POSTPONE(1)						      General Commands Manual						       POSTPONE(1)

NAME

       postpone - schedules a command to be executed later when a lockfile disappears

SYNOPSIS

       postpone [-dfv] [-lwLoO FILE] command args ...

DESCRIPTION

       Postpone  schedules a command to be executed later when a lockfile disappears. Intended usage is when a command is executed possibly multi-
       ple times, but running once is sufficient. This happens in maintainer scripts of Debian packages, for example, the menu system will need to
       run  update-menus  every time a menu file is installed or removed, but if several packages are processed at once, a single update-menus run
       at the end is sufficient. In fact, postpone is a generalization of similar code found in update-menus that  is  already	able  to  postpone
       itself. Another prominent use case are TeX packages that need to rebuild several indices.

       On  startup,  postpone  will look if any of the lock files given with --wait exists. If not, it will run command immediately in foreground.
       Otherwise, it will fork to background, waiting for the lock file(s) to disappear. At the same time, the --lock file is created to  indicate
       to other postpone instances that command is already scheduled for execution; the other instances will exit without further action.

       Additionally,  --extra-lock  will  be  acquired	just before the execution of command, this allows the serialization of several (different)
       background jobs.

       All lockfiles are optional. See below for examples.

OPTIONS

       -w FILE or --wait FILE
	      Wait for FILE before running command. Can be given several times.  This activates the main feature of postpone  --  delay  execution
	      until some other task has exited (specifically, released its fcntl() lock).

       -l FILE or --lock FILE
	      Creates  lockfile  FILE.	If postpone is called again with the same FILE, it will exit, assuming the task is already postponed. Make
	      sure that the (effects of the) commands are the same for the same lockfile.

       -L FILE or --extra-lock FILE
	      Before running command, acquire lock on FILE. Use this to serialize several (most likely different) postponed commands.

       -d or --debian
	      Equivalent to --wait /var/lib/dpkg/lock --extra-lock /var/lib/dpkg/postpone.lock. Meant for use  in  maintainer  scripts	of  Debian
	      packages, will wait for dpkg and apt-get to exit and serialize the postponed commands.

       -o FILE or --output FILE
	      Redirect stdout and stderr output to FILE when running in background.  (Discarded otherwise.) If FILE ends with "XXXXXX", mkstemp is
	      used.

       -O FILE or --all-output FILE
	      Like --output, but always redirect stdout and stderr output to FILE.

       -f or --foreground
	      Do not detach while waiting for locks.

       -v or --verbose
	      Verbose output. Repeat for debugging output.

       --help Print options summary and exit.

       --version
	      Print version information and exit.

       --version-string
	      Print version information in machine readable form and exit.

EXAMPLES

       Example usage in a texlive postinst maintainer script:

	   if [ -x /usr/bin/postpone ] ; then
	       postpone --verbose --lock /var/run/updmap-sys.lock --debian 
		   --all-output /tmp/updmap.XXXXXXXX 
		   updmap-sys
	   else
	       updmap-sys
	   fi

FILES

       /var/lib/dpkg/lock
	      Dpkg's default lockfile.

       /var/lib/dpkg/postpone.lock
	      Default extra lockfile when running with --debian.

BUGS

       Postpone currently only uses fcntl() to acquire locks on --wait, usage with applications using flock() will fail gracefully.  (The  command
       will run immediately.) --lock and --extra-lock use flock().

COMPATIBILITY

       For  compatibility,  using  unknown  options does not abort the program.  Applications can use --version-string if they need to compare the
       postpone version number.

SEE ALSO

       update-menus(1), fcntl(2), flock(2), mkstemp(3), dpkg(1).

COPYRIGHT

       Copyright (C) 2007  Christoph Berg

       Based on code from:
       Debian menu system -- update-menus
       update-menus/update-menus.cc

       Copyright (C) 1996-2003	Joost Witteveen
       Copyright (C) 2002-2004	Bill Allombert and Morten Brix Pedersen

       This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as  published	by
       the Free Software Foundation; either version 2 of the License, or (at your option) any later version.

       This  program  is  distributed  in  the	hope  that  it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MER-
       CHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for more details.

								   June 9th 2007						       POSTPONE(1)