locatedb man page on DigitalUNIX

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       locatedb - front-compressed file name database

       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

       There can be multiple databases.	  Users	 can  select  which  databases
       locate  searches	 using an environment variable or command line option;
       see locate(1L).	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 period‐
       ically, typically nightly; see updatedb(1L).

       updatedb runs a program called 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	encod‐
       ing) 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  locate  checks for to ensure that the database file has the cor‐
       rect 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  follow‐
       ing databases will be wrong.

       There is also an old database format, used by Unix locate and find pro‐
       grams 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 off‐
       set-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.

       Input to frcode:

       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)

       find(1L), locate(1L), locatedb(5L), xargs(1L) Finding Files (on-line in
       Info, or printed)


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