121 lines
2.8 KiB
Groff
121 lines
2.8 KiB
Groff
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.Dd June 29, 1997
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.Dt SHLOCK 1
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.Os
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.Sh NAME
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.Nm shlock
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.Nd create or verify a lock file for shell scripts
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.Sh SYNOPSIS
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.Nm shlock
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.Fl f
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.Ar lockfile
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.Op Fl p Ar PID
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.Op Fl u
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.Op Fl v
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.Sh DESCRIPTION
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The
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.Nm shlock
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command can create or verify a lock file on behalf of a shell or
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other script program.
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When it attempts to create a lock file, if one already exists,
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.Nm shlock
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verifies that it is or is not valid.
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If valid,
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.Nm shlock
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will exit with a non-zero exit code.
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If invalid,
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.Nm shlock
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will remove the lock file, and
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create a new one.
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.Pp
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.Nm shlock
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uses the
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.Xr rename 2
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system call to make the final target lock file, which is an atomic
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operation (i.e. "dot locking", so named for this mechanism's original
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use for locking system mailboxes).
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It puts the process ID ("PID") from the command line into the
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requested lock file.
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.Pp
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.Nm shlock
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verifies that an extant lock file is still valid by
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using
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.Xr kill 2
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with a zero signal to check for the existence of the process that
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holds the lock.
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.Pp
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The
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.Fl f
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argument with
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.Ar lockfile
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is always required.
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.Pp
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The
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.Fl p
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option with
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.Ar PID
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is given when the program is to create a lock file; when absent,
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.Nm shlock
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will simply check for the validity of the lock file.
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.Pp
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The
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.Fl u
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option causes
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.Nm shlock
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to read and write the PID as a binary pid_t, instead of as ASCII,
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to be compatible with the locks created by UUCP.
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.Pp
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The
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.Fl v
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option causes
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.Nm shlock
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to be verbose about what it is doing.
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.Sh RETURN VALUES
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A zero exit code indicates a valid lock file.
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.Sh EXAMPLES
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.Ss BOURNE SHELL
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.Bd -literal
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#!/bin/sh
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lckfile=/tmp/foo.lock
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if shlock -f ${lckfile} -p $$
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then
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# do what required the lock
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rm ${lckfile}
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else
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echo Lock ${lckfile} already held by `cat ${lckfile}`
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fi
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.Ed
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.Ss C SHELL
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.Bd -literal
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#!/bin/csh -f
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set lckfile=/tmp/foo.lock
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shlock -f ${lckfile} -p $$
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if ($status == 0) then
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# do what required the lock
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rm ${lckfile}
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else
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echo Lock ${lckfile} already held by `cat ${lckfile}`
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endif
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.Ed
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.Pp
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The examples assume that the filesystem where the lock file is to
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be created is writeable by the user, and has space available.
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.Sh HISTORY
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.Nm shlock
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was written for the first Network News Transfer Protocol (NNTP)
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software distribution, released in March 1986.
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The algorithm was suggested by Peter Honeyman,
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from work he did on HoneyDanBer UUCP.
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.Sh AUTHOR
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Erik E. Fair <fair@clock.org>
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.Sh BUGS
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Does not work on NFS or other network filesystem on different
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systems because the disparate systems have disjoint PID spaces.
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.Pp
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Cannot handle the case where a lock file was not deleted, the
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process that created it has exited, and the system has created a
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new process with the same PID as in the dead lock file.
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The lock file will appear to be valid even though the process is
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unrelated to the one that created the lock in the first place.
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Always remove your lock files after you're done.
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