NAME

       dotlockfile - Utility to manage lockfiles

SYNOPSIS

       dotlockfile -l [-r retries] [-i interval] [-p] [-q] <-m | lockfile>
       dotlockfile -l [-r retries] [-i interval] [-p] [-q] <-m | lockfile> [-P] cmd args ...
       dotlockfile -u | -t

DESCRIPTION

       dotlockfile  is  a command line utility to reliably create, test and remove lockfiles.  It
       creates lockfiles reliably on local and NFS filesystems,  because  the  crucial  steps  of
       testing  for  a  preexisting lockfile and creating it are performed atomically by a single
       call to link(2).  Manpage lockfile_create(3) describes the used algorithm.

       dotlockfile is installed with attribute SETGID mail and thus can also be used to lock  and
       unlock mailboxes even if the mailspool directory is only writable by group mail.

       The  name dotlockfile comes from the way mailboxes are locked for updates on a lot of UNIX
       systems.  A lockfile is created with the same filename as the mailbox but with the  string
       ".lock" appended.

       The names dotlock and lockfile were already taken – hence the name dotlockfile :).

OPTIONS

       -l     Create  a  lockfile  if no preexisting valid lockfile is found, else wait and retry
              according to option -r.  Retry interval can be explicitly set with option -i.  This
              option (-l) is the default, so it can be left off.

              A lockfile is treated as valid,
              •  if it holds the process-id of a running process,
              •  or  if  it does not hold any process-id and has been touched less than 5 minutes
              ago (timestamp is younger than 5 minutes).

       -r retries
              The number of times dotlockfile retries to acquire the lock if it failed the  first
              time  before  giving  up.   The  initial sleep after failing to acquire the lock is
              5 seconds.  After each retry the  sleep  interval  is  increased  incrementally  by
              5 seconds  up  to  a maximum sleep of 60 seconds between tries unless overridden by
              -i.  The default number of retries is 5.  To try only once, use  "-r  0".   To  try
              indefinitely, use "-r -1".

       -i interval
              Sets a consistent retry interval.

       -u     Remove a lockfile.

       -t     Touch  an  existing  lockfile  (update the timestamp).  Useful for lockfiles on NFS
              filesystems.  For lockfiles on local filesystems the -p option is preferable.

       -p     Write the process-id of the calling process (or dotlockfile itself if a command  is
              executed) into the lockfile.  Also when testing for an existing lockfile, check the
              contents for the process-id of a running process to verify if the lockfile is still
              valid.  Obviously useful only for lockfiles on local filesystems.

       -m     Lock  or  unlock the current users mailbox.  The path to the mailbox is the default
              system mailspool directory (usually /var/mail) with the  username  as  gotten  from
              getpwuid()  appended.   If  the  environment  variable  $MAIL  is set, that is used
              instead.  Then the string ".lock" is  appended  to  get  the  name  of  the  actual
              lockfile.

       -q     Don't  print  warnings  or  errors to the standard error output. Used internally by
              liblockfile when it spawns dotlockfile as a helper program.

       -P     On successful "lock and spawn command", don't  exit  with  status  zero,  but  pass
              through the exit value of the spawned command.

       lockfile
              The  lockfile  to be created or removed.  Must not be specified if the -m option is
              given.

       command argument ...
              Create lockfile, run the command , wait for it to exit, and remove lockfile.

RETURN VALUE

       Zero on success, and non-zero on failure.  When locking  (the  default,  or  with  the  -l
       option)  dotlockfile  returns  the same values as the library function lockfile_create(3).
       Unlocking a non-existent lockfile is not an error.

       Unless the -P option was supplied, when a command is executed, the return value  does  not
       correspond  with  that  of  the  command  that  was  run.   If  locking  and unlocking was
       successful, the exit status is zero.

NOTES

       The lockfile is created exactly as named on the command line.  The  extension  ".lock"  is
       not automatically appended.

       This  utility  is  a  lot  like  the  lockfile(1)  utility included with procmail, and the
       mutt_dotlock(1) utility included with mutt.  However the  command-line  arguments  differ,
       and  so  does  the  return  status.  It is believed, that dotlockfile is the most flexible
       implementation, since it automatically detects when it needs to use privileges to  lock  a
       mailbox, and does it safely.

       The above mentioned lockfile_create(3) manpage is present in the liblockfile-dev package.

BUGS

       None known.

SEE ALSO

       lockfile_create(3), maillock(3)

AUTHOR

       Miquel van Smoorenburg

                                         January 10, 2017                          DOTLOCKFILE(1)