Ubuntu Manpage: locatedb - front-compressed file name database

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

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       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  ©  1994-2022  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.