mbox(5) User Manuals mbox(5)NAMEmbox - Format for mail message storage.
DESCRIPTION
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 RFC 822. 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. The sender
address is expected to be an addrspec as defined in appendix D of RFC
822.
The date is expected to be formatted according to the following syntax
(represented in the augmented Backus-Naur formalism used by RFC 822):
mbox-date = weekday month day time [ timezone ] year
weekday = "Mon" / "Tue" / "Wed" / "Thu" / "Fri"
/ "Sat" / "Sun"
month = "Jan" / "Feb" / "Mar" / "Apr" / "May"
/ "Jun" / "Jul" / "Aug" / "Sep"
/ "Oct" / "Nov" / "Dec"
day = 1*2DIGIT
time = 1*2DIGIT ":" 1*2DIGIT [ ":" 1*2DIGIT ]
timezone = ( "+" / "-" ) 4DIGIT
year = ( 4DIGIT / 2DIGIT )
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 prepared to accept
non-numeric timezone information such as "CET DST" for Central European
Time, dailight saving time.
Example:
From roessler@does-not-exist.org Fri Jun 23 02:56:55 2000
In order to avoid mis-interpretation of lines in message bodies which
begin with the four characters "From", followed by a space character,
the character ">" is commonly prepended in front of such lines.
LOCKING
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:
· 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' caches.
· flock(2) locking is mostly used on BSD-based systems.
· 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
resides. 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 addi‐
tionally verified using stat(2) calls. If the link has suc‐
ceeded, 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) sytem 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
obtained. When one of the individual locking methods fails, an appli‐
cation 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.
FILES
/var/spool/mail/user
user's incoming mail folder.
~user/mbox
user's archived mail messages, in his home directory.
~user/Mail/
A directory in user's home directory which is commonly used to
hold mbox format folders.
SEE ALSOelm(1), fcntl(2), flock(2), link(2), local(8), mail(1), maildir(5),
mail.local(8), mutt(1), mutt_dotlock(1), pine(1), procmail(1), send‐
mail(8)
D. Crocker, Standard for the format of ARPA Internet text messages, RFC
822
M. R. Horton, UUCP mail interchange format standard, RFC 976
AUTHOR
The present document was written by Thomas Roessler <roessler@does-not-
exist.org>.
HISTORY
The mbox format occured in Version 6 AT&T Unix.
A variant of this format was documented in RFC 976.
Unix August 2000 mbox(5)
