Finding whitespaces in string variable shell script

Question

I have a string variable which could have values like below.

my_string=" "
my_string="   "
my_string="name1"
my_string="name 2"

I have to identify if my_string has only spaces/whitespaces and exit the program when it has only whitespaces. If it has one or two spaces inbetween, it's a valid string.

How to check if the string has only whitespaces and exit the shell script based on that.

Thank you.

"Shell" meaning /bin/sh or bash? (bash has built-in support for extended regular expressions) — Charles Duffy
– Charles Duffy, Commented Mar 30, 2022 at 18:58
(also, what do you mean "between"? Between individual words in the string? Between the start and end of the string?) — Charles Duffy
– Charles Duffy, Commented Mar 30, 2022 at 18:59
...have you tried to follow the advice given in preexisting questions like Check if a string matches a regex in bash script or Count occurances of a char in a string using bash? If so, where did you get stuck? — Charles Duffy
– Charles Duffy, Commented Mar 30, 2022 at 19:01
If you want to do pattern matches in /bin/sh, the case statement is your friend. It's not as powerful as a real regex engine, but if you don't want to use grep, it's what you have (and it's often powerful enough). I can't speak beyond that because I still don't understand your requirements in any detail. — Charles Duffy
– Charles Duffy, Commented Mar 30, 2022 at 19:21

fpmurphy · Accepted Answer · 2022-03-31 14:16:30Z

3

Your question is somewhat unclear. However the following example should point you in the right direction.

#!/bin/sh

#my_string=""
my_string="     "
#my_string="   "
#my_string="name1"
#my_string="name 2"

case "$my_string" in
  "")             echo "string is empty";;
  *[![:space:]]*) echo "string does not contain only whitespace";;
  *)              echo "string contains only whitespace"; exit 1;;
esac

If you want to test only for spaces and tabs rather than all whitespace, you should use [:blank:] instead of [:space:].

edited Mar 31, 2022 at 14:16

answered Mar 31, 2022 at 4:06

fpmurphy

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6 Comments

David C. Rankin Over a year ago

Nice use of case. Since your interpreter is bash, for bonus points the example could be my_array=( "" " " " " "name1" "name 2"); for my_string in "${my_array[@]}"; do case ... done. Not a critique, just a tip.

user1934428 Over a year ago

Good solution, but requires bash, while the OP is looking for a POSIX shell solution.

fpmurphy Over a year ago

@user1934428 What part of the case conditional construct is non-POSIX?

user1934428 Over a year ago

I thought it was the [![:space:]] glob pattern. Or is this already present in POSIX?

fpmurphy Over a year ago

@user1934428. Err, glob is a pathname generator that implements the rules defined in the Shell and Utilities volume of POSIX.1-2017, Section 2.13, Pattern Matching Notation, with optional support for rule 3 in the Shell and Utilities volume of POSIX.1-2017, Section 2.13.3, Patterns Used for Filename Expansion.

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user1934428 · Accepted Answer · 2022-03-31 07:06:47Z

0

Since you are searching for a solution in POSIX shell, I would do something like this:

if [ "$(echo $my_string | tr -d ' ')" = '' ]
then
  echo The string is blank
fi

answered Mar 31, 2022 at 7:06

user1934428

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2 Comments

fpmurphy Over a year ago

This is very inefficient as it requires the use of tr which is not a shell builtin. Your example also fails for a tab. POSIX regards a tab as whitespace!

user1934428 Over a year ago

Right, I was thinking of space characters only. Even worse, I now notice that the OP said: If it has one or two spaces inbetween, it's a valid string. This makes me wonder whether a b is considered valid or not.

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