Provided by: maildir-utils_1.12.9-1_amd64 
NAME
mu-index - index e-mail messages stored in Maildirs
SYNOPSIS
mu [COMMON-OPTIONS] index
DESCRIPTION
mu index is the mu command for scanning the contents of Maildir directories and storing the results in a
Xapian database. The data can then be queried using mu-find(1).
Before the first time you run mu index, you must run mu init to initialize the database.
index understands Maildirs as defined by Daniel Bernstein for qmail(7). In addition, it understands
recursive Maildirs (Maildirs within Maildirs), Maildir++. It also supports VFAT-based Maildirs which use
! or ; as the separators instead of :.
E-mail messages which are not stored in something resembling a maildir leaf-directory (cur and new) are
ignored, as are the cache directories for notmuch and gnus, and any dot-directory.
Symlinks are followed, and the directories can be spread over multiple filesystems; however note that
moving files around is much faster when multiple filesystems are not involved. Be careful to avoid self-
referential symlinks!
If there is a file called .noindex in a directory, the contents of that directory and all of its
subdirectories will be ignored. This can be useful to exclude certain directories from the indexing
process, for example directories with spam-messages.
If there is a file called .noupdate in a directory, the contents of that directory and all of its
subdirectories will be ignored. This can be useful to speed up things you have some maildirs that never
change.
.noupdate does not affect already-indexed messages: you can still search for them. .noupdate is ignored
when you start indexing with an empty database (such as directly after mu init).
There also the option --lazy-check which can greatly speed up indexing; see below for details.
The first run of mu index may take a few minutes if you have a lot of mail (tens of thousands of
messages). Fortunately, such a full scan needs to be done only once; after that it suffices to index the
changes, which goes much faster. See the `PERFORMANCE (i,ii,iii)' below for more information.
The optional `phase two' of the indexing-process is the removal of messages from the database for which
there is no longer a corresponding file in the Maildir. If you do not want this, you can use -n,
--nocleanup.
When mu index catches one of the signals SIGINT, SIGHUP or SIGTERM (e.g., when you press Ctrl-C during
the indexing process), it attempts to shutdown gracefully; it tries to save and commit data, and close
the database etc. If it receives another signal (e.g., when pressing Ctrl-C once more), mu index will
terminate immediately.
INDEX OPTIONS
--lazy-check
In lazy-check mode, mu does not consider messages for which the time-stamp (ctime) of the directory in
which they reside, has not changed since the previous time this directory was checked.
This is much faster than the non-lazy check, but won't update messages that have changed (rather than
having been added or removed), since merely editing a message does not update the directory time-stamp.
Of course, you can run mu-index occasionally without --lazy-check, to pick up such messages.
Furthermore, in lazy-check mode, files which have a ctime smaller than the time the previous indexing
operation was completed, are ignored. This helps for the use-case where new messages can appear in big
maildirs.
--nocleanup
Disable the database cleanup that mu does by default after indexing.
--reindex
Perform a complete reindexing of all the messages in the maildir.
--muhome
Use a non-default directory to store and read the database, write the logs, etc. By default, mu uses the
XDG Base Directory Specification (e.g. on GNU/Linux this defaults to ~/.cache/mu and ~/.config/mu).
Earlier versions of mu defaulted to ~/.mu, which now requires --muhome=~/.mu.
The environment variable MUHOME can be used as an alternative to --muhome. The latter has precedence.
COMMON OPTIONS
-d, --debug
Makes mu generate extra debug information, useful for debugging the program itself. Debug information
goes to the standard logging location; see mu(1).
-q, --quiet
Causes mu not to output informational messages and progress information to standard output, but only to
the log file. Error messages will still be sent to standard error. Note that mu index is much faster with
--quiet, so it is recommended you use this option when using mu from scripts etc.
--log-stderr
Causes mu to not output log messages to standard error, in addition to sending them to the standard
logging location.
--nocolor
Do not use ANSI colors. The environment variable NO_COLOR can be used as an alternative to --nocolor.
-V, --version
Prints mu version and copyright information.
-h, --help
Lists the various command line options.
ENCRYPTION
mu index does not decrypt messages, and only the metadata (such as headers) of encrypted messages makes
it to the database. mu view and mu4e can decrypt messages, but those work with the message directly and
the information is not added to the database.
PERFORMANCE
indexing in ancient times (2009?)
As a non-scientific benchmark, a simple test on the author's machine (a Thinkpad X61s laptop using Linux
2.6.35 and an ext3 file system) with no existing database, and a maildir with 27273 messages:
$ sudo sh -c 'sync && echo 3 > /proc/sys/vm/drop_caches'
$ time mu index --quiet
66,65s user 6,05s system 27% cpu 4:24,20 total
(about 103 messages per second)
A second run, which is the more typical use case when there is a database already, goes much faster:
$ sudo sh -c 'sync && echo 3 > /proc/sys/vm/drop_caches'
$ time mu index --quiet
0,48s user 0,76s system 10% cpu 11,796 total
(more than 56818 messages per second)
Note that each test flushes the caches first; a more common use case might be to run mu index when new
mail has arrived; the cache may stay quite `warm' in that case:
$ time mu index --quiet
0,33s user 0,40s system 80% cpu 0,905 total
which is more than 30000 messages per second.
indexing in 2012
As per June 2012, we did the same non-scientific benchmark, this time with an Intel i5-2500 CPU @
3.30GHz, an ext4 file system and a maildir with 22589 messages. We start without an existing database.
$ sudo sh -c 'sync && echo 3 > /proc/sys/vm/drop_caches'
$ time mu index --quiet
27,79s user 2,17s system 48% cpu 1:01,47 total
(about 813 messages per second)
A second run, which is the more typical use case when there is a database already, goes much faster:
$ sudo sh -c 'sync && echo 3 > /proc/sys/vm/drop_caches'
$ time mu index --quiet
0,13s user 0,30s system 19% cpu 2,162 total
(more than 173000 messages per second)
indexing in 2016
As per July 2016, we did the same non-scientific benchmark, again with the Intel i5-2500 CPU @ 3.30GHz,
an ext4 file system. This time, the maildir contains 72525 messages.
$ sudo sh -c 'sync && echo 3 > /proc/sys/vm/drop_caches'
$ time mu index --quiet
40,34s user 2,56s system 64% cpu 1:06,17 total
(about 1099 messages per second).
indexing in 2022
A few years later and it is June 2022. There's a lot more happening during indexing, but indexing became
multi-threaded and machines are faster; e.g. this is with an AMD Ryzen Threadripper 1950X (16 cores) @
3.399GHz.
The instructions are a little different since we have a proper repeatable benchmark now. After building,
$ sudo sh -c 'sync && echo 3 > /proc/sys/vm/drop_caches'
% THREAD_NUM=4 build/lib/tests/bench-indexer -m perf
# random seed: R02Sf5c50e4851ec51adaf301e0e054bd52b
1..1
# Start of bench tests
# Start of indexer tests
indexed 5000 messages in 20 maildirs in 3763ms; 752 μs/message; 1328 messages/s (4 thread(s))
ok 1 /bench/indexer/4-cores
# End of indexer tests
# End of bench tests
Things are again a little faster, even though the index does a lot more now (text-normalization, and pre-
generating message-sexps). A faster machine helps, too!
recent releases
Indexing the the same 93000-message mail corpus with the last few releases:
┌───────────────────────────────────────────────────────────────────────┐
│ release time (sec) notes │
├───────────────────────────────────────────────────────────────────────┤
│ 1.4 160s │
│ 1.6 178s │
│ 1.8 97s │
│ 1.10 120s adds html indexing, sexp-caching │
│ 1.11 (master) 96s adds language-guessing, batch-size=50000 │
└───────────────────────────────────────────────────────────────────────┘
Quite some variation!
Over time new features / refactoring can change the timings quite a bit. At least for now, the latest
code is both the fastest and the most featureful!
EXIT CODE
This command returns 0 upon successful completion, or a non-zero exit code otherwise.
0. success
2. no matches found. Try a different query
11. database schema mismatch. You need to re-initialize mu, see mu-init(1)
19. failed to acquire lock. Some other program has exclusive access to the mu database
99. caught an exception
REPORTING BUGS
Please report bugs at https://github.com/djcb/mu/issues.
AUTHOR
Dirk-Jan C. Binnema <djcb@djcbsoftware.nl>
COPYRIGHT
This manpage is part of mu 1.12.9.
Copyright © 2008-2025 Dirk-Jan C. Binnema. License GPLv3+: GNU GPL version 3 or later
https://gnu.org/licenses/gpl.html. This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
SEE ALSO
maildir(5), mu(1), mu-init(1), mu-find(1), mu-cfind(1)
MU INDEX(1)