From: mike@... (Mike Stok)
Date: 2002-03-06T23:52:32+09:00
Subject: Re: Improvement to system()

In article <CFFFA2FE61C8D656.68DF0AF2ECCDB283.D9A57F9E27ECD964@lp.airnews.net>,
Yohanes Santoso wrote:
> Hi,
> 
> I just got bitten by return value of system() because it does not
> distinguish invalid command and error code resulted from executing
> command.
> 
> irb(main):014:0> system "/bin/falsed" 
> false
> irb(main):015:0> system "/bin/false" 
> false
> 
> Quickly, why did the first one returns false, and why did the second one
> returns false?
> 
> Did you instantly catch the typo? Sometimes the command to be executed
> is determined at some other place. Thus, if system(*cmd) returns
> false, there's no easy way to determine why it fails.
> 
> A solution is to use File.stat.executable? But even directories are
> executable.
> 
> Any better solution?


You can look at $? e.g.

irb(main):001:0> $?
nil
irb(main):002:0> system "/bin/false"
false
irb(main):003:0> $?
256
irb(main):004:0> system "/bin/true"
true
irb(main):005:0> $?
0
irb(main):006:0> system "/bin/banana"
false
irb(main):007:0> $?
32512

The contents of $? can be viewed as 2 8 bit values, 

  exit_status = ($? & 0xff00) >> 8
  exit_flags = $? & 255

(gory details can be found in various header files like
/usr/include/bits/waitstatus.h on linux)

If the program you were calling really did an "exit(127)" then you might have
to dig deeper, but $? contains some extra information,

Hope this helps.

Mike

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