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mh-folders - Man Page

storage format used by nmh message system


nmh stores messages in the files and directories of the host filesystem according to the following rules:

one folder per directory

An nmh folder corresponds to a directory.  There are no limits on folder names beyond those of the host filesystem.

one message per file

The file name is a positive integer.  Other files containing metadata or arbitrary names can exist in a folder; while the preference is that non-message files begin with “.”, all files that are not positive integers must be ignored by an MH-compatible implementation.  However, implementations are free to indicate to the user the existence of non-message files that are not prefixed with a “.”.

The filename for a new message is one greater than the highest numbered message in the folder; its full path can be accessed by the pseudo-sequence new (e.g., mhpath new). New messages are only permitted to be added to a folder at the end of the message number range.

To add a new message to a folder, the recommended sequence is:

  • Create a temporary file in the desired folder.
  • Attempt to link the temporary file to the new message number.
  • If successful, remove the temporary file.  If the link fails, increment the message number and try again.

There is one context file.  Its default location is in the user's Path and its default name is context, but these can be overridden by the $MHCONTEXT environment variable. context has the following format:

Current-Folder: +folder
atr-sequence-path: m[-n] [...]

where folder is the directory name of the current folder.  Lines beginning with “atr” are used for private sequences. sequence is the name of the private sequence, path is the full path to the folder with the private sequence, and m[-n] is a message number or range of message numbers in the sequence.


There is one sequences file in each nmh folder.  Its default name is .mh_sequences, but this can be overridden by the “mh-sequences” profile entry. sequences has the following format:

sequence: m[-n] [...]

showing the (possibly empty) message numbers and/or ranges of message numbers in each sequence.  The cur sequence has at most just a single message number, not a range.

Sequence names have a maximum size of 998 characters.  Each line is also limited to a maximum of 998 characters, but RFC 822 continuation rules apply; sequences can be continued across multiple lines by prefixing continuation lines with a whitespace character.

If an implementation finds messages in a sequence that do not exist, the sequence file should be updated to remove the missing messages from the sequence.  If a sequence contains no messages, it should be removed from the sequence file.  The exception to this is the cur sequence, which can refer to a nonexistent message.


nmh programs read and write the context and sequences files, and lock these files when accessing them.  There should not be a need to access these files directly; instead, programs such as flist, folder, mark, pick, and rcvstore should be used to query and update their contents.  Any program outside of nmh that accesses these files must be sure to lock them using the same locking method as nmh. The default data locking method is selected when nmh is configured and can be accessed as a string using mhparam datalocking. By default, fcntl locking is used, but this may be overridden by the datalocking profile entry.

A second, possibly different, locking method is used by inc(1) when accessing the user's mail spool file or by nmh programs that open any mbox file.  This locking method can be overridden when nmh is configured, or in the nmh mts configuration file, and can be accessed as a string using mhparam spoollocking. By default, kernel-level locking is used if appropriate for the platform, and it is for popular platforms.  That default should also be the same as used by the mail program, if provided on the platform.


nmh folders can be given arbitrary names, with one exception: folders should not be given all-numeric names.  This limitation results from nmh messages themselves being stored in numerically named files -- allowing folders to be named similarly would make nmh slower, and introduce usage ambiguities.



The user's context.


Overrides the above context.


Public sequences for <folder>.

See Also

flist(1), folder(1), mail(1), mark(1), mhparam(1), mhpath(1), mh-profile(5), mh-sequence(5), mh-tailor(5), pick(1), rcvstore(1)

Referenced By

mh-chart(7), nmh(7).

2016-02-25 nmh-1.8