On Sat, 11 Mar 2006, Terry Carroll wrote:
> I gotta admit, this took me by surprise, too, but my guess is that once
> the head command is done, it closes the pipe it's reading from, which is
> being filled by grep; grep takes the hint and terminates, closing the pipe
> it's reading from, which is being filled by strings; and strings takes the
> hint and terminates, even though it hasn't gotten through the entire file.
Just for the heack of it, I wrote a tiny Python echo program, and
interposed it in the pipe between the strings and grep command:
while 1:
line = raw_input()
print line
The command line now looks like this:
strings 00003193.DAT | python echo.py | grep Newsgroups: | head
(the .DAT file is an Agent newsgroup file; the idea here is that by
grepping for the first few lines with "Newsgroups:", I can tell what
newsgroup the .DAT file is associated with.)
I get:
> strings 00003193.DAT | python echo.py | grep Newsgroups: | head -10
Newsgroups: comp.lang.python.announce
Newsgroups: comp.lang.python.announce
Newsgroups: comp.lang.python.announce
Newsgroups: comp.lang.python.announce
Newsgroups: comp.lang.python.announce
Newsgroups: comp.lang.python.announce
Newsgroups: comp.lang.python.announce
Newsgroups: comp.lang.python.announce
Newsgroups: comp.lang.python.announce
Newsgroups: comp.lang.python.announce
Traceback (most recent call last):
File "echo.py", line 3, in ?
print line
IOError: [Errno 22] Invalid argument
close failed: [Errno 22] Invalid argument
My guess is that the "Invalid argument" here was a write attempting to
write to a closed file, the pipe. The string and grep commands (or IO
routines called by them) probably detect this and close gracefully; the
end result being that a set of piped commands only lasts as long as the
shortest-lived consumer, and the upstream producers shut down when they
can no longer write to the pipe.
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