====== Installing packages on a machine to match another one ======

Sometimes you want to set up a new server and have the same packages installed on it as are installed on an existing one.

There are several tedious ways of doing this, and also a few pretty simple ones.  Here are two of the simple ones.

===== Firstly =====

Get a list of the packages which are installed on the existing machine.

==== If the machine is running: ====

<code>dpkg --get-selections >packages.txt</code>

==== If the machine is not running but you have a backup: ====

<code>dpkg --admindir=/pathtobackup/var/lib/dpkg --get-selections >packages.txt</code>

==== Either way... ====

This will create a file called **packages.txt** containing a list of all the packages which have been installed //or uninstalled// on your machine, in a format suitable for transferring to another machine in order to make it match.

If you do not want the list to uninstall packages, you can filter the output simply:

<code>dpkg --get-selections | grep "\sinstall$" >packages.txt</code>

The file will now contain only the list of packages which have been installed (and will therefore not uninstall anything from the new machine).

===== Secondly =====

Transfer the list to the new machine and tell it to install the same things.

Copy the text file you created above by whatever means is most convenient, and then use:

<code>/usr/lib/dpkg/methods/apt/update /var/lib/dpkg/
dpkg --set-selections <packages.txt
apt-get dselect-upgrade</code>

===== Alternatively =====

There's a tool named [[http://chris.olstrom.com/software/apt-clone/|apt-clone]] which will apparently do the above for you (disclaimer: I have not tried it).

On both machines you install the **apt-clone** package (which is a standard Debian/Devuan package) and then on the origin machine you do:<code>apt-clone clone source</code>(you can put anything you like in place of **source**), which will create a file named **source.apt-clone-tar-gz**.

Copy this file to the destination machine and use<code>apt-clone restore source.apt-clone.tar.gz</code>(putting whatever you used previously in place of **source**).

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