in reply to Re^3: Using backquotes to echo results in "Bad file descriptor"
in thread Using backquotes to echo results in "Bad file descriptor"

Yes, I realize now that I'm not doing my error checking correctly by using $! but my next question (read: obsession) is why do the perl C libraries set errno to "Bad file descriptor" whenever perl executes a shell command using backquotes that sends data to STDOUT?

Consider this:

perl -e '$res = `echo test`; print $!."\n";'

versus this:

perl -e '$res = `echo test>/dev/null`; print $!."\n";'

In the first example, $! is "Bad file descriptor". In the latter example, $! is empty. So whatever the answer is, it has something to do with whether or not the command has output. You can replace "echo test" with any shell command that produces STDOUT info.

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Re^5: Using backquotes to echo results in "Bad file descriptor"
by ikegami (Patriarch) on Mar 04, 2009 at 19:09 UTC

    Try using strace to find out.

    Nevermind, I just did, and I don't see a system call returning that error.

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Re^5: Using backquotes to echo results in "Bad file descriptor"
by kennethk (Abbot) on Mar 04, 2009 at 19:32 UTC

    Not entirely sure, but an interesting test case I found is the following:

    perl -e 'print `echo test`; print $!."\n";'

    shows $! to be empty. I am guessing the issue is that echo is attempting to output to a non-existent terminal, and this error is a result of an internal perl call to determine if the output is being piped to an existing terminal. Note that

    perl -e 'print $res = `echo test`; print $!."\n";'

    does yield $! eq "Bad file descriptor".

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      So... when perl is outputing the data using print, it doesn't cause the error, but when it's sending it to a scalar, it isn't really a terminal, so it causes $! to have that error. I can go with that. Makes a lot of sense actually. Thank you!
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