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mbox(5)				 User Manuals			       mbox(5)

       mbox - Format for mail message storage.

       This  document describes	the format traditionally used by Unix hosts to
       store mail messages locally.  mbox files	typically reside in  the  sys-
       tem's  mail  spool, under various names in users' Mail directories, and
       under the name mbox in users' home directories.

       An mbox is a text file containing an arbitrary number  of  e-mail  mes-
       sages.  Each message consists of	a postmark, followed by	an e-mail mes-
       sage formatted according	to RFC822, RFC2822. The	file format  is	 line-
       oriented. Lines are separated by	line feed characters (ASCII 10).

       A  postmark  line consists of the four characters "From", followed by a
       space character,	followed by the	 message's  envelope  sender  address,
       followed	 by whitespace,	and followed by	a time stamp. This line	is of-
       ten called From_	line.

       The sender address is expected to be addr-spec as  defined  in  RFC2822
       3.4.1.  The  date  is expected to be date-time as output	by asctime(3).
       For compatibility reasons with legacy software, two-digit years greater
       than  or	 equal	to  70 should be interpreted as	the years 1970+, while
       two-digit years less  than  70  should  be  interpreted	as  the	 years
       2000-2069.  Software  reading  files in this format should also be pre-
       pared to	accept non-numeric timezone information	such as	"CET DST"  for
       Central European	Time, daylight saving time.


	>From Fri Jun 23 02:56:55 2000

       In  order  to  avoid misinterpretation of lines in message bodies which
       begin with the four characters "From", followed by a  space  character,
       the  mail  delivery  agent  must	quote any occurrence of	"From "	at the
       start of	a body line.

       There are two different quoting schemes,	the first (MBOXO) only	quotes
       plain  "From  "	lines in the body by prepending	a '>' to the line; the
       second (MBOXRD) also quotes already quoted "From	" lines	by  prepending
       a '>' (i.e. ">From ", ">>From ",	...). The later	has the	advantage that
       lines like

	>From the command line you can use the '-p' option

       aren't dequoted wrongly as a MBOXRD-MDA would turn the line into

	>>From the command line	you can	use the	'-p' option

       before storing it. Besides MBOXO	and MBOXRD there is also MBOXCL	 which
       is MBOXO	with a "Content-Length:"-field with the	number of bytes	in the
       message body; some MUAs (like mutt(1)) do automatically transform MBOXO
       mailboxes into MBOXCL ones when ever they write them back as MBOXCL can
       be read by any MBOXO-MUA	without	any problems.

       If the modification-time	(usually determined via	stat(2)) of a nonempty
       mbox  file  is greater than the access-time the file has	new mail. Many
       MUAs place a Status: header in each message to indicate which  messages
       have already been read.

       Since mbox files	are frequently accessed	by multiple programs in	paral-
       lel, mbox files should generally	not be accessed	without	locking.

       Three different locking mechanisms (and combinations  thereof)  are  in
       general use:

       o      fcntl(2)	locking	is mostly used on recent, POSIX-compliant sys-
	      tems. Use	of this	locking	method is, in particular, advisable if
	      mbox  files  are accessed	through	the Network File System	(NFS),
	      since it seems the only way to reliably invalidate NFS  clients'

       o      flock(2) locking is mostly used on BSD-based systems.

       o      Dotlocking  is used on all kinds of systems. In order to lock an
	      mbox file	named folder, an application first creates a temporary
	      file with	a unique name in the directory in which	the folder re-
	      sides. The application then tries	to use the link(2) system call
	      to  create  a hard link named folder.lock	to the temporary file.
	      The success of the link(2) system	call  should  be  additionally
	      verified	using  stat(2)	calls.	If the link has	succeeded, the
	      mail folder is considered	dotlocked. The temporary file can then
	      safely be	unlinked.

	      In  order	 to  release the lock, an application just unlinks the
	      folder.lock file.

       If multiple methods are combined, implementors should make sure to  use
       the  non-blocking variants of the fcntl(2) and flock(2) system calls in
       order to	avoid deadlocks.

       If multiple methods are combined, an mbox file must not	be  considered
       to  have	 been successfully locked before all individual	locks were ob-
       tained. When one	of the individual locking methods fails,  an  applica-
       tion should release all locks it	acquired successfully, and restart the
       entire locking procedure	from the beginning, after a suitable delay.

       The locking mechanism used on a particular system is a matter of	 local
       policy,	and  should be consistently used by all	applications installed
       on the system which access mbox files. Failure to do so may  result  in
       loss of e-mail data, and	in corrupted mbox files.

	      $LOGNAME's incoming mail folder.

	      user's archived mail messages, in	his $HOME directory.

	      A	 directory in user's $HOME directory which is commonly used to
	      hold mbox	format folders.

       mutt(1),	fcntl(2), flock(2), link(2), stat(2), asctime(3),  maildir(5),
       mmdf(5),	RFC822,	RFC976,	RFC2822

       Thomas	  Roessler    <>,	Urs    Janssen

       The mbox	format occurred	in Version 6 AT&T Unix.
       A variant of this format	was documented in RFC976.

Unix			      February 19th, 2002		       mbox(5)


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