DOKK / manpages / debian 10 / inn / shlock.1.en
SHLOCK(1) General Commands Manual SHLOCK(1)

shlock - create lock files for use in shell scripts

shlock -p pid -f name [ -b ] [ -u ] [ -c ]

Shlock tries to create a lock file named name and write the process ID pid into it. If the file already exists, shlock will read the process ID from the file and test to see if the process is currently running. If the process exists, then the file will not be created.

Shlock exits with a zero status if it was able to create the lock file, or non-zero if the file refers to currently-active process.

Process IDs are normally read and written in ASCII. If the ``-b'' flag is used, then they will be written as a binary int. For compatibility with other systems, the ``-u'' flag is accepted as a synonym for ``-b'' since binary locks are used by many UUCP packages.
If the ``-c'' flag is used, then shlock will not create a lock file, but will instead use the file to see if the lock is held by another program. If the lock is valid, the program will exit with a non-zero status; if the lock is not valid (i.e., invoking shlock without the flag would have succeeded), then the program will exit with a zero status.

The following example shows how shlock would be used within a shell script:

LOCK=/run/innd/LOCK.send
trap 'rm -f ${LOCK} ; exit 1' 1 2 3 15
if shlock -p $$ -f ${LOCK} ; then


    # Do appropriate work
else


    echo Locked by `cat ${LOCK}`
fi

Written by Rich $alz <rsalz@uunet.uu.net> after a description of HDB UUCP locking given by Peter Honeyman. This is revision 1.9, dated 1996/10/29.