Listing Processes with pgrep

Listing all the processes owned by the user raam can be done like this (the -l switch causes the output to include the process name):

$ pgrep -l -u raam
9614 screen
9628 bash
9644 irssi
16165 bash
16297 rtorrent
19462 ssh
19515 bash
19526 ssh
20964 sshd

You can also filter the list of results by appending a full (or partial) process name to the command:

$ pgrep -l -u raam bash
9628 bash
16165 bash
19515 bash

Killing Processes with pkill

The pkill command does basically the same thing as pgrep, except it kills the processes instead of listing them. This is useful if you have a user with several dead processes, or if you were deleting a user and you wanted to kill any running processes first.

Killing all the processes owned by the user raam looks like this:

$ pkill -u raam

And once again, if you only wanted to kill all the bash processes owned by raam, you would append the process name to the command:

$ pkill -u raam bash

As always, check the man pages for pgrep and pkill for more information and switch options.

echo "00:11:24:3e:a5:78" | egrep "^([0-9a-fA-F]{2}\:){5}[0-9a-fA-F]{2}$"

In the event that there was a problem getting the MAC address (e.g., faulty NIC or unstable device driver), I generate a random hostname instead of basing the hostname generation on the MAC. Here’s how I validated the MAC in the script:

if [ `echo $ACTIVE_INTERFACE_MAC | egrep "^([0-9a-fA-F]{2}\:){5}[0-9a-fA-F]{2}$"` ]; then
	# generate unique hostname based on MAC
else
	# generate random-character hostname
fi

For systems that are only accessed remotely via SSH, such a scenario can be painful. What is needed is a daemon that watches the link status of the Ethernet jack and reconfigures the network (or sends out another DHCP request) when it detects a cable is plugged in (or the power to the router is restored).

ifplugd does exactly that:

ifplugd is a Linux daemon which will automatically configure your ethernet device when a cable is plugged in and automatically unconfigure it if the cable is pulled.

On a Debian system, installing and configuring ifplugd is relatively simple using apt-get install ifplugd. Once its been installed, it needs to be configured by editing /etc/default/ifplugd. The most basic configuration is to simply set INTERFACES="auto" and HOTPLUG_INTERFACES="all". This configuration tells ifplugd to watch all network interfaces for a new link status and automatically reconfigure them using the Debian network configuration defined in /etc/network/interfaces.

I recently needed to automate the install and configuration of ifplugd on many remote Linux systems, so I wrote this simple script.

Download: install-ifplugd.tar.gz

#!/bin/sh

#########################################
# Author: Raam Dev
#
# This script installs ifplugd and configures
# it to automatically attempt to restore any
# lost connections.
#
# Must be run as root!
#########################################

# Check if we're running this as root
if [ $EUID -ne 0 ]; then
   echo "This script must be run as root" 1>&2
   exit 1
fi

# Files used when configuring ifplugd
OUTFILE=/tmp/outfile.$$
CONFIG_FILE=/etc/default/ifplugd

# Update package list and install ifplugd, assuming yes to any questions asked
# (to insure the script runs without requiring manual intervention)
apt-get update --assume-yes ; apt-get install --assume-yes ifplugd

# Configure ifplugd to watch all interfaces and automatically attempt configuration
sed 's/INTERFACES=\"\"/INTERFACES=\"auto\"/g' < $CONFIG_FILE > $OUTFILE
mv $OUTFILE $CONFIG_FILE

sed 's/HOTPLUG_INTERFACES=\"auto\"/HOTPLUG_INTERFACES=\"all\"/g' < $CONFIG_FILE > $OUTFILE
mv $OUTFILE $CONFIG_FILE

If you’re interested in doing more with ifplugd, check out this article.

vi /path/to/some/file.conf
[make some edits]
:w
VIM Message: E45: 'readonly' option is set (add ! to override)
:q!
$ sudo vi /path/to/some/file.conf
[make all my edits AGAIN]
:w

I have gone through this process so many times that I knew there must be an easy fix for it. (I know about sudo !! for running the previous command, but I only recently started developing the habit of using it.) After forgetting to use sudo while editing a configuration file yet again this morning, I finally decided to search Google and find a solution. Here it is:

vi /path/to/some/file.conf
[make some edits]
:w
VIM Message: E45: 'readonly' option is set (add ! to override)
:w !sudo tee %

The :w !sudo tee % command tells VIM to write the file (w) but run the sudo command first (!sudo) and read the writing of the file from standard input to standard output (tee) using the same filename as the one we’re editing (%).

After saving the file as root, you’ll get this message: “W12: Warning: File “/private/etc/smb.conf” has changed and the buffer was changed in Vim as well”. You’ll be given the option to reload it, but since you were already looking at the new version it doesn’t much matter which option you choose (OK or Reload).

And last but not least, if you don’t want to remember the syntax for this command, you can map the command in your ~/.vimrc file:

cmap w!! w !sudo tee % >/dev/null

Now, if you forget to edit a file with sudo, you can simply type :w!! to fix the problem!

$ sudo touch /forcefsck

Alternatively (and depending on your version of Linux) you may also be able to pass an option to the shutdown command:

$ shutdown -rF now

When using a static IP configuration on Linux, you normally add the DNS servers to the /etc/resolv.conf. However, if you try to add a DNS server to /etc/resolv.conf under a DHCP configuration, you’ll notice that your static entry disappears as soon as the DHCP client runs (usually on boot). To prevent this, you need to tell the DHCP client to prepend the static DNS server(s) to /etc/resolv.conf before adding the ones provided from the DHCP server (if any).

The configuration file you’ll need to edit is the same on both Debian and Ubuntu, however depending on your setup the location of the file may vary. Here are the two common places I’ve found the file:

Debian: /etc/dhclient.conf
Ubuntu: /etc/dhcp3/dhclient.conf

Open the file in your favorite editor and add one of two lines at the top, separating multiple DNS servers with a comma and ending the entry with a semi-colon:

If you simply want to add static DNS servers to be used in addition to the ones provided by DHCP, use a prepend entry:

prepend domain-name-servers 208.67.222.222, 208.67.220.220;

If you want to override the DNS servers provided by DHCP entirely and force the system to use the ones you provide, use the supersede entry:

supersede domain-name-servers 208.67.222.222, 208.67.220.220;

Before these static DNS servers will to be appended to your /etc/resolv.conf file, you’ll need to re-run the DHCP client. The easiest way to do this is by running /etc/init.d/networking restart (sudo required) or you can try running the dhclient command.

After re-running the DHCP client, check your /etc/resolv.conf file to confirm the static DNS servers have been added.