Writing my own shell in C, how do I run Unix executables?

1. Check out getenv(3) and its relatives. You probably don't actually need to directly do any operations on directories, but if you do, you can use opendir(3), readdir(3), closedir(3), etc.
2. Use fork(2) and execl(3) or one of its relatives (execve(2) is the base-level API).

The man pages themselves or a simple google search for any of these functions will turn up plenty of examples to help you along.

You split the command string up into separate strings (command name, arguments), and you store them in a null terminated array of character pointers. You then fork and pass the command name and the command + arguments to execvp(), unless that's verboten. You will eventually have to worry about pipes and I/O redirection too, but the basic idea remains the same.

char *args[MAXARGS];args[0] = "cat";args[1] = "foo.txt";args[2] = 0;if ((pid = fork()) == 0){    execvp(args[0], args);    ...print error...    exit(1);    /* execvp() does not return unless there's an error */}else if (pid < 0)    ...fork failed...else    ...wait for child to finish - or start other commands, or ...

You might dynamically allocate args in a real shell. It gets one step trickier if you have to supply a custom environment to the executed program. Basically, you'll assemble a copy of the environment, and then after forking, set the global environ to the new value - but if the new environment contains a new value for PATH, that will affect the search for the command by execvp(). So you may end up writing your own variant of execvp() because of that. If so, take a leaf out of the BSD (MacOS X) book; there is a function execvP():

 int execvP(const char *file, const char *search_path, char *const argv[]);

which allows you to specify the search path to use, even though you've fixed the environment. The most fundamental call is execve(); this is used by execvp() behind the scenes (possibly via execv()).