Unit 1: Terminal and Shell

What is a Shell?

The term CLI refers to a type of user interface. To realize this interface, Unix computing environments rely on another type of program called shell.

A shell usually works closely with a terminal to get inputs from the users, interpret the meaning of the inputs, execute the tasks (perhaps through the invocation of other programs), and returned the output back to the user through the terminal.

Note that a shell can run on its own without a terminal (it can read input from a file, and write the output to a file, for instance).

Command Prompt

A shell has a command prompt. It typically looks something like this, but will be different depending on the default configuration on your machine:

name@pe111:~$

The prompt is where you type in a command for the shell to interpret and execute.

In bash, the command prompt can be customized to include information such as the username, hostname, time, current working directory, etc. It is customary to use the $ sign as the final character of the prompt. In our examples, we will just show $ to incidate the command prompt.

Terminal Control Sequence

The following lists some of the most useful control sequences to know:

Ctrl+D : signal the end of input to a program. This is also used to exit from a shell (by telling the shell that you have no more input to send and you are done with it).

Ctrl+Z : suspend the current running program. This pauses the execution of the program (but not terminating it). In the bash shell, the most recently suspended program can resume executing in the background with the command bg or brought back to execution in the foreground again with the command fg.

Ctrl+C : terminate the current running program.

Ctrl+S : freeze the terminal. This is a legacy control command that pauses the output printing of a teletype machine. You shouldn't need to use this control sequence.

Ctrl+Q : resume the terminal. This is a legacy control command that resume the printing of a teletype machine. You shouldn't need to use this control sequence, unless you accidentally hit Ctrl+S

!!! note "Ctrl+Z vs. Ctrl+C" A common mistake for new students is to hit Ctrl+Z frequently if something goes wrong with their program -- this behavior leads to large number of suspended programs (which still occupy resources such as memory on the computer). The right sequence to use is Ctrl+C -- which terminates the program (and frees up the resources).

shortcut in non-Unix environment, many students accidentally hit this control sequence, causing their terminal to freeze. Don't panic if this happens. Just hit Ctrl+Q and things will be back to normal.