Between pressing Enter and seeing the output, several programs get to work, each with a job of its own.
The terminal passes on your keystrokes and shows the text that programs print. It does not understand commands itself.
The line is read by the shell — here, the program Bash. It splits the line into words: the first is the command name, and the rest go to the program itself — options, for example. By that name the shell finds a program and starts it. In uname -sr, the uname program asked the kernel and printed its name and release, and the terminal displayed that text.
When the program finishes, the shell prints the prompt — the student@ubuntu:~$ line before the cursor. The prompt means the shell is waiting for the next command. You do not type it.
The shell looks up a program name letter for letter, and upper and lower case differ: uname and Uname are two different names to it.
You typed Uname -r and got this output. What do you do?
Command 'Uname' not found, did you mean:
command 'uname' from deb coreutils (9.4-3ubuntu6.2)
Try: sudo apt install <deb name>-
The terminal only showed the text the shell printed. In a new terminal
Uname -ris still not found: the name is the same. -
The option never came into play: the shell found no
Unameprogram, so nobody read-r. With the same option,uname -rworks. -
The message names the package that holds a similar command,
uname, and it is already installed:uname -rworks. There is nothing to install; fix the name.
The message comes from the shell: it found no Uname program, because upper and lower case differ for it. Ubuntu adds a hint about a similar command and a package, but for a typo it is enough to fix the name and run the line again.
Key points
- The terminal passes on keys and shows text; the Bash shell parses the line.
- The shell looks up a program by its exact name and starts it.
- The prompt is the sign that the shell is waiting for a new command.