Find out whether a package is installed or not.

# dpkg -s [package-name]
$ dpkg -s apache2-mpm-worker
$ dpkg -l apache*

List files in a package which has been installed.

# dpkg -L [package-name]
$ dpkg -L python-openssl

Check the reason a package was installed, ie. find out the reverse dependencies of a package.

# apt-cache rdepends --installed [package-name]
$ apt-cache rdepends --installed nano

Update package list, then preform all upgrades.

$ sudo apt-get update
$ sudo apt-get upgrade

Create new user and grant root privileges.

$ sudo adduser newuser
$ sudo usermod -a -G sudo newuser        # add to sudo group

Delete a user.

$ sudo userdel newuser
$ sudo rm -rf /home/newuser

List all users.

$ cat /etc/passwd
$ lastlog

View current user uid and groups.

$ id

Add user to a group, eg: dialout group.

$ sudo usermod -a -G dialout

Change time zone.

$ sudo dpkg-reconfigure tzdata
$ less /etc/timezone            # verify timezone has changed
$ sudo service cron restart     # restart cron so it pick up the timezone change

Manual time synchronization.

$ sudo ntpdate ntp.ubuntu.com

Install time synchronization service.

$ sudo apt-get install ntp

Update hardware clock.

$ hwclock --set --date '`date`'

List all mounted and unmounted partitions.

$ sudo fdisk -l | less

List mounted partitions.

$ df -h

View size of a particular file or folder.

$ du -h myfile
$ du -sh myfolder

Find the full path of a command.

# which [command]
$ which python

View appended data as a file grows.

$ tail -f filename
$ less +F filename

Monitor a program status in real time.

# watch [command]
$ watch who

Get system environment.

$ printenv

Change server hostname.

$ sudo sh -c "echo new_host_name > /etc/hostname"
$ sudo /etc/init.d/hostname restart

View information of the linux distribution.

$ lsb_release -a

Check the OS installation date.

$ ls -ld /var/log/installer