* review fork / wait / exec

    fork -- create a copy of the current process (with some difference)
    exec -- execute a program; replace the currently running code with
            the code for a different program
    wait -- parent waits for child and gets the exit status

**  pid_t pid = fork();
    if ( pid == 0 ) {
        // child process here
    } else if ( pid > 0 ) {
        // parent process here
    } else {
        // error case
    }

** exec variants
    l -- list ... just list out the arguments with the last one always NULL
    v -- vector ... same as the list, just loaded into an array
    e -- environment ... specify the environment as a vector for the new
                         process to use
    p -- path ... search the path for a program by that name
    
    execve is the one system call, all the others wrap it

*** shell exec builtin

    takes all of the args _after_ exec and uses them as the vector for execvp

** int status = 0;
   
        if ( wait(&status) < 0 ) {
            // error condition
        }

        if ( WIFEXITED(status) ) {
            // WEXITSTATUS(status) is what was passed to exit in the child
        } else if ( WIFSIGNALED(status) ) {
            // WTERMSIG(status) is the signal number
        }

* review starting code
* diagram flow described in lecture 8

read VAR
-l /tmp

echo $VAR
-l /tmp

ls $VAR
argv[0] == "ls"
argv[1] == "-l"
argv[2] == "/tmp"
argv[3] == NULL

* assignment clarifications
** cd command requirements

    calls chdir
    handle the case of one arg and no args
    *** with no arg, cd goes to $HOME
    *** what if HOME doesn't have a value or is unset?

** read command
    
    requires a single argument that must be a valid variable name
    reads a single line of text from stdin (always)
    stores that as the var value

** unset command

    removes variable completely from the table in varlib.c

** source command (dot)

    open a separate file stream
    read commands from the new file stream
    then return to whatever file stream you were reading from before