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.

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

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 and defined are implemented for tied DBM hashes. Unlike a normal hash, a DBM hash does not distinguish between those two functions.

#!/usr/bin/perl -w
# userstats - generates statistics on who is logged in.
# call with an argument to display totals

use DB_File;

$db = '/tmp/userstats.db';       # where data is kept between runs

tie(%db, 'DB_File', $db)         or die "Can't open DB_File $db : $!\n";

if (@ARGV) {
    if ("@ARGV" eq "ALL") {
        @ARGV = sort keys %db;
    }
    foreach $user (@ARGV) {
            print "$user\t$db{$user}\n";
    }
} else {
    @who = `who`;                                   # run who(1)
    if ($?) {
        die "Couldn't run who: $?\n";               # exited abnormally
    }
    # extract username (first thing on the line) and update
    foreach $line (@who) {
        $line =~ /^(\S+)/;
        die "Bad line from who: $line\n" unless $1;
        $db{$1}++;
    }
}

untie %db;

We use who to get a list of users logged in. This typically produces output like:

gnat     ttyp1   May 29 15:39   (coprolith.frii.com)

If the userstats program is called without any arguments, it checks who's logged on and updates the database appropriately.

If the program is called with arguments, these are treated as usernames whose information will be presented. The special argument "ALL" sets @ARGV to a sorted list of DBM keys. For large hashes with many keys, this is prohibitively expensive  - a better solution would be to use the BTREE bindings of DB_File described in Recipe 14.6.

See Also

The documentation for the standard modules GDBM_File, NDBM_File, SDBM_File, DB_File, also in Chapter 7 of Programming Perl; perltie (1); the section on "Using Tied Variables" in Chapter 5 of Programming Perl; the discussion on the effect of your umask on file creation in Recipe 7.1; Recipe 13.15