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The most important concept to grasp is the Ubuntu packaging system, which may be familiar to those who have already used Debian. In essence, large parts of your system should be considered under the control of the packaging system. These include:
• /usr (excluding /usr/local)
• /var (you could make /var/local and be safe in there)
1. Under the SysV init system halt had the same effect as poweroff, but with systemd as init system their effects are different.
• /bin
• /sbin
• /lib
For instance, if you replace /usr/bin/perl, that will work, but then if you upgrade your perl package, the file you put there will be replaced. Experts can get around this by putting packages on “hold” using aptitude or apt-mark.
One of the best installation methods is apt. You can use the command line versions apt and apt-get, the full-screen text version aptitude, or the graphical version synaptic.
Note: apt will also let you merge main, contrib, and non-free so you can have export-restricted packages as well as standard versions.