[bash] What's a concise way to check that environment variables are set in a Unix shell script?

I've got a few Unix shell scripts where I need to check that certain environment variables are set before I start doing stuff, so I do this sort of thing:

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

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

which is a lot of typing. Is there a more elegant idiom for checking that a set of environment variables is set?

EDIT: I should mention that these variables have no meaningful default value - the script should error out if any are unset.

Rather than using external shell scripts I tend to load in functions in my login shell. I use something like this as a helper function to check for environment variables rather than any set variable:

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

I always used:

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

Not that much more concise, I'm afraid.

Under CSH you have $?STATE.

${MyVariable:=SomeDefault}

If MyVariable is set and not null, it will reset the variable value (= nothing happens).
Else, MyVariable is set to SomeDefault.

The above will attempt to execute ${MyVariable}, so if you just want to set the variable do:

MyVariable=${MyVariable:=SomeDefault}

Your question is dependent on the shell that you are using.

Bourne shell leaves very little in the way of what you're after.

BUT...

It does work, just about everywhere.

Just try and stay away from csh. It was good for the bells and whistles it added, compared the Bourne shell, but it is really creaking now. If you don't believe me, just try and separate out STDERR in csh! (-:

There are two possibilities here. The example above, namely using:

${MyVariable:=SomeDefault}

for the first time you need to refer to $MyVariable. This takes the env. var MyVariable and, if it is currently not set, assigns the value of SomeDefault to the variable for later use.

You also have the possibility of:

${MyVariable:-SomeDefault}

which just substitutes SomeDefault for the variable where you are using this construct. It doesn't assign the value SomeDefault to the variable, and the value of MyVariable will still be null after this statement is encountered.

${MyVariable:=SomeDefault}

If MyVariable is set and not null, it will reset the variable value (= nothing happens).
Else, MyVariable is set to SomeDefault.

The above will attempt to execute ${MyVariable}, so if you just want to set the variable do:

MyVariable=${MyVariable:=SomeDefault}

For future people like me, I wanted to go a step forward and parameterize the var name, so I can loop over a variable sized list of variable names:

#!/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

Try this:

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

Your question is dependent on the shell that you are using.

Bourne shell leaves very little in the way of what you're after.

BUT...

It does work, just about everywhere.

Just try and stay away from csh. It was good for the bells and whistles it added, compared the Bourne shell, but it is really creaking now. If you don't believe me, just try and separate out STDERR in csh! (-:

There are two possibilities here. The example above, namely using:

${MyVariable:=SomeDefault}

for the first time you need to refer to $MyVariable. This takes the env. var MyVariable and, if it is currently not set, assigns the value of SomeDefault to the variable for later use.

You also have the possibility of:

${MyVariable:-SomeDefault}

which just substitutes SomeDefault for the variable where you are using this construct. It doesn't assign the value SomeDefault to the variable, and the value of MyVariable will still be null after this statement is encountered.

This can be a way too:

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

http://unstableme.blogspot.com/2007/02/checks-whether-envvar-is-set-or-not.html

None of the above solutions worked for my purposes, in part because I checking the environment for an open-ended list of variables that need to be set before starting a lengthy process. I ended up with this:

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}

Surely the simplest approach is to add the -u switch to the shebang (the line at the top of your script), assuming you’re using bash:

#!/bin/sh -u

This will cause the script to exit if any unbound variables lurk within.

${MyVariable:=SomeDefault}

If MyVariable is set and not null, it will reset the variable value (= nothing happens).
Else, MyVariable is set to SomeDefault.

The above will attempt to execute ${MyVariable}, so if you just want to set the variable do:

MyVariable=${MyVariable:=SomeDefault}

For future people like me, I wanted to go a step forward and parameterize the var name, so I can loop over a variable sized list of variable names:

#!/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

The $? syntax is pretty neat:

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

This can be a way too:

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

http://unstableme.blogspot.com/2007/02/checks-whether-envvar-is-set-or-not.html

Your question is dependent on the shell that you are using.

Bourne shell leaves very little in the way of what you're after.

BUT...

It does work, just about everywhere.

Just try and stay away from csh. It was good for the bells and whistles it added, compared the Bourne shell, but it is really creaking now. If you don't believe me, just try and separate out STDERR in csh! (-:

There are two possibilities here. The example above, namely using:

${MyVariable:=SomeDefault}

for the first time you need to refer to $MyVariable. This takes the env. var MyVariable and, if it is currently not set, assigns the value of SomeDefault to the variable for later use.

You also have the possibility of:

${MyVariable:-SomeDefault}

which just substitutes SomeDefault for the variable where you are using this construct. It doesn't assign the value SomeDefault to the variable, and the value of MyVariable will still be null after this statement is encountered.

${MyVariable:=SomeDefault}

If MyVariable is set and not null, it will reset the variable value (= nothing happens).
Else, MyVariable is set to SomeDefault.

The above will attempt to execute ${MyVariable}, so if you just want to set the variable do:

MyVariable=${MyVariable:=SomeDefault}

In my opinion the simplest and most compatible check for #!/bin/sh is:

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

Again, this is for /bin/sh and is compatible also on old Solaris systems.

We can write a nice assertion to check a bunch of variables all at once:

#
# 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_var_not_null [-f] variable ...
#
function assert_var_not_null() {
  local fatal var num_null=0
  [[ "$1" = "-f" ]] && { shift; fatal=1; }
  for var in "$@"; do
    [[ -z "${!var}" ]] &&
      printf '%s\n' "Variable '$var' not set" >&2 &&
      ((num_null++))
  done

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

Sample invocation:

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

Output:

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

More such assertions here: https://github.com/codeforester/base/blob/master/lib/assertions.sh

bash 4.2 introduced the -v operator which tests if a name is set to any value, even the empty string.

$ 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

I always used:

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

Not that much more concise, I'm afraid.

Under CSH you have $?STATE.

Try this:

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

We can write a nice assertion to check a bunch of variables all at once:

#
# 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_var_not_null [-f] variable ...
#
function assert_var_not_null() {
  local fatal var num_null=0
  [[ "$1" = "-f" ]] && { shift; fatal=1; }
  for var in "$@"; do
    [[ -z "${!var}" ]] &&
      printf '%s\n' "Variable '$var' not set" >&2 &&
      ((num_null++))
  done

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

Sample invocation:

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

Output:

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

More such assertions here: https://github.com/codeforester/base/blob/master/lib/assertions.sh

This can be a way too:

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

http://unstableme.blogspot.com/2007/02/checks-whether-envvar-is-set-or-not.html

In my opinion the simplest and most compatible check for #!/bin/sh is:

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

Again, this is for /bin/sh and is compatible also on old Solaris systems.

Try this:

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

Surely the simplest approach is to add the -u switch to the shebang (the line at the top of your script), assuming you’re using bash:

#!/bin/sh -u

This will cause the script to exit if any unbound variables lurk within.

Try this:

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

None of the above solutions worked for my purposes, in part because I checking the environment for an open-ended list of variables that need to be set before starting a lengthy process. I ended up with this:

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}

Rather than using external shell scripts I tend to load in functions in my login shell. I use something like this as a helper function to check for environment variables rather than any set variable:

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

The $? syntax is pretty neat:

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

I always used:

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

Not that much more concise, I'm afraid.

Under CSH you have $?STATE.

This can be a way too:

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

http://unstableme.blogspot.com/2007/02/checks-whether-envvar-is-set-or-not.html