How to replace multiple spaces with a single space using Bash?

Question

I'd like to use bash to replace multiple adjacent spaces in a string by a single space. Example:

Original string:

"too         many       spaces."

Transformed string:

"too many spaces."

I've tried things like "${str//*( )/.}" or awk '{gsub(/[:blank:]/," ")}1' but I can't get it right.

Note: I was able to make it work with <CMD_THAT_GENERATES_THE_INPUT_STRINGH> | perl -lpe's/\s+/ /g' but I had to use perl to do the job. I'd like to use some bash internal syntax instead of calling an external program, if that is possible.

OP is probably looking for a bash way not necessarily asking for a `sed` or any other external tool solution. — anubhava, May 09 '18 at 18:32
Just with built-ins, `str="too many spaces.";shopt -s extglob; printf '%s\n' "${str//+([[:space:]])/ }"` — Inian, May 09 '18 at 18:32
Re: the `echo $string | perl` note: When IFS is at its default value, `echo $string` *itself* will replace runs of multiple spaces with a single space (along with other side effects, like replacing a wildcard in that string with a list of files) -- one needs to use `echo "$string"` to keep them in. (Even then, `echo` can still munge contents rather than emitting them exactly as they exist; `printf '%s\n' "$string"` is much more reliable). — Charles Duffy, May 09 '18 at 18:39
@codeforester: _Why_ have them? :) those tags are independent of the programming language used. Not adding any value as such — Inian, May 09 '18 at 18:40
@Inian: Even if those tags are independent of the programming language used, keeping them in question will only increase visibility of this question in search results. — anubhava, May 09 '18 at 18:42
How many people will have "string" or "replace" in their tags-to-watch list? Color me in the skeptic camp re: those tags having any value. — Charles Duffy, May 09 '18 at 18:44
@CharlesDuffy you may be right: [Do tags help in Google searches?](https://meta.stackoverflow.com/q/367648/6862601) — codeforester, May 09 '18 at 18:58
@CharlesDuffy, thank you for pointing out that echo already replaces the spaces! I edited my question to show that I wasn't using echo in my tests. I was using | perl .... And I wanted to substitute the spaces in the output from SOME_COMMAND — Bruno Negrão Zica, May 09 '18 at 19:35
Possible duplicate of [How to remove extra spaces in bash?](https://stackoverflow.com/q/13092360/608639) — jww, May 26 '19 at 22:45

James Brown · Accepted Answer · 2018-05-09T20:06:07.000

42

Using tr:

$ echo "too         many       spaces." | tr -s ' '
too many spaces

man tr:

-s, --squeeze-repeats
       replace each sequence of a repeated character that is listed  in
       the last specified SET, with a single occurrence of that charac‐
       ter

Edit: Oh, by the way:

$ s="foo      bar"
$ echo $s
foo bar
$ echo "$s"
foo      bar

Edit 2: On the performance:

$ shopt -s extglob
$ s=$(for i in {1..100} ; do echo -n "word   " ; done) # 100 times: word   word   word...
$ time echo "${s//+([[:blank:]])/ }" > /dev/null

real    0m7.296s
user    0m7.292s
sys     0m0.000s
$ time echo "$s" | tr -s ' ' >/dev/null

real    0m0.002s
user    0m0.000s
sys     0m0.000s

Over 7 seconds?! How is that even possible. Well, this mini laptop is from 2014 but still. Then again:

$ time echo "${s//+( )/ }" > /dev/null

real    0m1.198s
user    0m1.192s
sys     0m0.000s

edited May 09 '18 at 20:06

answered May 09 '18 at 18:26

James Brown

36,089
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I'd certainly go this route on a POSIX shell where bash extensions weren't available. That said, it's going to be much slower to run (with reasonable input sizes) than the approach given by @anubhava when one *can* use bash. – Charles Duffy May 09 '18 at 18:37
Added a test with a hundred words and 3 spaces between each. – James Brown May 09 '18 at 19:44
Over 7 seconds every time. `echo $BASH_VERSION 4.4.12(1)-release` – James Brown May 09 '18 at 20:00
2

...waitaminute, found it -- I didn't have extglob enabled. Okay, that's a legitimate result. Might add `shopt -s extglob` to make it copy-and-pasteable for folks trying to repro. – Charles Duffy May 09 '18 at 20:01

score 14 · Answer 2 · answered May 09 '18 at 18:32

Here is a way to do this using pure bash and extglob:

s="too         many       spaces."

shopt -s extglob
echo "${s//+([[:blank:]])/ }"

too many spaces.

Bracket expression [[:blank:]] matches a space or tab character
+([[:blank:]]) matches one or more of the bracket expression (requires extglob)

David C. Rankin · Answer 3 · 2018-05-09T19:16:48.380

Another simple sed expression using BRE is:

sed 's/[ ][ ]*/ /g'

For example:

$ echo "too         many       spaces." | sed 's/[ ][ ]*/ /g'
too many spaces.

There are a number of ways to skin the cat.

If the enclosed whitespace could consist of mixed spaces and tabs, then you could use:

sed 's/\s\s*/ /g'

And if you simply want to have bash word-splitting handle it, just echo your string without quotes, e.g.

$ echo "too         many       spaces." | while read line; do echo $line; done
too many spaces.

Continuing with that same thought, if your string with spaces is already stored in a variable, you can simply use echo unquoted within command substitution to have bash remove the additional whitespace for your, e.g.

$ foo="too         many       spaces."; bar=$(echo $foo); echo "$bar"
too many spaces.

How to replace multiple spaces with a single space using Bash?

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