uname

uname -m Display the type of the current hardware platform.

uname -n Display the name of the system.

uname -r Display the current release level of the operating system.

uname -s Display the name of the operating system implementation.

uname -vDisplay the version level of this release of the operating system.

uname -a Behave as though the options -m, -n, -r, -s, and -v were specified.

uptime & last reboot

uptime Show how long the system has been running.

last reboot Show when the last reboot was.

hostname

hostname Set or print the name of the current host system.

hostname -i Display all local IP addresses of the host system.

date & cal & timedatectl

date Display or set date and time.

cal Displays a calendar.

timedatectl May be used to query and change the system clock and its settings, and enable or disable time synchronization services.

whereis & which & whatis

whereis Checks the standard binary, and manual page directories for the specified programs, printing out the paths of any it finds.

which Locate a program file in the user’s path.

whatis Display online manual documentation pages.

whoami & finger

whoami Display effective user id.

finger All logged in users information lookup program.

finger [username] Show specific user information.

df & free

df Display free disk space.

free Memory allocation.

Terminal wise, you can run sudo lsof -i -P -n | grep LISTEN to see what processes are listening on ports on your local machine

The PATH is an important concept when working on the command line. It's a list
of directories that tell your operating system where to look for programs, so
that you can just write script instead of /home/me/bin/script or
C:\Users\Me\bin\script. But different operating systems have different ways to
add a new directory to it:

Windows

1. The first step depends which version of Windows you're using:
   • If you're using Windows 8 or 10, press the Windows key, then search for and
     select "System (Control Panel)".
   • If you're using Windows 7, right click the "Computer" icon on the desktop
     and click "Properties".
2. Click "Advanced system settings".
3. Click "Environment Variables".
4. Under "System Variables", find the PATH variable, select it, and click
   "Edit". If there is no PATH variable, click "New".
5. Add your directory to the beginning of the variable value followed by ; (a
   semicolon). For example, if the value was C:\Windows\System32, change it to
   C:\Users\Me\bin;C:\Windows\System32.
6. Click "OK".
7. Restart your terminal.

Mac OS X

1. Open the .bash_profile file in your home directory (for example,
   /Users/your-user-name/.bash_profile) in a text editor.
2. Add export PATH="your-dir:$PATH" to the last line of the file, where
   your-dir is the directory you want to add.
3. Save the .bash_profile file.
4. Restart your terminal.

Linux

1. Open the .bashrc file in your home directory (for example,
   /home/your-user-name/.bashrc) in a text editor.
2. Add export PATH="your-dir:$PATH" to the last line of the file, where
   your-dir is the directory you want to add.
3. Save the .bashrc file.
4. Restart your terminal.