Ubuntu Manpage: locatedb - front-compressed file name database

Provided by: locate_4.8.0-1ubuntu3_amd64

NAME

       locatedb - front-compressed file name database

DESCRIPTION

       This  manual  page  documents  the  format  of  file name databases for the GNU version of
       locate.  The file name databases contain lists of files that were in particular  directory
       trees when the databases were last updated.

       There  can  be multiple databases.  Users can select which databases locate searches using
       an environment variable or command line option; see locate(1).  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.   Normally,  file  name
       databases are updated by running the updatedb program periodically, typically nightly; see
       updatedb(1).

GNU LOCATE02 database format

This is the default format of databases produced by updatedb. The updatedb program runs
frcode to compress the list of file names using front-compression, 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 signed 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 signed byte (±127), the
byte has the value 0x80 (binary 10000000) and the actual count follows in a 2-byte word,
with the high byte first (network byte order). This count can also be negative (the sign
bit being in the first of the two bytes).

Every database begins with a dummy entry for a file called `LOCATE02', which locate checks
for to ensure that the database file has the correct format; it ignores the entry in doing
the search.

Databases cannot 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.

In the future, the data within the locate database may not be sorted in any particular
order. To obtain sorted results, pipe the output of locate through sort -f.

slocate database format

       The  slocate  program  uses  a  database format similar to, but not quite the same as, GNU
       locate.  The first byte of the database specifies its security  level.   If  the  security
       level  is  0, slocate will read, match and print filenames on the basis of the information
       in the database only.  However, if the security level byte is  1,  slocate  omits  entries
       from  its  output  if  the invoking user is unable to access them.  The second byte of the
       database is zero.  The second byte is followed by the first  database  entry.   The  first
       entry  in  the database is not preceded by any differential count or dummy entry.  Instead
       the differential count for the first item is assumed to be zero.

       Starting with the second entry (if any) in the database, data is interpreted  as  for  the
       GNU LOCATE02 format.

Old Locate Database format

There is also an old database format, used by Unix locate and find programs and earlier
releases of the GNU ones. updatedb runs programs called bigram and code to produce old-
format databases. The old format differs from the above description 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
≤ 30.

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 single character.

EXAMPLE

       Input to frcode:
       /usr/src
       /usr/src/cmd/aardvark.c
       /usr/src/cmd/armadillo.c
       /usr/tmp/zoo

       Length of the longest prefix of the preceding entry to share:
       0 /usr/src
       8 /cmd/aardvark.c
       14 rmadillo.c
       5 tmp/zoo

       Output from frcode,  with  trailing  nulls  changed  to  newlines  and  count  bytes  made
       printable:
       0 LOCATE02
       0 /usr/src
       8 /cmd/aardvark.c
       6 rmadillo.c
       -9 tmp/zoo

       (6 = 14 - 8, and -9 = 5 - 14)

REPORTING BUGS

       GNU findutils online help: <https://www.gnu.org/software/findutils/#get-help>
       Report any translation bugs to <https://translationproject.org/team/>

       Report any other issue via the form at the GNU Savannah bug tracker:
              <https://savannah.gnu.org/bugs/?group=findutils>
       General  topics about the GNU findutils package are discussed at the bug-findutils mailing
       list:
              <https://lists.gnu.org/mailman/listinfo/bug-findutils>

COPYRIGHT

       Copyright © 1994-2021 Free Software Foundation, Inc.  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.