How can I implement my own basic unix shell in C?

Question

I'm a newbie to process and thread management. My Shell should understand PATH environment variable. It can be set and modified. It runs in two ways -interactive & batch mode. Shell is capable of taking more than one job like ls;ps;wc file;cal. I want to get my hands dirty on signals too. So I should handle ^K , ^c as well.

I know I will have to use execs, forks and pipes but just can't get started.

Answer 1

Many of the Unix books that describe the main system calls also implement a shell to illustrate how and why you might use the various calls. Stevens and Rochkind are two such books:

• W Richard Stevens, Stephen A Rago Advanced Programming in the Unix Environment, 3rd Edn

• Marc J Rochkind Advanced Unix Programming, 2nd Edn

Answer 2

All the unix shells are open-source - so a good place to start may be to read the code.

If you're looking for a good starter article on the subject try Writing Your Own Shell from the Linux Gazette.

Another good starting point is to take a look at the source code of mini-shell just because its one of the smallest to get your head round.

Answer 3

Your main loop is:

• read a line (use fgets(3) for a simple shell, readline(3) for a fancy one)
• parse the command
• fork and execute the pipelines

To parse the command, there are two common choices. Write a recursive descent parser or use yacc(1) to generate one. It's a lot easier to bang out an initial parser using yacc, but you can totally get stuck debugging it and it really wants to be context-free. I prefer recursive descent but just about everyone else in the world prefers using yacc. (Technically, bison.) If the shell is really really simple, like a homework shell, yacc may be overkill.

To do the lexical analysis you can also roll your own or use flex.

You won't need to use any threads.