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

I think there is a Perl module to do this, but I can't seem to find a reference to it. What I want to do is exit backticks early. So, rather than returning 1000 rows, it only returns the first x number of rows. The point of my wanting to do this is speed of execution. Any ideas you can point me towards? Thanks!

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Re: Exiting backticks early
by salva (Canon) on Mar 07, 2018 at 19:23 UTC
    Open the child process as a pipe, read as many lines as you want, then kill it. 
    my $pid = open my $fh, '-|', $cmd, @args;
my @ten_lines;
for (0..9) {
   my $line = <$fh> // last;
   push @ten_lines, $line;
}
kill TERM => $pid;

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Re: Exiting backticks early
by LanX (Saint) on Mar 07, 2018 at 19:16 UTC
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      Well cat | head -n3 prints "cat: write error: No space left on device" on stderr and terminates when I type a fourth line so the writing process can be terminated (though I'm sure there are exceptions). Tested with cygwin on Windows BTW

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      Isn't killing the feeding process done by SIGPIPE handling? 
      $ perl -e ' @a = `cat /etc/passwd | head -3 `; print for @a'
root:x:0:0:root:/root:/bin/bash
daemon:x:1:1:daemon:/usr/sbin:/usr/sbin/nologin
bin:x:2:2:bin:/bin:/usr/sbin/nologin

$ ps auxww | fgrep 'cat /etc/passwd'
bobn     26404  0.0  0.0   9376   764 pts/10   S+   13:11   0:00 fgrep
+ --color=auto cat /etc/passwd
$

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      --Bob Niederman,

      All code given here is UNTESTED unless otherwise stated.

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