You need to read in a logical line, so a "\" followed by a new line continues.

Then you need to do macro expansion, so look for all "$" and expand as per environment variable.

I would then look for all commands between back quotes, run a subshell to execute the contents and expand the command with the stdout.

Then I would split the result based on pipes to work out what the actual processes are with arguments.

Then I would pull out the <,>,<< and >> tokens and do the appropriate IO redirection

Then I would look for a "&" and flag if I found that to say don't wait for result.

Finally I would setup the pipe line and do the chain of forks and execs.

Quite where you choose to detect "cd", "if", "while", "do" etc is a good question.