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Editorial Team
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Asked: 2026-06-02T11:29:33+00:00

In the code below p00 is a named pipe, created with mkfifo p00 .

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In the code below p00 is a named pipe, created with mkfifo p00.

In console 1, I ran:

% perl -ne 'print "PERL: $_"' < p00

Then, while the above blocked (as expected), in console 2 I ran

% seq 3 > p00

As a result, the following appeared in console 1:

PERL: 1
PERL: 2
PERL: 3
%

This was almost the result I had hoped to achieve, except for the fact that the perl script terminated after printing the third line.

I want the script to continue echoing lines (prefixed by “PERL: “) as soon as they become available, and block otherwise.

The following variant of the one-liner above superficially resembles the desired behavior:

perl -e 'while ( 1 ) { print "PERL: $_" while <>; sleep 1 }' < p00

…but it’s not the real deal, because it does not block while waiting for input, nor it echoes its input as soon as it is available.

NOTES:

  1. the motivation behind this question is education (mine, that is) and nothing more; I’m not trying to solve any practical problem; I’m just trying to learn more perl (and unix).
  2. I wasn’t sure if this question was more suited for unix.se.com; I’m more than happy to re-post it there if it is; just let me know.
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1 Answer

  1. Editorial Team

    After reading Maxim Yegorushkin’s comment I realized that all I had to do was get rid of the sleep 1 in the second version. I.e., this does exactly what I want:

    perl -e 'while ( 1 ) { print "PERL: $_" while <> }' < p00

    As Maxim wrote, the inner loop terminates upon receiving the EOF; then the outer loop returns the script to a blocking state, waiting for input… Doh!

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