Perl Cookbook

Reading Configuration Files

Problem

You want to allow the users of your program to change its behavior through configuration files.

Solution

Either process a file in trivial VAR=VALUE format, setting a hash key-value pair for each setting:

while (<CONFIG>) {
    chomp;                  # no newline
    s/#.*//;                # no comments
    s/^\s+//;               # no leading white
    s/\s+$//;               # no trailing white
    next unless length;     # anything left?
    my ($var, $value) = split(/\s*=\s*/, $_, 2);
    $User_Preferences{$var} = $value;
}

Or better yet, treat the config file as full Perl code:

do "$ENV{HOME}/.progrc";

Discussion

The first solution lets you read in config files in a trivial format like this (comments and blank lines are allowed):

# set class C net
NETMASK = 255.255.255.0
MTU     = 296
    
DEVICE  = cua1
RATE    = 115200
MODE    = adaptive

After you’re done, you can pull in a setting by something like $User_Preferences{"RATE"} to find the value 115200. If you wanted the config file to directly set a variable in your program using that name, instead of assigning to the hash, do this:

no strict 'refs';
$$var = $value;

and the $RATE variable would contain 115200.

The second solution uses do to pull in raw Perl code directly. When used with an expression instead of a block, do interprets the expression as a filename. This is nearly identical to using require, but without risk of taking a fatal exception. In the second format, the config file would look like:

# set class C net $NETMASK = '255.255.255.0'; $MTU = 0x128; # Brent, please turn on the modem $DEVICE ...