SHLOCK(1) BSD General Commands Manual SHLOCK(1)NAME
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 mech‐
anism'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 ⟨fair@clock.org⟩
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 created the
lock in the first place. Always remove your lock files after you're
done.
BSD June 29, 1997 BSD
