From: Justin Collins <justincollins@...>
Date: 2009-01-07T19:28:05+09:00
Subject: Re: A question about running external program

Zhao Yi wrote:
> I use this method to run an external program:
>
> output=`uname -r | sed 's/\([^\.]*\.[^\.]*\)\.*.*/\1/'`
> puts output
>
> ruby will ignore the sed command. Doesn't ruby support "|"?
>   

The sed command is not being ignored. The issue is that the backticks 
(`) do string interpolation before sending the command, which means your 
backslashes are removed before the commands are sent to the shell.

Take a look at this:

irb(main):001:0> puts `uname -r | sed 's/\([^\.]*\.[^\.]*\)\.*.*/\1/' && 
ps x | grep uname`
2.6.27.5-desktop-2mnb
24178 pts/8    S+     0:00 sh -c uname -r | sed 's/([^.]*.[^.]*).*.*/?/' 
&& ps x | grep uname
24182 pts/8    S+     0:00 grep uname
=> nil
irb(main):002:0> puts system 'uname -r | sed 
"s/\([^\.]*\.[^\.]*\)\.*.*/\1/" && ps x | grep uname'
2.6
24187 pts/8    S+     0:00 sh -c uname -r | sed 
"s/\([^\.]*\.[^\.]*\)\.*.*/\1/" && ps x | grep uname
24191 pts/8    S+     0:00 grep uname
true
=> nil
irb(main):003:0> puts `uname -r | sed 
's/\\([^\\.]*\\.[^\\.]*\\)\\.*.*/\\1/' && ps x | grep uname`
2.6
24201 pts/8    S+     0:00 sh -c uname -r | sed 
's/\([^\.]*\.[^\.]*\)\.*.*/\1/' && ps x | grep uname
24205 pts/8    S+     0:00 grep uname
=> nil

-Justin