How to unset environmental variable

For example, you set DJANGO=true environmental variable with command:

export DJANGO=true

To unset it run this command:

unset DJANGO

How to display all available package versions in Ubuntu

To display all available package versions use next command:

apt-cache madison

For example:

vagrant@vagrant:~$ apt-cache madison redis-server
redis-server | 2:3.0.2-3~bpo8+1 | http://http.debian.net/debian/ jessie-backports/main amd64 Packages
redis-server | 2:2.8.17-1+deb8u1 | http://security.debian.org/ jessie/updates/main amd64 Packages
redis-server | 2:2.8.17-1 | http://http.debian.net/debian/ jessie/main amd64 Packages
     redis | 2:2.8.17-1 | http://http.debian.net/debian/ jessie/main Sources
     redis | 2:2.8.17-1+deb8u1 | http://security.debian.org/ jessie/updates/main Sources