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.
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.
Your main loop is:
fgets(3) for a simple shell, readline(3) for a fancy one)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.
readline() be used in any case - it will make your life infinitely easier as long as you have to use said shell. But +1 for getting bogged down in yacc/bison. Good tools, but it's a hard task to simplify.
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
