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Full Discussion: UNIX fifo concurrent read from a named pipe
Top Forums Shell Programming and Scripting UNIX fifo concurrent read from a named pipe Post 303013524 by Chubler_XL on Thursday 22nd of February 2018 04:41:46 PM
Old 02-22-2018
Probably best to use a mutex/semaphore to ensure only 1 process only reads from the pipe at a time.

I don't know what OS you are using so not sure if lockfile(1) is available to you.

This solution uses a lock directory as mkdir is supposed to be atomic:

readpipe.sh
Code:
lockdir=lock.dir

get_book() {
   local DONE=0
   while ! mkdir $lockdir 2> /dev/null
   do
      sleep 0.02
   done

   IFS="," read -u7 var || DONE=1

   rmdir $lockdir
   return $DONE
}

exec 7< namepipe
while get_book
do
   echo "$var" >> log$$.txt
done
exec 7<&-

Here is my test:

Code:
$ cat naveen 
mkfifo namepipe
./readpipe.sh &
./readpipe.sh &
exec 3<> namepipe
cat books.txt >&3
# Give time for readers to get established
sleep 1
# Close the write end
exec 3>&-
# wait for readers to complete
wait

$ ./naveen 

$ wc -l books.txt log*
 200 books.txt
 106 log2838.txt
  94 log2839.txt
 400 total

$ diff <(sort books.txt) <(cat log* | sort)


Update: below is an example using flock(1):

Code:
lockfile=lock.file

get_book() {
  var=$( (
      if flock -w .007 -n 200
      then
        IFS="," read -u7 var || exit 1
        echo "$var"
      fi) 200>$lockfile )
}

exec 7< namepipe

while get_book
do
  if [ -n "$var" ]
  then
          # Do any required work with the book record here...
          echo "$var" >> log$$.txt
  fi
done
exec 7<&-

Last edited by Chubler_XL; 02-25-2018 at 11:56 PM.. Reason: Added sleep to writer before close - avoid 1 process hogging the fifo
 

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unix2dos

unix2dos(1)						      General Commands Manual						       unix2dos(1)

NAME

       unix2dos - UNIX to DOS text file format converter

SYNOPSYS

       unix2dos [options] [-c convmode] [-o file ...] [-n infile outfile ...]

       Options:

       [-hkqV] [--help] [--keepdate] [--quiet] [--version]

DESCRIPTION

       This manual page documents unix2dos, the program that converts text files in UNIX format to DOS format.

OPTIONS

       The following options are available:

       -h --help
	      Print online help.

       -k --keepdate
	      Keep the date stamp of output file same as input file.

       -q --quiet
	      Quiet mode. Suppress all warning and messages.

       -V --version
	      Prints version information.

       -c --convmode convmode
	      Sets conversion mode. Simulates unix2dos under SunOS.

       -o --oldfile file ...
	      Old file mode. Convert the file and write output to it. The program default to run in this mode. Wildcard names may be used.

       -n --newfile infile outfile ...
	      New  file  mode.	Convert the infile and write output to outfile. File names must be given in pairs and wildcard names should NOT be
	      used or you WILL lost your files.

EXAMPLES

       Get input from stdin and write output to stdout.

	      unix2dos

       Convert and replace a.txt. Convert and replace b.txt.

	      unix2dos a.txt b.txt

	      unix2dos -o a.txt b.txt

       Convert and replace a.txt in ASCII conversion mode.  Convert and replace b.txt in ISO conversion mode.

	      unix2dos a.txt -c iso b.txt

	      unix2dos -c ascii a.txt -c iso b.txt

       Convert and replace a.txt while keeping original date stamp.

	      unix2dos -k a.txt

	      unix2dos -k -o a.txt

       Convert a.txt and write to e.txt.

	      unix2dos -n a.txt e.txt

       Convert a.txt and write to e.txt, keep date stamp of e.txt same as a.txt.

	      unix2dos -k -n a.txt e.txt

       Convert and replace a.txt. Convert b.txt and write to e.txt.

	      unix2dos a.txt -n b.txt e.txt

	      unix2dos -o a.txt -n b.txt e.txt

       Convert c.txt and write to e.txt. Convert and replace a.txt.  Convert and replace b.txt. Convert d.txt and write to f.txt.

	      unix2dos -n c.txt e.txt -o a.txt b.txt -n d.txt f.txt

DIAGNOSTICS

BUGS

       The program does not work properly under MSDOS in stdio processing mode.  If you know why is that so, please tell me.

AUTHOR

       Benjamin Lin - ( blin@socs.uts.edu.au )

MISCELLANY

       Tested environment:

	      Linux 1.2.0 with GNU C 2.5.8

	      SunOS 4.1.3 with GNU C 2.6.3

	      MS-DOS 6.20 with Borland C++ 4.02

       Suggestions and bug reports are welcome.

SEE ALSO

       dos2unix(1)

1995.03.31							   unix2dos v2.2						       unix2dos(1)
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