Re: qx and $!

A comparison on how to run external programs and get meaningful errors from them:

print qx(date)     or die "Cannot run date:$!\n";
system (date) == 0 or die "Cannot run date:$?\n";
[download]

--
Regards,
Helgi Briem
hbriem AT f-prot DOT com

Comment on Re: qx and $! Download Code

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Re^2: qx and $! by ikegami (Patriarch) on Oct 29, 2006 at 20:19 UTC
That's not right. Both `qx` and `system` set `$!` when unable to launch the child. Both `qx` and `system` set `$?` to the child's error code. An error did not necessarily occur when `qx` return false, only when it returns undef. `$!` is not necessarily set when `qx` return false, only when it returns undef. Here is how one actually gets meaningful errors from them: `my $output = qx(date); die("Unable to launch external process: $!\n") if not defined $output; die("The external process did not run successfully (error code $?)\n") if $?;` [download] `my $rv = system(date); die("Unable to launch external process: $!\n") if $rv == -1; die("The external process did not run successfully (error code $?)\n") if $?;` [download]	[reply] [d/l] [select]

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Re^2: qx and $!
by ikegami (Patriarch) on Oct 29, 2006 at 20:19 UTC

That's not right.

Both qx and system set $! when unable to launch the child.
Both qx and system set $? to the child's error code.
An error did not necessarily occur when qx return false, only when it returns undef.
$! is not necessarily set when qx return false, only when it returns undef.

Here is how one actually gets meaningful errors from them:

my $output = qx(date);
die("Unable to launch external process: $!\n")
   if not defined $output;
die("The external process did not run successfully (error code $?)\n")
   if $?;
[download]

my $rv = system(date);
die("Unable to launch external process: $!\n")
   if $rv == -1;
die("The external process did not run successfully (error code $?)\n")
   if $?;
[download]

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