"}  # Not quite careful enough
bash: x: You must set x
$ unset x
$ printf "%sn" ${x:?"You must set x"}  # Not quite careful enough
bash: x: You must set x
$

[ -z "$STATE" ] && echo "Need to set STATE" && exit 1;

${MyVariable:=SomeDefault}

${MyVariable:-SomeDefault}

${MyVariable:=SomeDefault}

MyVariable=${MyVariable:=SomeDefault}

if [ "$MYVAR" = "" ]
then
   echo "Does not exist"
else
   echo "Exists"
fi

if [ "x$STATE" == "x" ]; then echo "Need to set State"; exit 1; fi

$ unset a
$ b=
$ c=
$ [[ -v a ]] && echo "a is set"
$ [[ -v b ]] && echo "b is set"
b is set
$ [[ -v c ]] && echo "c is set"
c is set

mapfile -t arr < variables.txt

EXITCODE=0

for i in "${arr[@]}"
do
   ISSET=$(env | grep ^${i}= | wc -l)
   if [ "${ISSET}" = "0" ];
   then
      EXITCODE=-1
      echo "ENV variable $i is required."
   fi
done

exit ${EXITCODE}

#
# assert if variables are set (to a non-empty string)
# if any variable is not set, exit 1 (when -f option is set) or return 1 otherwise
#
# Usage: assert_not_null [-f] variable ...
#
function assert_not_null() {
  local fatal var num_null=0
  [[ "$1" = "-f" ]] && { shift; fatal=1; }
  for var in "$@"; do
    [[ -z "${!var}" ]] &&
      printf '%sn' "Variable '$var' not set" >&2 &&
      ((num_null++))
  done

  if ((num_null > 0)); then
    [[ "$fatal" ]] && exit 1
    return 1
  fi
  return 0
}

one=1 two=2
assert_not_null one two
echo test 1: return_code=$?
assert_not_null one two three
echo test 2: return_code=$?
assert_not_null -f one two three
echo test 3: return_code=$? # this code shouldn't execute

test 1: return_code=0
Variable 'three' not set
test 2: return_code=1
Variable 'three' not set

#!/bin/bash
declare -a vars=(NAME GITLAB_URL GITLAB_TOKEN)

for var_name in "${vars[@]}"
do
  if [ -z "$(eval "echo $$var_name")" ]; then
    echo "Missing environment variable $var_name"
    exit 1
  fi
done

if (set -u; : $HOME) 2> /dev/null
...
...

is_this_an_env_variable ()
    local var="$1"
    if env |grep -q "^$var"; then
       return 0
    else
       return 1
    fi
 }

if [ $?BLAH == 1 ]; then
    echo "Exists";
else
    echo "Does not exist";
fi