Lesson 06 · Finding commands on PATH

Finding commands on PATH

Nobody types `/bin/ls` - they type `ls`. Today you teach your shell to find a bare command name by searching the directories in the PATH environment variable, the last piece that makes it feel like a real shell.

The goal

Resolve a bare command name to a full executable path by searching PATH; use a name containing a slash as-is.

Start here - the target

TO DO

Scenario: Resolving a command through PATH

Giventhe environment variable PATH is "/bin" and /bin/echo exists

Whenthe command name "echo" is resolved

Thenit resolves to "/bin/echo"

Anda name that contains a "/" (like "./prog") is used unchanged, without searching PATH

Background

The PATH environment variable is a colon-separated list of directories like /usr/local/bin:/usr/bin:/bin. When you type ls, the shell walks that list in order, and the first directory that contains an executable file named ls wins - that is the full path it hands to exec. This is why the order of directories in PATH matters, and why a program in an earlier directory “shadows” one of the same name later.

There is one exception: if the name already contains a slash - ./prog, bin/tool, /usr/bin/env - the shell treats it as a path directly and does not search PATH. That is the rule that lets you run a program in the current directory with ./prog. With this in place your shell is genuinely usable: it prompts, runs any command on your path, and reports status and errors.

(Some languages’ process-spawning calls will search PATH for you automatically. Build the resolver yourself anyway - a resolve(name) that returns the full path - so you understand exactly what that convenience is doing, and so your shell owns the decision rather than delegating it.)

Make it work

// for each dir in PATH, try dir + "/" + name
for (char *dir = strtok(path, ":"); dir; dir = strtok(NULL, ":")) {
    snprintf(full, sizeof full, "%s/%s", dir, name);
    if (access(full, X_OK) == 0) return full;   // found & executable
}
// if name already contains '/', skip the search and use it directly

CheckpointDONE

You can type `ls` instead of `/bin/ls`. Chapter one is done - your shell runs any program on your PATH. Commit and stop here.