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shlock(1) [osx man page]

SHLOCK(1)						    BSD General Commands Manual 						 SHLOCK(1)

shlock -- create or verify a lock file for shell scripts SYNOPSIS
shlock [-du] [-p PID] -f lockfile 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 link(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 -d option causes shlock to be verbose about what it is doing. 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. EXIT STATUS
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 file system where the lock file is to be created is writable 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. AUTHORS
Erik E. Fair <> BUGS
Does not work on NFS or other network file system 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

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SHLOCK(1)						    InterNetNews Documentation							 SHLOCK(1)

shlock - Create lock files for use in shell scripts SYNOPSIS
shlock [-b|-c|-u] -f name -p pid DESCRIPTION
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 whether the process is currently running. If the process exists, then the file will not be created. shlock exits with a zero status if it could create the lock file, or non-zero if the file refers to a currently active process. OPTIONS
-b Process IDs are normally read and written in ASCII. If the -b flag is used, then they will be written as a binary int. -c 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. -f name name is the name of the lock file shlock attempts to create. If the file already exists, it will read the process ID from the file and exit with a non-zero status if this process is currently active. -p pid pid is the process ID to write into the file name. -u For compatibility with other systems, the -u flag is accepted as a synonym for -b since binary locks are used by many UUCP packages. EXAMPLES
The following example shows how shlock would be used within a shell script: LOCK=<pathrun in inn.conf>/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 HISTORY
Written by Rich $alz <> for InterNetNews after a description of HDB UUCP locking given by Peter Honeyman, and improved by Berend Reitsma to solve a race condition. Converted to POD by Julien Elie. $Id: shlock.pod 8357 2009-02-27 17:56:00Z iulius $ INN 2.5.2 2009-05-21 SHLOCK(1)
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