Check if string is neither empty nor space in shell script

Question

I am trying to run the following shell script which is supposed to check if a string is neither space nor empty. However, I am getting the same output for all the 3 mentioned strings. I have tried using the "[[" syntax as well but to no avail.

Here is my code:

str="Hello World"
str2=" "
str3=""

if [ ! -z "$str" -a "$str"!=" " ]; then
        echo "Str is not null or space"
fi

if [ ! -z "$str2" -a "$str2"!=" " ]; then
        echo "Str2 is not null or space"
fi

if [ ! -z "$str3" -a "$str3"!=" " ]; then
        echo "Str3 is not null or space"
fi

I am getting the following output:

# ./checkCond.sh 
Str is not null or space
Str2 is not null or space

Are empty strings known as null strings in shell scripting?

Josiah Yoder
– Josiah Yoder

2020-10-24 14:52:42 +00:00
Commented Oct 24, 2020 at 14:52 — Josiah Yoder
– Josiah Yoder, Commented Oct 24, 2020 at 14:52

dogbane · Accepted Answer · 2012-11-22 09:40:50Z

202

You need a space on either side of the !=. Change your code to:

str="Hello World"
str2=" "
str3=""

if [ ! -z "$str" -a "$str" != " " ]; then
        echo "Str is not null or space"
fi

if [ ! -z "$str2" -a "$str2" != " " ]; then
        echo "Str2 is not null or space"
fi

if [ ! -z "$str3" -a "$str3" != " " ]; then
        echo "Str3 is not null or space"
fi

answered Nov 22, 2012 at 9:40

dogbane

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2 Comments

Shubhanshu Mishra Over a year ago

Thanks a lot, it works. But I wonder why the assignment doesn't use a space whereas a comparison does.

anishsane Over a year ago

^^It's syntax. The first word on the command line is the command & subsequent ones are arguments. var=value [command [args]] is the syntax, in which a variable is assigned value. for comparison, [ (/usr/bin/[) is the command & it requires var1, != & var2 to be 3 separate arguments. var1!=var2 is a single argument.

cobrexus · Accepted Answer · 2024-08-31 09:32:12Z

99

For checking the empty string in shell

if [ "$str" = "" ];then
   echo NULL
fi

OR

if [ ! "$str" ];then
   echo NULL
fi

edited Aug 31, 2024 at 9:32

cobrexus

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answered Nov 22, 2012 at 9:37

RajeshKashyap

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1 Comment

Jens Over a year ago

The shell's string equality operator is =. The == is a non-portable hack invented by shell authors to mess up the minds of young programmers.

elomage · Accepted Answer · 2018-10-30 20:51:22Z

44

Another quick test for a string to have something in it but space.

if [[ -n "${str// /}" ]]; then
    echo "It is not empty!"
fi

"-n" means non-zero length string.

Then the first two slashes mean match all of the following, in our case space(s). Then the third slash is followed with the replacement (empty) string and closed with "}". Note the difference from the usual regular expression syntax.

You can read more about string manipulation in bash shell scripting here.

edited Oct 30, 2018 at 20:51

answered May 23, 2016 at 18:36

elomage

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2 Comments

Alexander Mills Over a year ago

isn't [[ -n "$1" ]] the same as [[ ! -z "$1" "]]?

elomage Over a year ago

@AlexanderMills you are right, using -n would make it a bit shorter. I just updated the answer accordingly.

hyde · Accepted Answer · 2012-11-22 10:51:26Z

21

In case you need to check against any amount of whitespace, not just single space, you can do this:

To strip string of extra white space (also condences whitespace in the middle to one space):

trimmed=`echo -- $original`

The -- ensures that if $original contains switches understood by echo, they'll still be considered as normal arguments to be echoed. Also it's important to not put "" around $original, or the spaces will not get removed.

After that you can just check if $trimmed is empty.

[ -z "$trimmed" ] && echo "empty!"

edited Nov 22, 2012 at 10:51

answered Nov 22, 2012 at 9:43

hyde

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2 Comments

Sridhar Sarnobat Over a year ago

In bourne shell I end up with "--" as the value of trimmed.

Mr.C Over a year ago

According to this post, echo does not interpret -- to mean the end of options. In my Bash shell, I get the same result as Sridhar Sarnobat

Eugene Yarmash · Accepted Answer · 2017-04-21 20:07:09Z

12

To check if a string is empty or contains only whitespace you could use:

shopt -s extglob  # more powerful pattern matching

if [ -n "${str##+([[:space:]])}" ]; then
    echo '$str is not null or space'
fi

See Shell Parameter Expansion and Pattern Matching in the Bash Manual.

edited Apr 21, 2017 at 20:07

answered Nov 22, 2012 at 9:41

Eugene Yarmash

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Comments

Harsh Shandilya · Accepted Answer · 2017-05-01 11:40:43Z

2

[ $(echo $variable_to_test | sed s/\n// | sed s/\ //) == "" ] && echo "String is empty"

Stripping all newlines and spaces from the string will cause a blank one to be reduced to nothing which can be tested and acted upon

answered May 1, 2017 at 11:40

Harsh Shandilya

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1 Comment

Rémi.B Over a year ago

I think the g flag is missing in sed commands, to change all occurences: [ $(echo $variable_to_test | sed s/\n//g | sed s/\ //g) == "" ] && echo "String is empty"

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