exclamation mark to test variable is true or not in bash shell

Question

I know in shell the exclamation mark can invert the outcome of condition. Here I wanna use it to test variable is true or false .

#! /bin/bash
bat=false
if [         


        

Answer 1

See the POSIX specification of test for details of the portable use of the [ or test utility.

If you use [ ! $bar ] you are playing with fire.

• If $bar is an empty string, it degenerates to the one-argument form of test, with that argument being !, and exits successfully because ! is not an empty string.

• If $bar is not an empty string, then the arguments are ! and the value of $bar, which establishes that the non-empty string is non-empty, and then inverts the condition, so it exits with a failure status.

You should normally write [ ! "$bar" ] which will then work correctly if $bar is empty. You could (arguably should) use the explicit tests:

• [ -z "$bar" ] to test for an empty (zero-length) string, and
• [ -n "$bar" ] to test for a non-empty string

but the quotes are mandatory to avoid confusion. The non-empty and zero-length are mnemonic terms.

$ bar=
$ [ ! $bar ] || echo Test failed
$ bar=xyz
$ [ ! $bar ] || echo Test failed
Test failed
$ [ ! "$bar" ] || echo Test failed
Test failed
$ bar=
$ [ ! "$bar" ] || echo Test failed
$ [ -n "$bar" ] || echo Test failed
Test failed
$ [ -z "$bar" ] || echo Test failed
$ 

Some people prefer the Bash (Korn shell, …) operator [[ which behaves differently. You could experiment with variations on the theme of this code:

set -x
bar=
[ ! $bar ] || echo Test failed
bar=xyz
[ ! $bar ] || echo Test failed
[ ! "$bar" ] || echo Test failed
bar=
[ ! "$bar" ] || echo Test failed
[ -n "$bar" ] || echo Test failed
[ -z "$bar" ] || echo Test failed
bar=xyz
[ -n "$bar" ] || echo Test failed
[ -z "$bar" ] || echo Test failed

bar=
[[ ! $bar ]] || echo Test failed
bar=xyz
[[ ! $bar ]] || echo Test failed
[[ ! "$bar" ]] || echo Test failed
bar=
[[ ! "$bar" ]] || echo Test failed
[[ -n "$bar" ]] || echo Test failed
[[ -z "$bar" ]] || echo Test failed
bar=xyz
[[ -n "$bar" ]] || echo Test failed
[[ -z "$bar" ]] || echo Test failed
bar=
[[ -n $bar ]] || echo Test failed
[[ -z $bar ]] || echo Test failed
bar=xyz
[[ -n $bar ]] || echo Test failed
[[ -z $bar ]] || echo Test failed
set +x  # Unnecessary if you save this in a script file and don't dot (source) it

It will (probably) help you understand what's going on. The output is quite verbose, though.