run the <command> and replace the current shell process with the command.
can be used in the startup script to create a user that has access only to one program.
Once the use exist from the program it will be logged out (in case this command is in the startup script)
will run command that will not stop once we log out.
Actually what this command is doing is prevent the termination of the process when its parent process is dying.
type <command string>
show what type is the command, if it is a shell built-in, or executable (if executable include its location)
show the current, for how long the system is running, the average.
show how much memory is used and how much is used
will show the memory information is Gigabyte
execute fileName in the current shell (instead of subshell by default). This mean all variable, alilas and function that are created by the execution of fileName, will be aviliable in the current shell after the execuation.