Question

I’m attempting to write a menu driven modular perl script that will capture user input and automate the network configuration process. This script has to be able to install required Arch packages, configure AP mode, configure either DHCP or a static address for the user selected interface and give an option to enable bridging. (EDIT: The script also needs to be able to enable and configure the dhcpd service)

The part I’m stuck on right now is creating a backup of the rc.conf file, reading the file and editing the lines that need to be modified if a network interface has already been statically configured. This script is for use in ArchLinux, I did some searching around and didn’t find anything that met my needs specifically.

Using generic input for
$ip = 1.1.1.1; $Bcast = 2.2.2.2; $netmask = 3.3.3.3; $GW = 4.4.4.4;

I’ve spent about two hours reading about file I/O and tried several things that didn’t work including scrapping the multiple file IO method and using something similar to: while(<IS>){s/^interface.?=(.*)$/"interface=@if[0] \n"/;} with inputs for each of the values that need to be replaced and couldn’t get it to actually do anything.

   if (system ("cat","/etc/rc.conf","|","grep","interface")){   
    use File::Copy "cp";
    $filename = "/etc/rc.conf"; 
    $tempfile = "/etc/rc.tmp";  
    $bak = "/etc/rc.bak";
    cp($filename,$bak);
    open(IS, $filename); 
    open(OS, ">$tempfile"); 
    while(<IS>){ 
        if($_ =~ /^interface.?=(.*)$/){ print OS"interface=@if[0] \n";}
        if($_ =~ /^address.?=(.*)$/){ print OS "address=$ip\n";}
        if($_ =~/^netmask.?=(.*)$/){ print OS "netmask=$netmask\n";}
        if($_ =~/^broadcast.?=(.*)$/){ print OS "broadcast=$Bcast\n";}
        if($_ =~/^gateway.?=(.*)$/){ print OS "gateway=$GW\n"; }
        else {print OS $_;} 
    }
    close(IS); close(OS); 
    unlink($filename); rename($tempfile, $filename);
}

rc.conf before

#
# /etc/rc.conf - Main Configuration for Arch Linux

. /etc/archiso/functions

LOCALE_DEFAULT="en_US.UTF-8"
DAEMON_LOCALE_DEFAULT="no"
CLOCK_DEFAULT="UTC"
TIMEZONE_DEFAULT="Canada/Pacific"
KEYMAP_DEFAULT="us"
CONSOLEFONT_DEFAULT=
CONSOLEMAP_DEFAULT=
USECOLOR_DEFAULT="yes"

LOCALE="$(kernel_cmdline locale ${LOCALE_DEFAULT})"
DAEMON_LOCALE="$(kernel_cmdline daemon_locale ${DAEMON_LOCALE_DEFAULT})"
HARDWARECLOCK="$(kernel_cmdline clock ${CLOCK_DEFAULT})"
TIMEZONE="$(kernel_cmdline timezone ${TIMEZONE_DEFAULT})"
KEYMAP="$(kernel_cmdline keymap ${KEYMAP_DEFAULT})"
CONSOLEFONT="$(kernel_cmdline consolefont ${CONSOLEFONT_DEFAULT})"
CONSOLEMAP="$(kernel_cmdline consolemap ${CONSOLEMAP_DEFAULT})"
USECOLOR="$(kernel_cmdline usecolor ${USECOLOR_DEFAULT})"

MODULES=()

UDEV_TIMEOUT=30
USEDMRAID="no"
USEBTRFS="no"
USELVM="no"

HOSTNAME="archiso"

DAEMONS=(hwclock syslog-ng)

interface=eth0
address=192.168.0.99
netmask=255.255.255.0
broadcast=192.168.0.255
gateway=192.168.0.1

rc.conf after

#
# /etc/rc.conf - Main Configuration for Arch Linux

. /etc/archiso/functions

LOCALE_DEFAULT="en_US.UTF-8"
DAEMON_LOCALE_DEFAULT="no"
CLOCK_DEFAULT="UTC"
TIMEZONE_DEFAULT="Canada/Pacific"
KEYMAP_DEFAULT="us"
CONSOLEFONT_DEFAULT=
CONSOLEMAP_DEFAULT=
USECOLOR_DEFAULT="yes"

LOCALE="$(kernel_cmdline locale ${LOCALE_DEFAULT})"
DAEMON_LOCALE="$(kernel_cmdline daemon_locale ${DAEMON_LOCALE_DEFAULT})"
HARDWARECLOCK="$(kernel_cmdline clock ${CLOCK_DEFAULT})"
TIMEZONE="$(kernel_cmdline timezone ${TIMEZONE_DEFAULT})"
KEYMAP="$(kernel_cmdline keymap ${KEYMAP_DEFAULT})"
CONSOLEFONT="$(kernel_cmdline consolefont ${CONSOLEFONT_DEFAULT})"
CONSOLEMAP="$(kernel_cmdline consolemap ${CONSOLEMAP_DEFAULT})"
USECOLOR="$(kernel_cmdline usecolor ${USECOLOR_DEFAULT})"

MODULES=()

UDEV_TIMEOUT=30
USEDMRAID="no"
USEBTRFS="no"
USELVM="no"

HOSTNAME="archiso"

DAEMONS=(hwclock syslog-ng)

interface=eth0 
interface=eth0
address=1.1.1.1
address=192.168.0.99
netmask=3.3.3.3
netmask=255.255.255.0
broadcast=2.2.2.2
broadcast=192.168.0.255
gateway=4.4.4.4

Answer 1

I am not going to comment on the wisdom of the rest of your script, but you have:

if (system ("cat","/etc/rc.conf","|","grep","interface")){ 

system returns 0 on success.

So, you’ll enter the block only if that system call fails.

If fact, I am on a Windows system right now with no /etc/rc.conf (but cat and grep thanks to Cygwin. Running the following script:

#!/usr/bin/env perl

use strict; use warnings;

if (system ("cat","/etc/rc.conf","|","grep","interface")){
    print "*** it worked! ***\n";
    if ($? == -1) {
        print "failed to execute: $!\n";
    }
    elsif ($? & 127) {
        printf "child died with signal %d, %s coredump\n",
            ($? & 127),  ($? & 128) ? 'with' : 'without';
    }
    else {
        printf "child exited with value %d\n", $? >> 8;
    }
}

produces the output:

cat: /etc/rc.conf: No such file or directory
cat: |: No such file or directory
cat: grep: No such file or directory
cat: interface: No such file or directory
*** it worked! ***
child exited with value 1

That means system returned a failure code. Now, if you want to use shell piping and redirection, you should pass system a string, not a list, and check like this:

if (system ('cat /etc/rc.conf | grep interface') == 0) {

On the other hand, I would rather not trust shells propagating exit status.

The following should point you in a better direction:

#!/usr/bin/env perl

use strict;use warnings;

my %lookup = (
    eth0 => {
        address => '1.1.1.1',
        broadcast => '2.2.2.2',
        netmask => '3.3.3.3',
        gateway => '4.4.4.4',
    },
    wlan0 => {
        address => '5.5.5.5',
        broadcast => '6.6.6.6',
        netmask => '7.7.7.7',
        gateway => '8.8.8.8',
    },

);

while (my $line = <DATA>) {
    if (my ($interface) = ($line =~ /^interface=(\S+)/)) {
        print $line;
        if (exists $lookup{$interface}) {
            $line = process_interface(\*DATA, $lookup{$interface});
            redo;
        }
    }
    else {
        print $line;
    }
}

sub process_interface {
    my ($fh, $lookup) = @_;
    my $keys = join '|', sort keys %$lookup;

    while (my $line = <DATA>) {
        $line =~ s/\A($keys)=.+/$1=$lookup->{$1}/
            or return $line;
        print $line;
    }

    return;
}

__DATA__
#
# /etc/rc.conf - Main Configuration for Arch Linux

. /etc/archiso/functions

# stuff

interface=eth0
address=192.168.0.99
netmask=255.255.255.0
broadcast=192.168.0.255
gateway=192.168.0.1
interface=wlan0
address=192.168.0.99
netmask=255.255.255.0
broadcast=192.168.0.255
gateway=192.168.0.1

Output:

#
# /etc/rc.conf - Main Configuration for Arch Linux

. /etc/archiso/functions

# stuff

interface=eth0
address=1.1.1.1
netmask=3.3.3.3
broadcast=2.2.2.2
gateway=4.4.4.4
interface=wlan0
address=5.5.5.5
netmask=7.7.7.7
broadcast=6.6.6.6
gateway=8.8.8.8