Lesson 01 · The prompt and the read loop

The prompt and the read loop

Every shell is a loop - print a prompt, read a line, act on it, repeat. Today you build that loop and the way it ends, which is the skeleton every later lesson hangs code on.

The goal

Print a prompt, read one line of input at a time, and stop cleanly at end of input.

Start here - the target

TO DO

Scenario: The read loop consumes lines until EOF

Giveninput consisting of "one\ntwo\n" followed by end-of-input

Whenthe shell runs its read loop

Thenit yields the line "one", then the line "two" (newline stripped)

Andat end-of-input the loop stops and the shell exits with status 0

Background

A shell is a read-eval-print loop: it prints a prompt, reads a line you type, does what the line says, and loops back for the next one. Today only the “read” and “loop” halves exist - there is nothing to evaluate yet - but this loop is the spine of the whole project, so it is worth getting the shape right.

The one subtlety is how the loop ends. When input runs out - the user presses Ctrl-D at the terminal, or a script file reaches its last line - the read returns an end-of-input signal rather than a line. That is your cue to leave the loop and exit 0. Strip the trailing newline from each line as you read it; every later lesson assumes the line it receives has no \n on the end.

Make it work

// print a prompt, read a line, repeat until EOF
for (;;) {
    printf("$ ");
    if (!read_line(&line)) break;   // EOF -> stop
    // (nothing to do with the line yet)
}

CheckpointDONE

You have a running shell that prompts and reads lines until you press Ctrl-D. Commit and stop here.