Replace spaces with underscores via BASH

Question

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

Answer 1

您可以尝试以下操作：

str="${str// /_}"

Answer 2

$ 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.

Answer 3

Pure bash:

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

OR tr:

tr -s ' ' '_' <<< "$a"

Answer 4

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

Answer 5

Multiple spaces; 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//+( )/_}"