dbutil(1) Mail Avenger 0.8.4 dbutil(1)NAMEdbutil - database utility
SYNOPSISdbutil {-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).
SEE ALSOavenger(1)
The Mail Avenger home page: <http://www.mailavenger.org/>.
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
David Mazieres
Mail Avenger 0.8.4 2013-07-13 dbutil(1)
