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Full Discussion: Problem with pipes on infinite streams
Top Forums Shell Programming and Scripting Problem with pipes on infinite streams Post 302407979 by tokland on Saturday 27th of March 2010 12:04:56 PM
Old 03-27-2010
Problem with pipes on infinite streams
Here is an example code that shows the issue I have:

Code:
#!/bin/bash
counter() {
  seq 1000 | while read NUM; do
    echo $NUM
    echo "debug: $NUM" >&2 
    sleep 0.1 # slow it down so we know when this loop really ends
  done
}

counter | grep --line-buffered "[27]" | head -n1

Code:
debug: 1
debug: 2
2
debug: 3
debug: 4
debug: 5
debug: 6
debug: 7

If I understand it correctly, "head" finishes on the first match (as expected), but "grep" is not aware of it until it tries to write the next line (the second match). When it does, it finds out the pipe is closed so it also finishes.

That's normally not a problem, but if you have an infinite input stream containing only one match, it won't never stop. Any solution?
 

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LEARN ABOUT DEBIAN

mp4file

MP4FILE(1)							  MP4v2 Utilities							MP4FILE(1)

NAME

       mp4file - manual page for mp4file - MP4v2 2.0.0

SYNOPSIS

       mp4file [OPTION]... ACTION file...

DESCRIPTION

       For each mp4 file specified, perform the specified ACTION. An action must be specified. Some options are not applicable to some actions.

       ACTIONS

       --list list (summary information)

       --optimize
	      optimize mp4 structure

       --dump dump mp4 structure in human-readable format

       OPTIONS

       -y, --dryrun
	      do not actually create or modify any files

       -k, --keepgoing
	      continue batch processing even after errors

       -q, --quiet
	      equivalent to --verbose 0

       -d, --debug NUM
	      increase debug or long-option to set NUM

       -v, --verbose NUM
	      increase verbosity or long-option to set NUM

       -h, --help
	      print brief help or long-option for extended help

       --version
	      print version information and exit

       DEBUG LEVELS (for raw mp4 file I/O)

       0      supressed

       1      add warnings and errors (default)

       2      add table details

       3      add implicits

       4      everything

       VERBOSE LEVELS

       0      warnings and errors

       1      normal informative messages (default)

       2      more informative messages

       3      everything

mp4file - MP4v2 2.0.0						     June 2012								MP4FILE(1)
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