Go to the previous, next chapter.
The file name databases used by
locate contain lists of files that were in
particular directory trees when the databases were last updated. The file name of the
default database is determined when
configured and installed. The frequency with which the databases are updated and the
directories for which they contain entries depend on how often
run, and with which arguments.
There can be multiple file name databases. Users can select which databases
searches using an environment variable or a command line option. The system administrator
can choose the file name of the default database, the frequency with which the databases
are updated, and the directories for which they contain entries. File name databases are
updated by running the
updatedb program, typically nightly.
In networked environments, it often makes sense to build a database at the root of each
filesystem, containing the entries for that filesystem.
updatedb is then run
for each filesystem on the fileserver where that filesystem is on a local disk, to prevent
thrashing the network. Here are the options to
updatedb to select which
directories each database contains entries for:
- --localpaths='path...' Non-network directories to put in the database.
Default is /.
- --netpaths='path...' Network (NFS, AFS, RFS, etc.) directories to put
in the database. Default is none.
to not put in the database, which would otherwise be. Default is /tmp /usr/tmp
--output=dbfile The database file to build. Default is
system-dependent, but typically /usr/local/var/locatedb.
--netuser=user The user to search network directories as, using
The file name databases contain lists of files that were in particular directory trees
when the databases were last updated. The file name database format changed starting with
locate version 4.0 to allow machines with different byte orderings to
share the databases. The new GNU
locate can read both the old and new
database formats. However, old versions of
produce incorrect results if given a new-format database.
updatedb runs a program called
frcode to front-compress
the list of file names, which reduces the database size by a factor of 4 to 5.
Front-compression (also known as incremental encoding) works as follows.
The database entries are a sorted list (case-insensitively, for users' convenience).
Since the list is sorted, each entry is likely to share a prefix (initial string) with the
previous entry. Each database entry begins with an offset-differential count byte, which
is the additional number of characters of prefix of the preceding entry to use beyond the
number that the preceding entry is using of its predecessor. (The counts can be negative.)
Following the count is a null-terminated ASCII remainder---the part of the name that
follows the shared prefix.
If the offset-differential count is larger than can be stored in a byte (+/-127), the
byte has the value 0x80 and the count follows in a 2-byte word, with the high byte first
(network byte order).
Every database begins with a dummy entry for a file called LOCATE02, which
checks for to ensure that the database file has the correct format; it ignores the entry
in doing the search.
Databases can not be concatenated together, even if the first (dummy) entry is trimmed
from all but the first database. This is because the offset-differential count in the
first entry of the second and following databases will be wrong.
Sample input to
Length of the longest prefix of the preceding entry to share:
frcode, with trailing nulls changed to newlines and count
bytes made printable:
(6 = 14 - 8, and -9 = 5 - 14)
The old database format is used by Unix
programs and earlier releases of the GNU ones.
updatedb produces this format
if given the --old-format option.
updatedb runs programs called
produce old-format databases. The old format differs from the new one in the following
ways. Instead of each entry starting with an offset-differential count byte and ending
with a null, byte values from 0 through 28 indicate offset-differential counts from -14
through 14. The byte value indicating that a long offset-differential count follows is
0x1e (30), not 0x80. The long counts are stored in host byte order, which is not
necessarily network byte order, and host integer word size, which is usually 4 bytes. They
also represent a count 14 less than their value. The database lines have no termination
byte; the start of the next line is indicated by its first byte having a value In
addition, instead of starting with a dummy entry, the old database format starts with a
256 byte table containing the 128 most common bigrams in the file list. A bigram is a pair
of adjacent bytes. Bytes in the database that have the high bit set are indexes (with the
high bit cleared) into the bigram table. The bigram and offset-differential count coding
makes these databases 20-25% smaller than the new format, but makes them not 8-bit clean.
Any byte in a file name that is in the ranges used for the special codes is replaced in
the database by a question mark, which not coincidentally is the shell wildcard to match a