From: "Ara.T.Howard" <Ara.T.Howard@...>
Date: 2004-06-30T02:22:54+09:00
Subject: [RCR/BUG?] fix system


this seems broken:

   ~/tmp > cat a.rb
   STDERR.reopen STDOUT
   STDOUT.sync
   DIV = '=' * 79

   commands = [
     'no-such-command',
     'ls --no-such-option',
     'ls .',
   ]

   commands.each do |command|
     [false, true].each do |redirect|
       fork do
         puts DIV

         command << " >/dev/null 2>&1" if redirect

         puts "command <#{ command }>"

         system command

         puts "exit_status <#{ $?.exitstatus }>"
       end
       Process.wait
     end
   end

   ~/tmp > ruby a.rb
   ===============================================================================
   command <no-such-command>
   a.rb:18:in `system': No such file or directory - no-such-command (Errno::ENOENT)
           from a.rb:18
           from a.rb:13:in `fork'
           from a.rb:13
           from a.rb:13:in `each'
           from a.rb:12
           from a.rb:12:in `each'
           from a.rb:11
           from a.rb:11
   ===============================================================================
   command <no-such-command >/dev/null 2>&1>
   exit_status <127>
   ===============================================================================
   command <ls --no-such-option>
   ls: unrecognized option `--no-such-option'
   Try `ls --help' for more information.
   exit_status <1>
   ===============================================================================
   command <ls --no-such-option >/dev/null 2>&1>
   exit_status <1>
   ===============================================================================
   command <ls .>
   a.rb
   exit_status <0>
   ===============================================================================
   command <ls . >/dev/null 2>&1>
   exit_status <0>


the above behaviour of system means the following rules must be followed when
making system calls:

   - iff stderr IS NOT redirected you must be prepared to catch Errno::ENOENT in
     the event your command is not found.

   - iff stderr IS redirected you will NOT be informed of non-existent commands
     via an exception and must check $?.exitstatus manually for 127 to
     determine if the command was not found

   - in all cases you must check $?.exitstatus for success since your command
     may be found, but fail for other reasons and this will not raise any
     errors.

so, in summary you must still always check $?, but sometimes errors will be
thrown depending on the contents of the string passed to system.  am i the
only one who finds this confusing?  it seems like any code using system will
turn into spaghetti if it follows these rules...

-a
--
===============================================================================
| EMAIL   :: Ara [dot] T [dot] Howard [at] noaa [dot] gov
| PHONE   :: 303.497.6469
| A flower falls, even though we love it;
| and a weed grows, even though we do not love it. 
|   --Dogen
===============================================================================