1.\" 2.\" Copyright (C) 1998 John D. Polstra. All rights reserved. 3.\" 4.\" Redistribution and use in source and binary forms, with or without 5.\" modification, are permitted provided that the following conditions 6.\" are met: 7.\" 1. Redistributions of source code must retain the above copyright 8.\" notice, this list of conditions and the following disclaimer. 9.\" 2. Redistributions in binary form must reproduce the above copyright 10.\" notice, this list of conditions and the following disclaimer in the 11.\" documentation and/or other materials provided with the distribution. 12.\" 13.\" THIS SOFTWARE IS PROVIDED BY JOHN D. POLSTRA AND CONTRIBUTORS ``AS IS'' AND 14.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 15.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 16.\" ARE DISCLAIMED. IN NO EVENT SHALL JOHN D. POLSTRA OR CONTRIBUTORS BE LIABLE 17.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 18.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 19.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 20.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 21.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 22.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 23.\" SUCH DAMAGE. 24.\" 25.Dd November 25, 2023 26.Dt LOCKF 1 27.Os 28.Sh NAME 29.Nm lockf 30.Nd execute a command while holding a file lock 31.Sh SYNOPSIS 32.Nm 33.Op Fl knsw 34.Op Fl t Ar seconds 35.Ar file 36.Ar command 37.Op Ar arguments 38.Nm 39.Op Fl s 40.Op Fl t Ar seconds 41.Ar fd 42.Sh DESCRIPTION 43The 44.Nm 45utility acquires an exclusive lock on a 46.Ar file , 47creating it if necessary, 48.Bf Em 49and removing the file on exit unless explicitly told not to. 50.Ef 51While holding the lock, it executes a 52.Ar command 53with optional 54.Ar arguments . 55After the 56.Ar command 57completes, 58.Nm 59releases the lock, and removes the 60.Ar file 61unless the 62.Fl k 63option is specified. 64.Bx Ns -style 65locking is used, as described in 66.Xr flock 2 ; 67the mere existence of the 68.Ar file 69is not considered to constitute a lock. 70.Pp 71.Nm 72may also be used to operate on a file descriptor instead of a file. 73If no 74.Ar command 75is supplied, then 76.Ar fd 77must be a file descriptor. 78The version with a 79.Ar command 80may also be used with a file descriptor by supplying it as a path 81.Pa /dev/fd/N , 82where N is the desired file descriptor. 83The 84.Fl k 85option is implied when a file descriptor is in use, and the 86.Fl n 87and 88.Fl w 89options are silently ignored. 90This can be used to lock inside a shell script. 91.Pp 92If the 93.Nm 94utility is being used to facilitate concurrency between a number 95of processes, it is recommended that the 96.Fl k 97option be used. 98This will guarantee lock ordering, as well as implement 99a performance enhanced algorithm which minimizes CPU load associated 100with concurrent unlink, drop and re-acquire activity. 101It should be noted 102that if the 103.Fl k 104option is not used, then no guarantees around lock ordering can be made. 105.Pp 106The following options are supported: 107.Bl -tag -width ".Fl t Ar seconds" 108.It Fl k 109Causes the lock file to be kept (not removed) after the command 110completes. 111.It Fl s 112Causes 113.Nm 114to operate silently. 115Failure to acquire the lock is indicated only in the exit status. 116.It Fl n 117Causes 118.Nm 119to fail if the specified lock 120.Ar file 121does not exist. 122If 123.Fl n 124is not specified, 125.Nm 126will create 127.Ar file 128if necessary. 129.It Fl t Ar seconds 130Specifies a timeout for waiting for the lock. 131By default, 132.Nm 133waits indefinitely to acquire the lock. 134If a timeout is specified with this option, 135.Nm 136will wait at most the given number of 137.Ar seconds 138before giving up. 139A timeout of 0 may be given, in which case 140.Nm 141will fail unless it can acquire the lock immediately. 142When a lock times out, 143.Ar command 144is 145.Em not 146executed. 147.It Fl w 148Causes 149.Nm 150to open 151.Ar file 152for writing rather than reading. 153This is necessary on filesystems (including NFSv4) where a file which 154has been opened read-only cannot be exclusively locked. 155.El 156.Pp 157In no event will 158.Nm 159break a lock that is held by another process. 160.Sh EXIT STATUS 161If 162.Nm 163successfully acquires the lock, it returns the exit status produced by 164.Ar command . 165Otherwise, it returns one of the exit codes defined in 166.Xr sysexits 3 , 167as follows: 168.Bl -tag -width ".Dv EX_CANTCREAT" 169.It Dv EX_TEMPFAIL 170The specified lock file was already locked by another process. 171.It Dv EX_CANTCREAT 172The 173.Nm 174utility 175was unable to create the lock file, e.g., because of insufficient access 176privileges. 177.It Dv EX_UNAVAILABLE 178The 179.Fl n 180option is specified and the specified lock file does not exist. 181.It Dv EX_USAGE 182There was an error on the 183.Nm 184command line. 185.It Dv EX_OSERR 186A system call (e.g., 187.Xr fork 2 ) 188failed unexpectedly. 189.It Dv EX_SOFTWARE 190The 191.Ar command 192did not exit normally, 193but may have been signaled or stopped. 194.El 195.Sh EXAMPLES 196The first job takes a lock and sleeps for 5 seconds in the background. 197The second job tries to get the lock and timeouts after 1 second (PID numbers 198will differ): 199.Bd -literal -offset indent 200$ lockf mylock sleep 5 & lockf -t 1 mylock echo "Success" 201[1] 94410 202lockf: mylock: already locked 203.Ed 204.Pp 205The first job takes a lock and sleeps for 1 second in the background. 206The second job waits up to 5 seconds to take the lock and echoes the message on 207success (PID numbers will differ): 208.Bd -literal -offset indent 209$ lockf mylock sleep 1 & lockf -t 5 mylock echo "Success" 210[1] 19995 211Success 212[1]+ Done lockf mylock sleep 1 213.Ed 214Lock a file and run a script, return immediately if the lock is not 215available. Do not delete the file afterward so lock order is 216guaranteed. 217.Pp 218.Dl $ lockf -t 0 -k /tmp/my.lock myscript 219.Pp 220Protect a section of a shell script with a lock, wait up to 5 seconds 221for it to become available. 222Note that the shell script has opened the lock file 223.Fa /tmp/my.lock , 224and 225.Nm 226is performing the lock call exclusively via the passed in file descriptor (9). 227In this case 228.Fl k 229is implied, and 230.Fl w 231has no effect because the file has already been opened by the shell. 232This example assumes that 233.Ql > 234is implemented in the shell by opening and truncating 235.Pa /tmp/my.lock , 236rather than by replacing the lock file. 237.Bd -literal -offset indent 238( 239 lockf -s -t 5 9 240 if [ $? -ne 0 ]; then 241 echo "Failed to obtain lock" 242 exit 1 243 fi 244 245 echo Start 246 # Do some stuff 247 echo End 248) 9>/tmp/my.lock 249.Ed 250.Sh SEE ALSO 251.Xr flock 2 , 252.Xr lockf 3 , 253.Xr sysexits 3 254.Sh HISTORY 255A 256.Nm 257utility first appeared in 258.Fx 2.2 . 259.Sh AUTHORS 260.An John Polstra Aq Mt jdp@polstra.com 261