CODEWITHSUNDEEP
bashvariable-expansionparameter-expansion
Jimbo
Jimbo

Reputation: 51

Bash variable expansion with a '/'

Inside of a while read line loop, I see this variable expansion ${line/device name:}. I've tried running the script with my own input file and it just prints out the line.

Can you tell me what that expansion is doing?

Upvotes: 5

Views: 153

Answers (2)

user3159253
user3159253

Reputation: 17455

The variable name is line. / is for string substitution, that is "device name:" if exists anywhere in the $line is removed.

> line="a device name: some name"
> echo ${line/device name:}
a  some name

You may also see # and % substitutions, which stand for substitutions in the line begin and end. Also beware that such / substitution is a bash-specific feature (e.g. ash doesn't support it, % and # are seemingly portable), so you should use #!/bin/bash instead of #!/bin/sh as a hashbang in the beginning of your script.

Upvotes: 4

John Kugelman
John Kugelman

Reputation: 361605

It returns $line with the substring device name: removed. From the bash man page:

${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.

Upvotes: 3

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