SHLOCK(1)

NAME
shlock - create or verify a lock file for shell scripts

SYNOPSIS
shlock -f lockfile [-p PID] [-u] [-v]

DESCRIPTION
The shlock command can create or verify a lock file on behalf of a shell or other script program. When it attempts to create a lock file, if one already exists, shlock verifies that it is or is not valid. If valid, shlock will exit with a non-zero exit code. If invalid, shlock will remove the lock file, and create a new one.

shlock uses the rename(2) system call to make the final target lock file, which is an atomic operation (i.e. "dot locking", so named for this mechanism's original use for locking system mailboxes). It puts the process ID ("PID") from the command line into the requested lock file.

shlock verifies that an extant lock file is still valid by using kill(2) with a zero signal to check for the existence of the process that holds the lock.

The -f argument with lockfile is always required.

The -p option with PID is given when the program is to create a lock file; when absent, shlock will simply check for the validity of the lock file.

The -u option causes shlock to read and write the PID as a binary pid_t, instead of as ASCII, to be compatible with the locks created by UUCP.

The -v option causes shlock to be verbose about what it is doing.

RETURN VALUES
A zero exit code indicates a valid lock file.

EXAMPLES

  BOURNE SHELL

    #!/bin/sh
    lckfile=/tmp/foo.lock
    if shlock -f ${lckfile} -p $$
    then
    #	do what required the lock
    	rm ${lckfile}
    else
    	echo Lock ${lckfile} already held by `cat ${lckfile}`
    fi

  C SHELL

    #!/bin/csh -f
    set lckfile=/tmp/foo.lock
    shlock -f ${lckfile} -p $$
    if ($status == 0) then
    #	do what required the lock
    	rm ${lckfile}
    else
    	echo Lock ${lckfile} already held by `cat ${lckfile}`
    endif

The examples assume that the filesystem where the lock file is to be created is writeable by the user, and has space available.

HISTORY
shlock was written for the first Network News Transfer Protocol (NNTP) software distribution, released in March 1986. The algorithm was suggested by Peter Honeyman, from work he did on HoneyDanBer UUCP.

AUTHOR
Erik E. Fair <fair@clock.org>

BUGS
Does not work on NFS or other network filesystem on different systems because the disparate systems have disjoint PID spaces.

Cannot handle the case where a lock file was not deleted, the process that created it has exited, and the system has created a new process with the same PID as in the dead lock file. The lock file will appear to be valid even though the process is unrelated to the one that created the lock in the first place. Always remove your lock files after you're done.