SHLOCK(1) NetBSD Reference Manual SHLOCK(1) NNAAMMEE sshhlloocckk - create or verify a lock file for shell scripts SSYYNNOOPPSSIISS sshhlloocckk --ff _l_o_c_k_f_i_l_e [--pp _P_I_D] [--uu] [--vv] DDEESSCCRRIIPPTTIIOONN The sshhlloocckk 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, sshhlloocckk verifies that it is or is not valid. If valid, sshhlloocckk will exit with a non-zero exit code. If invalid, sshhlloocckk will re- move the lock file, and create a new one. sshhlloocckk 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 mech- anism's original use for locking system mailboxes). It puts the process ID ("PID") from the command line into the requested lock file. sshhlloocckk 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 --ff argument with _l_o_c_k_f_i_l_e is always required. The --pp option with _P_I_D is given when the program is to create a lock file; when absent, sshhlloocckk will simply check for the validity of the lock file. The --uu option causes sshhlloocckk 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 --vv option causes sshhlloocckk to be verbose about what it is doing. RREETTUURRNN VVAALLUUEESS A zero exit code indicates a valid lock file. EEXXAAMMPPLLEESS BBOOUURRNNEE SSHHEELLLL #!/bin/sh lckfile=/tmp/foo.lock if shlock -f ${lckfile} -p $$ then # do what required the lock rm ${lckfile} else echo Lock ${lckfile} already held by `cat ${lckfile}` fi CC SSHHEELLLL #!/bin/csh -f set lckfile=/tmp/foo.lock shlock -f ${lckfile} -p $$ 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 cre- ated is writeable by the user, and has space available. HHIISSTTOORRYY sshhlloocckk 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. AAUUTTHHOORR Erik E. Fair BBUUGGSS Does not work on NFS or other network filesystem on different systems be- cause 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. 4.4BSD June 29, 1997 2