TechniFul

Sometimes simply updating from "Ubuntu update" won't update/upgrade some of the required packages. We need to to it manually by adding necessary repositories. To upgrade git:

sudo add-apt-repository ppa:voronov84/andreyv
sudo apt-get update
sudo apt-get upgrade
sudo apt-get install git

If the first command result in error "couldn't connect to host"

Ubuntu will include openJDK by default. If you install some other version or Sun JDK, then the first installation is set to be the default one.

To check if there are any other java installations:

update-java-alternatives -l

Change the default to any of them:

update-java-alternatives -s NAME-OF-IT

Ubuntu has dropped including Sun/Oracle Java in its repository, and added OpenJDK. But, the former is more reliable than latter, and is also must for some applications.
Here is a way to add Sun java 6:

sudo add-apt-repository ppa:webupd8team/java
sudo apt-get update
sudo apt-get install oracle-java6-installer

Possible error : "couldn't connect to host"

To make this version as default: here.

If you want to install java 7, instead of last command try

sudo apt-get install oracle-java7-installer

If you are under proxy network or seen a error like

Cannot add PPA: 'ppa:webupd8team/java'.
Please check that the PPA name or format is correct.

and if you are sure that the PPA is working, the try

sudo -E add-apt-repository ppa:webupd8team/java

and proceed with remaining commands.

This error will be resulted mostly because of the user permissions. To go through it, we need the simulate the initial login mode:

sudo -i

Now run the commands which resulted in the above error, but without prefixing with 'sudo'. Remember to go out of this mode after running your commands. To go out:

exit

To add a repository ppa:

sudo add-apt-repository ppa:user/ppa-name
sudo apt-get update

Sometimes adding a repository may result in error related to missing key. Then copy that key and run following command

sudo apt-key adv --recv-keys --keyserver keyserver.ubuntu.com KEY

Do this for each of the missing keys.
Now re-run the above two commands.

If you are under proxy network or seen a error like

Cannot add PPA: 'ppa:user/ppa-name'.
Please check that the PPA name or format is correct.

and if you are sure that the PPA is working, the try

sudo -E add-apt-repository ppa:user/ppa-name

Set the proxy in "Network Proxy" setings and click "apply system-wide", and also at the following locations. Some terminal applications need them.

sudo gedit /etc/apt/apt.conf.d/95proxies

Acquire::http::proxy "http://username:passwd@proxy:port/";
Acquire::ftp::proxy "ftp://username:passwd@proxy:port/";
Acquire::https::proxy "https://username:passwd@proxy:port/";
sudo gedit .bashrc
export http_proxy=http://username:passwd@proxy:port
export https_proxy=http://username:passwd@proxy:port
export HTTP_PROXY=http://username:passwd@proxy:port
export HTTPS_PROXY=http://username:passwd@proxy:port

Requires both CAPITALS and small ones. Some applications accepts only CAPITALS.

Here is a open source software repository, which contains almost every version of the packages. This will be useful for all those who need any older version of the package, which sometimes is difficult to find. So, I'm sharing the link here: