How to tell if a string is not defined in a bash shell script?

Question

If I want to check for the null string I would do

[ -z $mystr ]

but what if I want to check whether the variable has been defined at all? Or is there no distinction in bash scripting?

Answer 1

I think the answer you are after is implied (if not stated) by Vinko's answer, though it is not spelled out simply. To distinguish whether VAR is set but empty or not set, you can use:

if [ -z "${VAR+xxx}" ]; then echo VAR is not set at all; fi
if [ -z "$VAR" ] && [ "${VAR+xxx}" = "xxx" ]; then echo VAR is set but empty; fi

You probably can combine the two tests on the second line into one with:

if [ -z "$VAR" -a "${VAR+xxx}" = "xxx" ]; then echo VAR is set but empty; fi

However, if you read the documentation for Autoconf, you'll find that they do not recommend combining terms with '-a' and do recommend using separate simple tests combined with &&. I've not encountered a system where there is a problem; that doesn't mean they didn't used to exist (but they are probably extremely rare these days, even if they weren't as rare in the distant past).

You can find the details of these, and other related shell parameter expansions, the test or [ command and conditional expressions in the Bash manual.

---

I was recently asked by email about this answer with the question:

You use two tests, and I understand the second one well, but not the first one. More precisely I don't understand the need for variable expansion

if [ -z "${VAR+xxx}" ]; then echo VAR is not set at all; fi

Wouldn't this accomplish the same?

if [ -z "${VAR}" ]; then echo VAR is not set at all; fi

Fair question - the answer is 'No, your simpler alternative does not do the same thing'.

Suppose I write this before your test:

VAR=

Your test will say "VAR is not set at all", but mine will say (by implication because it echoes nothing) "VAR is set but its value might be empty". Try this script:

(
unset VAR
if [ -z "${VAR+xxx}" ]; then echo JL:1 VAR is not set at all; fi
if [ -z "${VAR}" ];     then echo MP:1 VAR is not set at all; fi
VAR=
if [ -z "${VAR+xxx}" ]; then echo JL:2 VAR is not set at all; fi
if [ -z "${VAR}" ];     then echo MP:2 VAR is not set at all; fi
)

The output is:

JL:1 VAR is not set at all
MP:1 VAR is not set at all
MP:2 VAR is not set at all

In the second pair of tests, the variable is set, but it is set to the empty value. This is the distinction that the ${VAR=value} and ${VAR:=value} notations make. Ditto for ${VAR-value} and ${VAR:-value}, and ${VAR+value} and ${VAR:+value}, and so on.

---

As Gili points out in his answer, if you run bash with the set -o nounset option, then the basic answer above fails with unbound variable. It is easily remedied:

if [ -z "${VAR+xxx}" ]; then echo VAR is not set at all; fi
if [ -z "${VAR-}" ] && [ "${VAR+xxx}" = "xxx" ]; then echo VAR is set but empty; fi

Or you could cancel the set -o nounset option with set +u (set -u being equivalent to set -o nounset).

Answer 2

~> if [ -z $FOO ]; then echo "EMPTY"; fi
EMPTY
~> FOO=""
~> if [ -z $FOO ]; then echo "EMPTY"; fi
EMPTY
~> FOO="a"
~> if [ -z $FOO ]; then echo "EMPTY"; fi
~> 

-z works for undefined variables too. To distinguish between an undefined and a defined you'd use the things listed here or, with clearer explanations, here.

Cleanest way is using expansion like in these examples. To get all your options check the Parameter Expansion section of the manual.

Alternate word:

~$ unset FOO
~$ if test ${FOO+defined}; then echo "DEFINED"; fi
~$ FOO=""
~$ if test ${FOO+defined}; then echo "DEFINED"; fi
DEFINED

Default value:

~$ FOO=""
~$ if test "${FOO-default value}" ; then echo "UNDEFINED"; fi
~$ unset FOO
~$ if test "${FOO-default value}" ; then echo "UNDEFINED"; fi
UNDEFINED

Of course you'd use one of these differently, putting the value you want instead of 'default value' and using the expansion directly, if appropriate.

Answer 3

Advanced bash scripting guide, 10.2. Parameter Substitution:

• ${var+blahblah}: if var is defined, 'blahblah' is substituted for the expression, else null is substituted
• ${var-blahblah}: if var is defined, it is itself substituted, else 'blahblah' is substituted
• ${var?blahblah}: if var is defined, it is substituted, else the function exists with 'blahblah' as an error message.

to base your program logic on whether the variable $mystr is defined or not, you can do the following:

isdefined=0
${mystr+ export isdefined=1}

now, if isdefined=0 then the variable was undefined, if isdefined=1 the variable was defined

This way of checking variables is better than the above answer because it is more elegant, readable, and if your bash shell was configured to error on the use of undefined variables (set -u), the script will terminate prematurely.

Other useful stuff:

to have a default value of 7 assigned to $mystr if it was undefined, and leave it intact otherwise:

mystr=${mystr- 7}

to print an error message and exit the function if the variable is undefined:

: ${mystr? not defined}

Beware here that I used ':' so as not to have the contents of $mystr executed as a command in case it is defined.

Answer 4

A summary of tests.

[ -n "$var" ] && echo "var is set and not empty"
[ -z "$var" ] && echo "var is unset or empty"
[ "${var+x}" = "x" ] && echo "var is set"  # may or may not be empty
[ -n "${var+x}" ] && echo "var is set"  # may or may not be empty
[ -z "${var+x}" ] && echo "var is unset"
[ -z "${var-x}" ] && echo "var is set and empty"

Answer 5

https://stackoverflow.com/a/9824943/14731 contains a better answer (one that is more readable and works with set -o nounset enabled). It works roughly like this:

if [ -n "${VAR-}" ]; then
    echo "VAR is set and is not empty"
elif [ "${VAR+DEFINED_BUT_EMPTY}" = "DEFINED_BUT_EMPTY" ]; then
    echo "VAR is set, but empty"
else
    echo "VAR is not set"
fi

Answer 6

The explicit way to check for a variable being defined would be:

[ -v mystr ]

Answer 7

another option: the "list array indices" expansion:

$ unset foo
$ foo=
$ echo ${!foo[*]}
0
$ foo=bar
$ echo ${!foo[*]}
0
$ foo=(bar baz)
$ echo ${!foo[*]}
0 1

the only time this expands to the empty string is when foo is unset, so you can check it with the string conditional:

$ unset foo
$ [[ ${!foo[*]} ]]; echo $?
1
$ foo=
$ [[ ${!foo[*]} ]]; echo $?
0
$ foo=bar
$ [[ ${!foo[*]} ]]; echo $?
0
$ foo=(bar baz)
$ [[ ${!foo[*]} ]]; echo $?
0

should be available in any bash version >= 3.0

Answer 8

not to shed this bike even further, but wanted to add

shopt -s -o nounset

is something you could add to the top of a script, which will error if variables aren't declared anywhere in the script. The message you'd see is unbound variable, but as others mention it won't catch an empty string or null value. To make sure any individual value isn't empty, we can test a variable as it's expanded with ${mystr:?}, also known as dollar sign expansion, which would error with parameter null or not set.

Answer 9

The Bash Reference Manual is an authoritative source of information about bash.

Here's an example of testing a variable to see if it exists:

if [ -z "$PS1" ]; then
        echo This shell is not interactive
else
        echo This shell is interactive
fi

(From section 6.3.2.)

Note that the whitespace after the open [ and before the ] is not optional.

---

Tips for Vim users

I had a script that had several declarations as follows:

export VARIABLE_NAME="$SOME_OTHER_VARIABLE/path-part"

But I wanted them to defer to any existing values. So I re-wrote them to look like this:

if [ -z "$VARIABLE_NAME" ]; then
        export VARIABLE_NAME="$SOME_OTHER_VARIABLE/path-part"
fi

I was able to automate this in vim using a quick regex:

s/\vexport ([A-Z_]+)\=("[^"]+")\n/if [ -z "$\1" ]; then\r  export \1=\2\rfi\r/gc

This can be applied by selecting the relevant lines visually, then typing :. The command bar pre-populates with :'<,'>. Paste the above command and hit enter.

---

Tested on this version of Vim:

VIM - Vi IMproved 7.3 (2010 Aug 15, compiled Aug 22 2015 15:38:58)
Compiled by root@apple.com

Windows users may want different line endings.

Answer 10

Here is what I think is a much clearer way to check if a variable is defined:

var_defined() {
    local var_name=$1
    set | grep "^${var_name}=" 1>/dev/null
    return $?
}

Use it as follows:

if var_defined foo; then
    echo "foo is defined"
else
    echo "foo is not defined"
fi

Answer 11

call set without any arguments.. it outputs all the vars defined..
the last ones on the list would be the ones defined in your script..
so you could pipe its output to something that could figure out what things are defined and whats not