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Full Discussion: How to send data to previous program (pipe)?
Top Forums Shell Programming and Scripting How to send data to previous program (pipe)? Post 303012660 by Corona688 on Wednesday 7th of February 2018 10:58:40 AM
Old 02-07-2018
Quote:
Originally Posted by JackK
I didn't know about coprocesses before. In my case, I think, writing a parser with coprocesses is not needed and too complicated (for such a small requirement).
Once again, what you're asking for isn't exactly trivial.

You can communicate with the previous process with a named pipe, but this will be a bit complicated to set up and prone to deadlocks afterwards (who's waiting for whom?) With all those caveats, here is how you do it:

Code:
#!/bin/bash

exec 1>&3 # Save stdout into FD 3 if we need it
mkfifo /tmp/$$

echo "fifo is /tmp/$$"

(       # Process A.  stdout writes to pipe.
        exec < /tmp/$$ # Redirect stdin from named pipe

        for X in 1 2 3
        do
                read ASDF
                echo "string: $ASDF"
        done
) | (
        # Process B.  stdin reads from pipe.
        exec > /tmp/$$  # Redirect stdout into named pipe
        rm -f /tmp/$$

        for X in 1 2 3
        do
                echo $X
                read REPLY
                echo "Reply was '$REPLY'" >&3 # Write to old stdout
        done
)

The sticky part is opening the named pipe. That has to be done in different subshells because exec < namedpipe and exec > namedpipe must happen simultaneously: If two processes aren't trying to open it, the will force your program to wait. This for example will not work:

Code:
exec 5<namedpipe # Will wait forever since the line below it will not run
exec 6>namedpipe

$$ is just a convenient random-ish number.
Last edited by Corona688; 02-07-2018 at 12:04 PM..
 

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LEARN ABOUT DEBIAN

dh-exec-install

DH-EXEC-INSTALL(1)						      dh-exec							DH-EXEC-INSTALL(1)

NAME

       dh-exec-install - Install (and possibly rename) files.

SYNOPSIS

       #! /usr/bin/dh-exec
       debian/default.conf => /etc/my-package/start.conf
       usr/bin/*

DESCRIPTION

       Being a sub-command of dh-exec(1), this program must not be ran directly, but through dh-exec, which automatically runs all available
       sub-commands if run bare; or explicitly with dh-exec --with=install.

       It is meant to be used for dh_install(1) files, and those alone. If it finds that its input is not such a file, it will do nothing, but
       echo back the contents.

       The purpose of the program is to extend dh_install(1)'s functionality, by allowing to specify a destination filename.

       This can be accomplished by a special syntax: the " => " mark between a source and a destination means that the source file should be
       installed with the specified destination name.

       For obvious reasons, the source must not be a wildcard, and the destination in this case must be a file, and not a directory.

       All other non-comment lines are left alone.

RESTRICTIONS

       Due to the way executable scripts are called from debhelper(1), there is no way to know what options were used for the original
       dh_install(1). This means, that the --sourcedir option of dh_install(1) will not work correctly when dh-exec-install is in use.

IMPLEMENTATION

       Internally, the renaming happens by creating a temporary directory under debian/tmp/, and copying (or moving, if the source was under
       debian/tmp/ to begin with) the file there, with the new name.

       This is done this way to allow dh_install(1) to do the real copying, and allow its options to continue working, even when renaming is
       involved.

       The temporary directory is put under debian/tmp so that it will be cleaned by dh_prep(1) when the clean target gets to run. Thus, no extra
       code is needed anywhere to clean up the renamed files.

ENVIRONMENT

       DH_EXEC_SCRIPTDIR
	   Indicates which directory the command-specific scripts should be sought for. If not specified, scripts will be searched for in
	   /usr/share/dh-exec/.

FILES

       $DH_EXEC_SCRIPTDIR/dh-exec-install-*
	   The various scripts for the higher-level program.

SEE ALSO

       debhelper(1), dh-exec(1), dh_install(1)

AUTHOR

       dh-exec-install is copyright (C) 2011-2012 by Gergely Nagy <algernon@madhouse-project.org>.

								    2012-05-03							DH-EXEC-INSTALL(1)
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