Bash variable expansion with a '/'

Question

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?

Answer 1

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 (eg 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.

Answer 2

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

${parameter/pattern/string}
         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.