Weird open() return code in pipe mode

saintmike has asked for the wisdom of the Perl Monks concerning the following question:

If you launch an external program via open() and a trailing pipe "|" character to read from its STDOUT, open() will only return a false value if it can't find the program specified:

open PIPE, "pfrrzt |" or die "failed!";
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This will correctly print an error message and die() afterwards:

Can't exec "pfrrzt": No such file or directory 
at ./test line 9.
failed! at ./t line 9.
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But if the command redirects its standard output to /dev/null, open will return a true value instead:

open PIPE, "pfrrzt >/dev/null |" or die "failed!";
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which doesn't trigger the die() statement.

Interestingly, the popen() function used internally won't catch a non-existing program name and return an OK pointer to a FILE struct either way.

What's the magic at the Perl level that causes the behavior mentioned above? ~

Comment on Weird open() return code in pipe mode Select or Download Code

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Re: Weird open() return code in pipe mode by ikegami (Patriarch) on Sep 21, 2007 at 04:13 UTC
It must act like `system`, where the shell (`/bin/sh`) is only involved if necessary. It's not involved for `open PIPE, "pfrrzt \|"` since the command contain no special shell characters. Perl is not able to launch `pfrrzt`, so `open` returns an error. On the other hand, `open PIPE, "pfrrzt >/dev/null \|"` does involve special shell characters (`>`), so `/bin/sh` is loaded to execute the command. Perl is able to successfully execute `/bin/sh`, so `open` returns the child's (the shell's) pid. I think you can check if the pipe is open using `eof`. If it's not open, you can `close` the pipe and then get the child's error code via `$?`.	[reply] [d/l] [select]
Re: Weird open() return code in pipe mode by duff (Parson) on Sep 21, 2007 at 04:08 UTC
At a guess, I'd say that when you add the string ">/dev/null", perl has to spawn a shell to make that work,so open returns the PID of that shell process. Without the ">/dev/null", perl probably just does a fork+exec(program). Again, just a guess. duff	[reply]
Re: Weird open() return code in pipe mode by graff (Chancellor) on Sep 22, 2007 at 01:52 UTC
I cannot imagine what you would hope to accomplish using an open statement like this: `open PIPE, "pfrrzt >/dev/null \|" or die "failed!";` [download] Given that the command's stdout is being redirected (whether to /dev/null or any other file), nothing would be left for delivery to the pipe. Were you expecting perl DWIMery to do something "coherent" in this case? I think if there were an executable command called "pfrrzt" in your PATH, the results would still not be what you were hoping for.	[reply] [d/l]