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.
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, will simply check for the validity of the lock file.
The -u option causes 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 to be verbose about what it is doing.
#!/bin/sh lckfile=/tmp/foo.lock if shlock -f ${lckfile} -p $$ then # do what required the lock rm ${lckfile} else echo Lock ${lckfile} already held by `cat ${lckfile}` fi
#!/bin/csh -f set lckfile=/tmp/foo.lock shlock -f ${lckfile} -p $$ 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.
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.