Ubuntu Manpage: dbutil - database utility

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NAME

       dbutil - database utility

SYNOPSIS

       dbutil {-d | --dump} dbfile

       dbutil {-q | --query} [-t] dbfile key

       dbutil {-u | --update} [-n] dbfile key [value]

       dbutil {-x | --delete} dbfile key

       dbutil -t [date | [+|-]interval]

DESCRIPTION

The dbutil program maintains a database of key-value pairs that can be queried and updated from the
command line. For each such pair in the database, it also keeps an expiration time, so that unused
entries can be purged from the database. dbutil must be given an option specifying in which mode to run
the program. The following modes are available:

--dump (-d)
Prints the contents of the database. Each database entry is printed in one of the the following two
formats, depending on whether the record has an expiration time:

key value

key value (expiration-time)

--query (-q)
Prints the value of a particular key in the database. If the -t flag is also specified, prints the
expiration time of the record. In addition, the --expire flag can be specified to update the
expiration time on the record. Exits 0 if the key was found, 1 if the key was not in the database,
or 2 if there is a system error.

--update (-u)
Sets the value of a key in the database to a particular value. If no value is supplied, sets the
value to the empty string (which is not the same as deleting the record). The --expire flag can also
be specified to set an expiration time on the record.

Ordinarily, this option overwrites any previous value in the database. If the -n option is supplied,
dbutil will not overwrite a previously stored value in the database (and will not update the
expiration time on the record). Exits 0 if the key was found, 1 if -n was specified and the key was
already in the database, or 2 if there is a system error.

--expire={date | [+|-]interval}
This option can be specified in conjunction with --update or --query to set an expiration time on the
record. The option has two formats. You can either specify an absolute time, as the number of
seconds since Jan 1, 1970 GMT, or you can specify an offset from the current time with the format:

[+|-]countunits

Where + means in the future, - means in the past, count is a number, and units is one of the
following characters:

s - seconds
m - minutes
h - hours
D - days
W - weeks
M - months
Y - years

For example --expire=+36D means the record will be deleted in 36 days. If you always look up key
with the command:

dbutil --query --expire=+36D key

then the key will only expire if you do not look it up within 36 days.

Note that dbutil keeps a sorted list of the records by time of last access. Thus, purging old
records is not an inherently expensive operation, and happens automatically whenever you modify the
database.

--nosync (-N)
Ordinarily, dbutil synchronously flushes the database file to disk after making any modifications, to
minimize the window of vulnerability in which a crash could corrupt the database (if the --dbhome
option is not supplied). Synchronously flushing the database file is slow, however. This option
suppresses that behavior, and can be used to build lookup tables efficiently. For example, you might
have a script that builds a file x.db by issuing the following commands:

#!/bin/sh -e
rm -f x.db~
dbutil -Nu x.db~ key1 val1
dbutil -Nu x.db~ key2 val2
# ...
dbutil -Nu x.db~ keyn valn
dbutil -u @ @
mv -f x.db~ x.db

--delete (-x)
Deletes a particular key from the database (if the database contains the key). Exits 0 if the key
was found, 1 if the key was not in the database, or 2 if there was a system error.

-t [date|interval]
With no options, prints the number of seconds since Jan 1, 1970, GMT. With an argument that takes
the same format as --expire, prints the expiration time as an absolute number of seconds since 1970.
Not really a database function, but useful hen you want to store a timestamp in the database.

Note that -t can also be combined with the --query option, in which case it causes dbutil to print
the expiration time of the key, rather than its value.

dbutil attempts to minimize the damage from an inopportune crash by flushing the database file to disk
whenever it is modified. However, there is still a small window in which your database can be
irrevocably corrupted. This may be alright if you are just using the database to store "soft state".

If you want the database to be recoverable under any circumstances, you must use write-ahead logging, in
which case dbutil needs to keep a directory with database logs, not just a single database file. The
following option specifies where to keep the log files. It must be used in conjunction with the other
options for each mode except -t:

--dbhome=dbhome
Specifies that database log files should be kept in directory dbhome (which will be created if it
does not already exist). Note that database files with relative pathnames will also be stored in
this directory. It is highly recommended that you use relative pathnames so as to store database
files and log files together. Otherwise, you run the risk of accessing a logged database without the
--dbhome option and trashing its contents.

ENVIRONMENT

       DB_HOME
           When  set,  specifies  a  directory  in  which  to  keep log files, so as to make the database crash-
           recoverable.  This is equivalent to specifying the option --dbhome=$DB_HOME (except that  any  actual
           --dbhome argument will override the environment variable).

BUGS

       If  you  do  not  use the --dbhome option or DB_HOME environment variable and your machine crashes at the
       wrong time, you can lose your whole database.

       The --dbhome may or may not work if the directory is stored on NFS; it depends on the NFS implementation.

       If you access the database from multiple machines simultaneously, you will likely corrupt  the  database.
       Accessing from multiple processes on one machine is fine, because dbutil does locking.

       If  you  ever  access  the same database file with and without the --dbhome option (or DB_HOME), you will
       probably irrevocably trash it.  For that reason, databases with relative pathnames are actually stored in
       the log directory.

AUTHOR