From: Ashley Yakeley <ashley@...>
Date: 2001-05-09T15:10:13+09:00
Subject: [ruby-talk:14897] Backslashes in Backquotes

Is there an easy way in Ruby to execute a string as a command in a 
subshell and return the output text? 'System' doesn't seem to have this 
kind of redirection of output.

I know, I'm probably missing something obvious. But I've been through 
the documentation and I couldn't find it...

My first attempt looked like this:

    def subshell(command)
        `command`
    end

That doesn't work, because while `` does execute in a subshell, it also 
(annoying) quotes its content like a string literal. So I tried this:

    def subshell(command)
        eval '`'+command+'`'
    end

You'd think that would work, right? But no, Ruby insists on doing this 
weird control-character interpretation of \1, \2 etc. Try this from irb:

irb(main):001:0> print `echo \1`

nil
irb(main):002:0> `echo \1`
"\001\n"
irb(main):003:0> print `echo \\1`
\1
nil
irb(main):004:0> print `echo '\1'`
\
nil
irb(main):005:0> print `echo '\\1'`
\\1
nil

...which is not what sh does:

localhost% echo \1
1
localhost% echo \\1
\1
localhost% echo '\1'
\1
localhost% echo '\\1'
\\1


So, there's no obvious way of passing the string "echo \1" to the shell. 
This is a trivial example, but it's a real pain for something like a 
regular expression passed as a command-line argument to sed.

-- 
Ashley Yakeley, Seattle WA