Null & empty string comparison in Bash [duplicate]

Question

I don't set any values for $pass_tc11; so it is returning null while echoing. How to compare it in if clause?

Here is my code. I don't want "Hi" to be printed...

-bash-3.00$ echo $pass_tc11

-bash-3.00$ if [ "pass_tc11" != "" ]; then
> echo "hi"
> fi
hi
-bash-3.00$

There is a difference between a variable being empty and a variable being unset. From the question's title, this appears to be the distinction you are trying to make, but it is unclear from the question if indeed you care (or are even aware of) this distinction. Do you care about that distinction? If so test -z will not help. — William Pursell
– William Pursell, Commented Jan 28, 2014 at 14:12

fedorqui · Accepted Answer · 2016-09-01 06:10:12Z

101

First of all, note you are not using the variable correctly:

if [ "pass_tc11" != "" ]; then
#     ^
#     missing $

Anyway, to check if a variable is empty or not you can use -z --> the string is empty:

if [ ! -z "$pass_tc11" ]; then
   echo "hi, I am not empty"
fi

or -n --> the length is non-zero:

if [ -n "$pass_tc11" ]; then
   echo "hi, I am not empty"
fi

From man test:

-z STRING

the length of STRING is zero

-n STRING

the length of STRING is nonzero

Samples:

$ [ ! -z "$var" ] && echo "yes"
$

$ var=""
$ [ ! -z "$var" ] && echo "yes"
$

$ var="a"
$ [ ! -z "$var" ] && echo "yes"
yes

$ var="a"
$ [ -n "$var" ] && echo "yes"
yes

edited Sep 1, 2016 at 6:10

answered Jan 28, 2014 at 13:47

fedorqui

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4 Comments

fedorqui Over a year ago

@logan, man test is the man page to check file types and compare values.

fedorqui Over a year ago

@logan I think it can be good to use -z for this case. It can be more clear for the one reading the code that you want to see if the var is empty. I would leave the string comparison for the cases in which you are comparing with a non-empty string.

CoR Over a year ago

Why do we need quoted $var, "$var"? Why $var does not work in if test?

fedorqui Over a year ago

@CoR it is good to get used to quoting in all cases, to prevent then having situations like v="aa bb"; [ ! -z $v ] && echo "yes" that gives a -bash: [: aa: binary operator expected error.

espr · Accepted Answer · 2014-01-28 16:30:35Z

3

fedorqui has a working solution but there is another way to do the same thing.

Chock if a variable is set

#!/bin/bash
amIEmpty='Hello'
# This will be true if the variable has a value
if [ $amIEmpty ]; then
    echo 'No, I am not!';
fi

Or to verify that a variable is empty

#!/bin/bash      
amIEmpty=''
# This will be true if the variable is empty
if [ ! $amIEmpty ]; then
    echo 'Yes I am!';
fi

tldp.org has good documentation about if in bash:
http://tldp.org/LDP/Bash-Beginners-Guide/html/sect_07_01.html

answered Jan 28, 2014 at 16:30

espr

311 bronze badge

1 Comment

bobbogo Over a year ago

Hmmm. Fails if you do amIEmpty='b < a'. You need to quote the test: if [ "$amIEmpty" ] …. (I much prefer the -n.)

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