.Dd June 29, 1997
.Dt SHLOCK 1
.Os
.Sh NAME
.Nm shlock
.Nd create or verify a lock file for shell scripts
.Sh SYNOPSIS
.Nm shlock
.Fl f
.Ar lockfile
.Op Fl p Ar PID
.Op Fl u
.Op Fl v
.Sh DESCRIPTION
The
.Nm 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,
.Nm shlock
verifies that it is or is not valid.
If valid,
.Nm shlock
will exit with a non-zero exit code.
If invalid,
.Nm shlock
will remove the lock file, and 
create a new one.
.Pp
.Nm shlock
uses the
.Xr 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.
.Pp
.Nm shlock
verifies that an extant lock file is still valid by
using
.Xr kill 2
with a zero signal to check for the existence of the process that
holds the lock.
.Pp
The
.Fl f
argument with
.Ar lockfile
is always required.
.Pp
The
.Fl p
option with
.Ar PID
is given when the program is to create a lock file; when absent,
.Nm shlock
will simply check for the validity of the lock file.
.Pp
The
.Fl u
option causes
.Nm 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.
.Pp
The
.Fl v
option causes
.Nm shlock
to be verbose about what it is doing.
.Sh RETURN VALUES
A zero exit code indicates a valid lock file.
.Sh EXAMPLES
.Ss BOURNE SHELL
.Bd -literal
#!/bin/sh
lckfile=/tmp/foo.lock
if shlock -f ${lckfile} -p $$
then
#	do what required the lock
	rm ${lckfile}
else
	echo Lock ${lckfile} already held by `cat ${lckfile}`
fi
.Ed
.Ss C SHELL
.Bd -literal
#!/bin/csh -f
set lckfile=/tmp/foo.lock
shlock -f ${lckfile} -p $$
if ($status == 0) then
#	do what required the lock
	rm ${lckfile}
else
	echo Lock ${lckfile} already held by `cat ${lckfile}`
endif
.Ed

.Pp
The examples assume that the filesystem where the lock file is to
be created is writeable by the user, and has space available.
.Sh HISTORY
.Nm 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.
.Sh AUTHOR
Erik E. Fair <fair@clock.org>
.Sh BUGS
Does not work on NFS or other network filesystem on different
systems because the disparate systems have disjoint PID spaces.
.Pp
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.