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

by

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).

More Related Contents:

  1. How do I split a string on a delimiter in Bash?
  2. Correct Bash and shell script variable capitalization
  3. How to convert a string to lower case in Bash?
  4. How to urlencode data for curl command?
  5. Why equal to operator does not work if it is surrounded by space?
  6. sudo echo “something” >> /etc/privilegedFile doesn’t work [duplicate]
  7. How can I declare and use Boolean variables in a shell script?
  8. Test for non-zero length string in Bash: [ -n “$var” ] or [ “$var” ]
  9. How can I suppress all output from a command using Bash?
  10. Random number from a range in a Bash Script
  11. How to get a password from a shell script without echoing
  12. How to get the part of a file after the first line that matches a regular expression
  13. How to split one string into multiple strings separated by at least one space in bash shell?
  14. Copy folder recursively, excluding some folders
  15. Passing arguments by reference
  16. Get just the filename from a path in a Bash script [duplicate]
  17. How to resolve symbolic links in a shell script
  18. Why equal to operator does not work if it is not surrounded by space?
  19. Extract version number from a string
  20. Is there a way to avoid positional arguments in bash?
  21. What does `kill -0 $pid` in a shell script do?
  22. How do I tell if a regular file does not exist in Bash?
  23. How can I compare numbers in Bash?
  24. Capturing Groups From a Grep RegEx
  25. Shell redirection i/o order
  26. Colorized grep — viewing the entire file with highlighted matches
  27. How to pass in password to pg_dump?
  28. How do I remove newlines from a text file?
  29. How to schedule to run first Sunday of every month
  30. How to update one file in a zip archive

Leave a Comment