Provided by: locate_4.6.0+git+20160126-2_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)
SEE ALSO
find(1), locate(1), locatedb(5), xargs(1), Finding Files (on-line in Info, or printed)
BUGS
The best way to report a bug is to use the form at
http://savannah.gnu.org/bugs/?group=findutils. The reason for this is that you will then
be able to track progress in fixing the problem. Other comments about locate(1) and
about the findutils package in general can be sent to the bug-findutils mailing list. To
join the list, send email to bug-findutils-request@gnu.org.
LOCATEDB(5)