# A question about running external program **URL:** \ **Category:** Ruby\ **Created:** [January 7, 2009, 9:30am UTC](https://www.ruby-forum.com/t/a-question-about-running-external-program/157088 "2009-01-07T09:30:43Z")\ **Posts on this page:** 5\ **Page:** 1
**Author:** ![youhaodeyi](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/y/b9bd4f/32.png) [@youhaodeyi](https://www.ruby-forum.com/u/youhaodeyi)\ **Post date:** [January 7, 2009, 9:30am UTC](https://www.ruby-forum.com/t/a-question-about-running-external-program/157088/1 "2009-01-07T09:30:43Z")
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 “|”? ---
**Author:** ![youhaodeyi](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/y/b9bd4f/32.png) [@youhaodeyi](https://www.ruby-forum.com/u/youhaodeyi)\ **Post date:** [January 7, 2009, 11:28am UTC](https://www.ruby-forum.com/t/a-question-about-running-external-program/157088/2 "2009-01-07T11:28:23Z")
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 “|”? That’s not a smart question. - Show what happens if you run the same script at the command line - Show what ‘output’ actually contains when doing this in Ruby - Show what version of Ruby you are using, and under what platform FWIW, it works just fine for me under Ubuntu Hardy with ruby-1.8.6 compiled from source: $ ruby -v ruby 1.8.6 (2007-09-24 patchlevel 111) [i486-linux] $ irb irb(main):001:0\> output=`uname -r | sed 's/\([^\.]*\.[^\.]*\)\.*.*/\1/'` =\> “2.6.24-22-386\n” ---
**Author:** ![youhaodeyi](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/y/b9bd4f/32.png) [@youhaodeyi](https://www.ruby-forum.com/u/youhaodeyi)\ **Post date:** [January 7, 2009, 11:43am UTC](https://www.ruby-forum.com/t/a-question-about-running-external-program/157088/3 "2009-01-07T11:43:06Z")
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 ---
**Author:** ![youhaodeyi](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/y/b9bd4f/32.png) [@youhaodeyi](https://www.ruby-forum.com/u/youhaodeyi)\ **Post date:** [January 7, 2009, 1:01pm UTC](https://www.ruby-forum.com/t/a-question-about-running-external-program/157088/4 "2009-01-07T13:01:35Z")
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 “|”? Instead of strugling with shell-escaping, you can do the regexp part in Ruby itself: output = `uname -r` version = output[/some-regexp/] ---
**Author:** ![youhaodeyi](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/y/b9bd4f/32.png) [@youhaodeyi](https://www.ruby-forum.com/u/youhaodeyi)\ **Post date:** [January 7, 2009, 2:03pm UTC](https://www.ruby-forum.com/t/a-question-about-running-external-program/157088/5 "2009-01-07T14:03:47Z")
Justin C. wrote: > irb(main):003:0\> puts `uname -r | sed 's/\\([^\\.]*\\.[^\\.]*\\)\\.*.*/\\1/' && ps x | grep uname` I think that’s not quite right, because \1 is still converted into a single character with ASCII code 1, rather than being passed to sed as \1 irb(main):013:0\> `echo 's/\\([^\\.]*\\.[^\\.]*\\)\\.*.*/\\1/'` =\> “s/\([^\.]_\.[^\.]_\)\._._/\001/\n” So as far as I can see, you need to use \\1 to get a literal \ followed by 1 irb(main):014:0\> `echo 's/\\([^\\.]*\\.[^\\.]*\\)\\.*.*/\\\\1/'` =\> “s/\([^\.]_\.[^\.]_\)\._._/\1/\n” Escapes like . and ( are OK, but for consistency you can escape them all in the same way: irb(main):015:0\> `echo 's/\\\\([^\\\\.]*\\\\.[^\\\\.]*\\\\)\\\\.*.*/\\\\1/'` =\> “s/\([^\.]_\.[^\.]_\)\._._/\1/\n”