int fork()  //Create a process, return child’s PID.
int exit(int status)    //Terminate the current process; status reported to wait(). No return.
int wait(int *status)   //Wait for a child to exit; exit status in *status; returns child PID.
int kill(int pid)   //Terminate process PID. Returns 0, or -1 for error.
int getpid()    //Return the current process’s PID.
int sleep(int n)    //Pause for n clock ticks.
int exec(char *file, char *argv[])  //Load a file and execute it with arguments; only returns if error.
char *sbrk(int n)   //Grow process’s memory by n bytes. Returns start of new memory.
int open(char *file, int flags)     //Open a file; flags indicate read/write; returns an fd (file descriptor).
int write(int fd, char *buf, int n)     //Write n bytes from buf to file descriptor fd; returns n.
int read(int fd, char *buf, int n)  //Read n bytes into buf; returns number read; or 0 if end of file.
int close(int fd)   //Release open file fd.
int dup(int fd)     //Return a new file descriptor referring to the same file as fd.
int pipe(int p[])   //Create a pipe, put read/write file descriptors in p[0] and p[1].
int chdir(char *dir)    //Change the current directory.
int mkdir(char *dir)    //Create a new directory.
int mknod(char *file, int, int)     //Create a device file.
int fstat(int fd, struct stat *st)  //Place info about an open file into *st.
int stat(char *file, struct stat *st)   //Place info about a named file into *st.
int link(char *file1, char *file2)  //Create another name (file2) for the file file1.
int unlink(char *file)  //Remove a file.