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14.1. Making and Using a DBM FileProblemYou want to create, populate, inspect, or delete values in a DBM database. SolutionUse dbmopenuse 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 tieuse 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 DiscussionAccessing 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 Example 14.1: userstats#!/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 See AlsoThe 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 |