1. halt : Brings the system down immediately
2. init 0 : Powers off the system using predefined scripts to synchronize and clean up the system prior to shutting down
3. init 6 : Reboots the system by shutting it down completely and then restarting it
4. poweroff : Shuts down the system by powering off
5. reboot : Reboots the system
6. shutdown : Shuts down the system
7. cal : This command will print a calendar for a specified month and/or year.
8. cat : This command outputs the contents of a text file. You can use it to read brief files or to concatenate files together.
9. cd : This command changes your current directory location. By default, your Unix login session begins in your home directory.
10. chmod : This command changes the permission information associated with a file. Every file (including directories, which Unix treats as files) on a Unix system is stored with records indicating who has permission to read, write, or execute the file, abbreviated as r, w, and x. These permissions are broken down for three categories of user: first, the owner of the file; second, a group with which both the user and the file may be associated; and third, all other users. These categories are abbreviated as u for owner (or user), g for group, and o for other.
11. cp : This command copies a file, preserving the original and creating an identical copy. If you already have a file with the new name, cp will overwrite and destroy the duplicate. For this reason, it's safest to always add -i after the cp command, to force the system to ask for your approval before it destroys any files. The general syntax for cp is:
12. date : The date command displays the current day, date, time, and year.
13. df : This command reports file system disk usage (i.e., the amount of space taken up on mounted file systems). For each mounted file system, df reports the file system device, the number of blocks used, the number of blocks available, and the directory where the file system is mounted.
14. du : This command reports disk usage (i.e., the amount of space taken up by a group of files). The du command descends all subdirectories from the directory in which you enter the command, reporting the size of their contents, and finally reporting a total size for all the files it finds.
15. find : The find command lists all of the files within a directory and its subdirectories that match a set of conditions. This command is most commonly used to find all of the files that have a certain name.
16. jobs : This command reports any programs that you suspended and still have running or waiting in the background (if you had pressed Ctrl-z to suspend an editing session, for example). For a list of suspended jobs, enter:
17. kill : Use this command as a last resort to destroy any jobs or programs that you suspended and are unable to restart. Use the jobs command to see a list of suspended jobs. To kill suspended job number three, for example, enter:
18. lpr and lp : These commands print a file on a printer connected to the computer network. The lpr command is used on BSD systems, and the lp command is used in System V. Both commands may be used on the UITS systems.
19. ls : This command will list the files stored in a directory. To see a brief, multi-column list of the files in the current directory, enter:
20. man : This command displays the manual page for a particular command. If you are unsure how to use a command or want to find out all its options, you might want to try using man to view the manual page.