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? O

Answer 1

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.

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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.

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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.