Perl Cookbook

Making and Using a DBM File

Problem

You want to create, populate, inspect, or delete values in a DBM database.

Solution

Use dbmopen or tie to open the database and make it accessible through a hash. Then use the hash as you normally would. When you’re done, call dbmclose or untie.

dbmopen

use DB_File; 		# optional; overrides default
dbmopen %HASH, $FILENAME, 0666 		# open database, accessed through %HASH
    or die "Can't open $FILENAME:$!\n";

$V = $HASH{$KEY};            		# retrieve from database
$HASH{$KEY} = $VALUE; 			# put value into database
if (exists $HASH{$KEY}) {  		# check whether in database
    # ...
}
delete $HASH{$KEY};            		# remove from database
dbmclose %HASH;            		# close the database

tie

use DB_File;     	# load database module

tie %HASH, "DB_File", $FILENAME		# open database, to be accessed
    or die "Can't open $FILENAME:$!\n";    # through %HASH

$V = $HASH{$KEY};            		# retrieve from database
$HASH{$KEY} = $VALUE; 			# put value into database
if (exists $HASH{$KEY}) { 		# check whether in database
    # ...
}
delete $HASH{$KEY};            		# delete from database
untie %HASH;            		# close the database

Discussion

Accessing a database as a hash is powerful but easy, giving you a persistent hash that sticks around after the program using it has finished running. It’s also much faster than loading in a new hash every time; even if the hash has a million entries, your program starts up virtually instantaneously.

The program in Example 14.1 treats the database as though it were a normal hash. You can even call keys or each on it. Likewise, exists ...