man postpone (1): 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 multiple 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.

COPYRIGHT

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

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 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.