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Suppose i have a string, $str. I want $str to be edited such that all the spaces in it are replaced by underscores.

Example

a="hello world"

I want the final output of 

echo "$a"

to be hello_world

Ayush Mishra
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5 Answers5

83

You could try the following:

str="${str// /_}"
William Hay
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30
$ a="hello world"
$ echo ${a// /_}
hello_world

According to bash(1):

${parameter/pattern/string}

Pattern substitution. The pattern is expanded to produce a pattern just as in pathname expansion. Parameter is expanded and the longest match of pattern against its value is replaced with string. If pattern begins with /, all matches of pattern are replaced
with string. Normally only the first match is replaced. If pattern begins with #, it must match at the beginning of the expanded value of parameter. If pattern begins with %, it must match at the end of the expanded value of parameter. If string is null, matches of pattern are deleted and the / following pattern may be omitted. If parameter is @ or *, the substitution operation is applied to each positional parameter in turn, and the expansion is the resultant list. If parameter is an array variable subscripted with @ or *, the substitution operation is applied to each member of the array in turn, and the expansion is the resultant list.

falsetru
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14

Pure bash:

a="hello world"
echo "${a// /_}"

OR tr:

tr -s ' ' '_' <<< "$a"
anubhava
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    +1 for just pure bash syntax!. Would like to know what's it doing behind scenes. Looks like it is doing 'sed' replacement operations. Any bash manual states for using such syntax?. – user1587504 Oct 29 '13 at 14:45
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    @user1587504: I believe `falsetru` has added relevant man section for this `Pattern substitution` – anubhava Oct 29 '13 at 14:47
10

With sed reading directly from a variable:

$ sed 's/ /_/g' <<< "$a"
hello_world

And to store the result you have to use the var=$(command) syntax:

a=$(sed 's/ /_/g' <<< "$a")

For completeness, with awk it can be done like this:

$ a="hello my name is"
$ awk 'BEGIN{OFS="_"} {for (i=1; i<NF; i++) printf "%s%s",$i,OFS; printf "%s\n", $NF}' <<< "$a"
hello_my_name_is
fedorqui
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  • Neither of these appears to deal with more than one space. – William Hay Oct 29 '13 at 14:46
  • @WilliamHay you are right, didn't check with more than one space. Updated to use `sed .../g`, as default was doing it just once. – fedorqui Oct 29 '13 at 14:49
  • @fedorqui +1 for letting know usage of direct variable without echo piping to sed. I donot see direct help for this: '<<<' . Where is it defined in man pages. Could you please point me? – user1587504 Oct 29 '13 at 14:50
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    @user1587504 they are called "Here Strings". See http://linux.die.net/abs-guide/x15683.html for more reference. They are useful. – fedorqui Oct 29 '13 at 14:53
  • @fedorqui . Thanks! for letting me know. I did used one with '<<' to check some EOF character when using isql inside a script. Usage of <<< is nice. .. – user1587504 Oct 29 '13 at 14:56
7

Multiple spaces to one underscore

This can easily be achieved with a GNU shell parameter expansion. In particular:

${parameter/pattern/string}

If pattern begins with /, all matches of pattern are replaced with string.

with +(pattern-list)

Matches one or more occurrences of the given patterns.

Hence:

$ a='hello world    example'

$ echo ${a// /_}
hello_world____example

$ echo ${a//+( )/_}
hello_world_example

However, for this to work in a bash script two amendments need to be made:

  1. The parameter expansion requires encapsulation in double quotes " " to prevent word splitting with the input field separator $IFS.
  2. The extglob shell option needs to be enabled using the shopt builtin, for extended pattern matching operators to be recognised.

The bash script finally looks like this:

#!/usr/bin/env bash
shopt -s extglob
a='hello world    example'
echo "${a//+( )/_}"
Serge Stroobandt
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