Its essential to download packages once but if you are reinstalling ubuntu on new machine you need not download these packages all over again if you have installed required packages on one of the machine you can use the content of below listed directory for installation
/var/cache/apt/archives
this directory contains the debian packages download by the apt-get.
So have fun reuse the content for each install and save time and bandwidth.
The syntax is as follows:
dpkg -i package-name-here.deb
dpkg --install package-name-here.deb
dpkg -i -R /path/to/dir/name/with/lots/of/dot-deb-files/
dpkg -i --recursive /path/to/dir/name/with/lots/of/dot-deb-files/
Where,
-i or --install : Install the package.
-R or --recursive : Recursively installed all *.deb files found at specified directories and all of its sub-directories. /path/to/dir/name/with/lots/of/dot-deb-files/ must refer to a directory instead of package-name-here.deb file name.
How do I install .deb file?
To install a single package file called /tmp/package.deb type the following commands. Open a terminal and then cd to directory where package.deb is kept. In this example, package.deb is kept in /tmp directory:
$ cd /tmp
Type the following command to install the package:
$ sudo dpkg -i package.deb
OR
# dpkg -i package.deb
Install all packages from /nfs/pkgs/ubuntu-builds directory
Recursively handle and install all regular files matching pattern *.deb found at /nfs/pkgs/ubuntu-builds/
directory and all of its subdirectories, type:
$ sudo dpkg -i -R /nfs/pkgs/ubuntu-builds/
OR
# dpkg -i -R /nfs/pkgs/ubuntu-builds/
follow all these commands and it will work for you.
man apt-cache
is your friend.