roscd command not found

If you face the error " roscd command not found " , after ROS installation, this means that your .bashrc doesn’t contain the link to setup.sh, so open the .bashrc file with the following command

gedit .bashrc

then add the following line

source /opt/ros/groovy/setup.bash

as I was working with groovy else add your distribution name like fuerte, electric or hydro

updating the rosdistro that is out of date

While I was installing the ROS (Groovy) , I face the error that said unable to process source or error loading sources or operation time out. I found out that my ROS distribution is out of date so I have to update it. so the following command can be used.

sudo easy_install rosdep

The error obtained previously before was as follows
reading in sources list data from /etc/ros/rosdep/sources.list.d
ERROR: unable to process source [https://github.com/ros/rosdistro/raw/master/rosdep/osx-homebrew.yaml]:
    <urlopen error timed out> (https://github.com/ros/rosdistro/raw/master/rosdep/osx-homebrew.yaml)
Hit https://github.com/ros/rosdistro/raw/master/rosdep/gentoo.yaml
Hit https://github.com/ros/rosdistro/raw/master/rosdep/base.yaml
ERROR: error loading sources list:
    The read operation timed out

Un Installing ROS completely from system in Ubuntu

In order to un-install ROS completely from system the following command can be used

sudo apt-get remove ros-*

or if you need to remove specific version then Like for electric

sudo apt-get remove ros-electric-* 

You have to edit .bashrc to remove all the calls or variables set in there for ROS, 

Remove any setup scripts also delete the workspaces created when working with ROS.

If there is some unused dependecies left then the following can be used

sudo apt-get autoremove