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Re: why complain about a broken pipe?
On Wed, 8 Feb 2012 01:44:51 -0500 (EST)
Mouse <mouse%Rodents-Montreal.ORG@localhost> wrote:
> > I posted my original query because I object to pax (and xargs)
> > complaining about EPIPE. I don't see the need. The script can't
> > capture it,
> Actually, scripts _can_ capture it. stderr can be redirected.
Oh, yes, I know. To split the hair ever so finely, "it" in that
sentence is meant to refer to EPIPE. It doesn't show up in the
shell's $? variable or similar, and can't be acted on
> xargs is, if you will, acting as a
> (severely special-case) shell in that it's running other
> user-specified processes; as such, it's well within its bailiwick to
> report on abnormal terminations.
Oh, no. xargs is no shell at all! It's a mere re-arranger made
necessary by mistaken choices, an ugly elbow-joint in the pipe.
$ find /usr | xargs ls -F
could as well be expressed as
$ ls -F $(find /usr)
$ find /usr | ls -F
but for accidents of history. As such, xargs has no more right to talk
to the user than the "|" character, and as much duty to report the exit
> Well, if you don't have SIGPIPE ignored, caught, or blocked, you
> don't last long enough to even _get_ EPIPE
Ah. I would have stumbled on that eventually, but I appreciate the
education. You're quite right, of course: pax.c ignores SIGPIPE around
It needs to because "pax -w" writes an archive to standard output by
default. Connect it to ssh to copy files across a network. Which, I
admit, is kinda important to get right.
$ pax -w * | hexdump -C | head -1
00000000 1b 5b 3f 31 3b 32 63 1b 5b 3f 31 3b 32 63 1b 5b |.[?1;2c.[?
pax: Failed write to archive volume: 1 (Broken pipe)
ATTENTION! pax archive volume change required.
Ready for archive volume: 2
Input archive name or "." to quit pax.
Archive name > ^C
pax: Signal caught, cleaning up.
Typing "." yields a list of files not processed, something that would
be a lot harder to do without ignoring SIGPIPE. ;-) And, as we'll
hear a lot in the coming months here in the US, I approve of this
message. About not being able to write the archive volume, that is.
The table of contents is another matter....
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