8 Responses to “Bash: test for undefined variable”

1. Anonymous says:

   July 23, 2008 at 8:32 pm

   defined()
   {
   [ ${!1-X} == ${!1-Y} ]
   }

   Example:
   $ defined FOO || echo notset
   notset
   $ FOO=X
   $ defined FOO || echo notset
   $ FOO=
   $ defined FOO || echo notset
   $ unset FOO
   $ defined FOO || echo notset
   notset
   $ defined BASH_VERSION[2] || echo notset
   $ defined BASH_VERSION[20] || echo notset
   notset
   $
2. ChrisGarrod says:

   October 7, 2008 at 6:00 pm

   Thanks for both bits of info. I really like function defined, but don’t understand how it works. !1 ?? very curious. By the way, on my BASH, only BASH_VERSINFO is an array:

   # 10:56:58 garrod@sioprism 2.15 $ cd ~ && set | grep BASH
   BASH=/bin/bash
   BASH_VERSINFO=([0]=”2″ [1]=”05b” [2]=”0″ [3]=”1″ [4]=”release” [5]=”ia64-redhat-linux-gnu”)
   BASH_VERSION=’2.05b.0(1)-release’
3. Rob says:

   October 22, 2008 at 9:06 pm

   It works even better with some double quotes. Try this:

   defined()
   {
   [ "${!1-one} == "${!1-two}" ]
   }

   By way of explanation, it uses three bash expansions. First, you get the arguments to a function via ${1}. Second, you get indirection by adding the bang. ${!a} returns the value of the variable named by a. Third, it uses default values, just like your original code. I usually use “one” and “two” instead of “X” and “Y”. It doesn’t mater, so long as the values are different.

   (I wasn’t the previous commiter. I’m very indebted to both the original post and the previous comment. It was very helpful to me!)
4. Sven says:

   November 25, 2008 at 10:23 pm

   Thanks, this helped me a lot!
5. Panos says:

   May 1, 2009 at 12:55 pm

   This suffices:

   defined() {
   [[ ! -z "${1}" ]]
   }

   $ defined “${SOMEVAR}” || echo “notset”
   notset
   $ export SOMEVAR=”sometext”
   $ defined “${SOMEVAR}” || echo “notset”
6. Jochen says:

   June 5, 2009 at 8:15 pm

   Not it doesn’t suffice.

   $ defined “${SOMEVAR}” || echo “notset”
   notset
   $ export SOMEVAR=”sometext”
   $ defined “${SOMEVAR}” || echo “notset”
   $ export SOMEVAR=
   $ defined “${SOMEVAR}” || echo “notset”

   It’ll show “notset” when a variable is defined but empty.

   -z only tells you whether a variable has content, not whether it’s set or not.
7. Jochen says:

   June 5, 2009 at 8:16 pm

   Ugh of course I cut off the bottom line in the paste.

   $ export SOMEVAR=
   $ defined “${SOMEVAR} || echo “notset”
   notset
8. Niranjan says:

   January 25, 2010 at 1:20 pm

   A VAR could take on teh following values:

   VAR=
   VAR=”" (Same as above)
   VAR=” “, and
   VAR=”something” (which is same as #3, VAR=” “)

   The solution has to defend against all these options.

   Assuming (Big assumption) that VAR’s value is always greater than a single alpha-numeric, one can test for the length of VAR.