NAME

       dotlockfile - Utility to manage lockfiles

SYNOPSIS

       /usr/bin/dotlockfile [-l [-r retries] |-u|-t|-c] [-p] [-m|lockfile]

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.  This option is the default.

              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 intervall is increased incrementally by 5 seconds up to a maximum  sleep  of  60 seconds
              between  tries.   The  default  number  of  retries  is  5.  To try only once, use "-r 0".  To try
              indefinitely, use "-r -1".

       -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.

       -c     For debugging only:  Check for the existence of a valid lockfile.

              Note:   Testing  for  a  preexisting lockfile and writing of the lockfile must be done by the same
              "dotlockfile -l" or "dotlockfile -m" command, else the lockfile creation cannot be reliable.

       -p     Write the process-id of the calling process 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.

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

RETURN VALUE

       Zero  on  success,  and  non-zero  on  failure.  For the -c option, sucess means that a valid lockfile is
       already present.  When locking (the default, or with the -l option) dotlockfile returns the  same  values
       as the library function lockfile_create(3).  Unlocking a non-existant lockfile is not an error.

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 priviliges 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, miquels@cistron.nl

                                                September 8, 2009                                 DOTLOCKFILE(1)