Lesson 11 · type - Build a Shell

type

How does the shell decide what a name means - a builtin, or a program on disk? The `type` builtin answers that question out loud, and building it ties together the builtin table and PATH search you already have.

The goal

Implement `type <name>` to report whether a name is a shell builtin or an executable found on PATH.

Start here - the target

TO DO

Scenario: Classifying a command name

Giventhe builtin table contains "cd" and /bin/ls exists on PATH

Whenthe command "type cd" runs

Thenit prints "cd is a shell builtin"

Andrunning "type ls" prints "ls is /bin/ls"

Background

type is the shell explaining its own decision procedure. Given a name, it reports how the shell would run it: as a builtin if the name is in the builtin table, otherwise as the executable it finds by searching PATH, and if neither, it reports “not found.” It is the tool you reach for when ls behaves unexpectedly and you want to know which ls you are actually running.

Notice you are not writing new machinery today - you are composing two pieces you already have: the builtin lookup from lesson 7 and the PATH search from lesson 6. The order matters and mirrors how the shell itself dispatches: builtins are checked first, so a builtin always shadows a program of the same name. Building type is a good gut-check that both halves of command resolution are solid before you move on to parsing.

Make it work

if (is_builtin(name))
    printf("%s is a shell builtin\n", name);
else if ((p = path_lookup(name)))     // reuse lesson 6's PATH search
    printf("%s is %s\n", name, p);
else
    printf("%s: not found\n", name);   // status 1

CheckpointDONE

`type` explains how your shell resolves any name. Chapter two is done. Commit and stop here.