shlock(1) [opendarwin man page]
SHLOCK(1) BSD General Commands Manual 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 sug- gested 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 cre- ated the lock in the first place. Always remove your lock files after you're done. BSD
June 29, 1997 BSD
Check Out this Related Man Page
lockfile-progs(1) Lockfile programs lockfile-progs(1) NAME
lockfile-progs - command-line programs to safely lock and unlock files and mailboxes (via liblockfile). SYNOPSIS
mail-lock [--use-pid] [--retry retry-count] mail-unlock mail-touchlock [--oneshot] lockfile-create [--use-pid] [--retry retry-count] [--lock-name] filename lockfile-remove [--lock-name] filename lockfile-touch [--oneshot] [--lock-name] filename lockfile-check [--use-pid] [--lock-name] filename DESCRIPTION
Lockfile-progs provides a set a programs that can be used to lock and unlock mailboxes and files safely (via liblockfile): mail-lock - lock the current user's mailbox mail-unlock - unlock the current user's mailbox mail-touchlock - touch the lock on the current user's mailbox lockfile-create - lock a given file lockfile-remove - remove the lock on a given file lockfile-touch - touch the lock on a given file lockfile-check - check the lock on a given file By default, the filename argument refers to the name of the file to be locked, and the name of the lockfile will be filename .lock. How- ever, if the --lock-name argument is specified, then filename will be taken as the name of the lockfile itself. Each of the mail locking commands attempts to lock /var/spool/mail/<user>, where <user> is the name associated with the effective user ID, as determined by via geteuid(2). Once a file is locked, the lock must be touched at least once every five minutes or the lock will be considered stale, and subsequent lock attempts will succeed. Also see the --use-pid option and the lockfile_create(3) manpage. The lockfile-check command tests whether or not a valid lock already exists. OPTIONS
-q, --quiet Suppress any output. Success or failure will only be indicated by the exit status. -v, --verbose Enable diagnostic output. -l, --lock-name Do not append .lock to the filename. This option applies to lockfile-create, lockfile-remove, lockfile-touch, or lockfile-check. -p, --use-pid Write the current process id (PID) to the lockfile whenever a lockfile is created, and use that pid when checking a lock's validity. See the lockfile_create(3) manpage for more information. This option applies to lockfile-create, lockfile-remove, lockfile-touch, and lockfile-check. -o, --oneshot Touch the lock and exit immediately. This option applies to lockfile-touch and mail-touchlock. When not provided, these commands will run forever, touching the lock once every minute until killed. -r retry-count, --retry retry-count Try to lock filename retry-count times before giving up. Each attempt will be delayed a bit longer than the last (in 5 second incre- ments) until reaching a maximum delay of one minute between retries. If retry-count is unspecified, the default is 9 which will give up after 180 seconds (3 minutes) if all 9 lock attempts fail. EXAMPLES
Locking a file during a lengthy process: lockfile-create /some/file lockfile-touch /some/file & # Save the PID of the lockfile-touch process BADGER="$!" do-something-important-with /some/file kill "${BADGER}" lockfile-remove /some/file EXIT STATUS
0 For lockfile-check this indicates that a valid lock exists, otherwise it just indicates successful program execution. Not 0 For lockfile-check a non-zero exit status indicates that the specified lock does not exist or is not valid. For other programs it indicates that some problem was encountered. SEE ALSO
maillock(3) touchlock(3) mailunlock(3) lockfile_create(3) lockfile_remove(3) lockfile_touch(3) lockfile_check(3) AUTHOR
Written by Rob Browning <rlb@defaultvalue.org> 0.1.12 2008-02-10 lockfile-progs(1)