Final Project: mini-shell — argv tokenizer

Challenge

Tokenise a command line into an argv vector the way a shell does, honouring double-quoted strings.

Task

Implement int shell_split(const char *line, char **argv, int max_argv) that splits line into tokens and stores a freshly allocated copy of each token in argv.

Input

• line: a NUL-terminated command line. Tokens are separated by spaces/tabs; a run inside "..." is one token (the quotes are stripped, and whitespace inside them is kept).
• argv, max_argv: the output array and its capacity (pointer slots).

Output

Stores up to max_argv - 1 token pointers in argv[0..ret-1] — each made with strdup, so the caller frees them — then sets argv[ret] = NULL. Does not modify line. Returns the token count, or -1 if a quote is never closed.

Example

shell_split("ls -la /tmp", argv, 16)          ->   3   argv = {"ls","-la","/tmp"}
shell_split("echo \"hello world\"", argv, 16)   ->   2   argv = {"echo","hello world"}
shell_split("", argv, 16)                      ->   0
shell_split("oops \"never ends", argv, 16)     ->   -1   (unterminated quote)

Edge cases

• The empty line yields 0 tokens.
• An unterminated " returns -1 (and frees any tokens already allocated).

Rules

• Do not modify line; copy each token with strdup (don't use strtok).
• Always NULL-terminate argv after the last token.

Why this matters

A POSIX shell is at heart a tokenizer + a fork/exec loop. The trickiest bit is splitting a command line into argv while respecting quotes — every shell on every UNIX-like OS does this dance, and getting it right is a great pointer/string exercise.

Input format

A NUL-terminated command line, an output array argv, and its capacity max_argv. Tokens split on whitespace; "..." groups one token without the quotes.

Output format

strdup'd token pointers in argv[0..ret-1], then argv[ret]=NULL. Returns the token count, or -1 on an unterminated quote.

Constraints

Do not modify line (don't use strtok). Write at most max_argv-1 tokens. Always NULL-terminate argv.

Starter code

#include <stddef.h>
int shell_split(const char *line, char **argv, int max_argv) { /* TODO */ return 0; }

Common mistakes

Mishandling escape sequences (we don't require them — keep it simple); forgetting to NULL-terminate argv; not freeing tokens on the error path.

Edge cases to handle

Empty line returns 0. Trailing whitespace. Two consecutive spaces. Quoted empty string "".

Complexity

O(n) time, O(tokens) memory.

Background lessons

• State machines for parsers