These functions help in reading user input and managing the command line interface.

• readline(char *prompt): Reads a line of input from the user, displaying the prompt.
• rl_clear_history(): Clears the command history.
• add_history(char *line): Adds the given line to the command history.
• rl_on_new_line(): Resets the internal state to indicate a new line.
• rl_replace_line(const char *text, int clear_undo): Replaces the current input line with the given text.
• rl_redisplay(): Refreshes and redraws the input line on the screen.

These functions help in printing formatted output to the user.

• printf(const char *format, ...): Prints formatted output to standard output.
• write(int fd, const void *buf, size_t count): Writes count bytes from buf to the file descriptor fd.

These functions manage dynamic memory allocation in your shell.

• malloc(size_t size): Allocates size bytes of memory.
• free(void *ptr): Frees the memory previously allocated by malloc.

These functions interact with the file system, handling files and directories.

• open(const char *pathname, int flags): Opens a file and returns a file descriptor.
• close(int fd): Closes the file descriptor fd.
• read(int fd, void *buf, size_t count): Reads count bytes from fd into buf.
• access(const char *pathname, int mode): Checks the access permissions for the file at pathname.
• unlink(const char *pathname): Deletes the file at pathname.
• getcwd(char *buf, size_t size): Gets the current working directory and stores it in buf.
• chdir(const char *path): Changes the current working directory to path.
• stat(const char *pathname, struct stat *statbuf): Retrieves information about the file at pathname.
• lstat(const char *pathname, struct stat *statbuf): Retrieves information about the link itself rather than the file it points to.
• fstat(int fd, struct stat *statbuf): Retrieves information about the file referred to by fd.
• opendir(const char *name): Opens the directory stream for name.
• readdir(DIR *dirp): Reads the next entry in the directory stream dirp.
• closedir(DIR *dirp): Closes the directory stream.

These functions create and manage processes.

• fork(): Creates a new process by duplicating the calling process.
• wait(int *status): Waits for a child process to terminate.
• waitpid(pid_t pid, int *status, int options): Waits for a specific child process to terminate.
• wait3(int *status, int options, struct rusage *rusage): Waits for a child process to terminate and provides resource usage information.
• wait4(pid_t pid, int *status, int options, struct rusage *rusage): Similar to waitpid but also returns resource usage information.
• kill(pid_t pid, int sig): Sends the signal sig to the process with ID pid.
• exit(int status): Terminates the current process with the given exit status.
• execve(const char *pathname, char *const argv[], char *const envp[]): Replaces the current process image with a new one.

These functions manage and respond to signals within your shell.

• signal(int signum, sighandler_t handler): Sets a handler function for the signal signum.
• sigaction(int signum, const struct sigaction *act, struct sigaction *oldact): Examines and changes the action taken on receipt of a specific signal.
• sigemptyset(sigset_t *set): Initializes the signal set set to be empty.
• sigaddset(sigset_t *set, int signum): Adds the signal signum to the signal set set.

These functions interact with and control terminal behavior.

• isatty(int fd): Checks if the file descriptor fd refers to a terminal device.
• ttyname(int fd): Returns the name of the terminal connected to fd.
• ttyslot(): Returns the index of the current terminal.
• ioctl(int fd, unsigned long request, ...): Performs device-specific I/O operations on the file descriptor fd.
• tcgetattr(int fd, struct termios *termios_p): Gets the parameters associated with the terminal referred to by fd.
• tcsetattr(int fd, int optional_actions, const struct termios *termios_p): Sets the parameters associated with the terminal referred to by fd.

These functions access and modify the shell environment.

• getenv(const char *name): Retrieves the value of the environment variable name.

These functions interact with the terminal capability database.

• tgetent(char *bp, const char *name): Gets the terminal entry for the terminal type name.
• tgetflag(char *id): Gets the value of the boolean capability id.
• tgetnum(char *id): Gets the numeric value of the capability id.
• tgetstr(char *id, char **area): Gets the string value of the capability id.
• tgoto(const char *cap, int col, int row): Returns a cursor movement string for the terminal.
• tputs(const char *str, int affcnt, int (*putc)(int)): Outputs the string str to the terminal, considering padding information.

These functions help handle errors within the shell.

• strerror(int errnum): Returns a string describing the error code errnum.
• perror(const char *s): Prints a description of the last error that occurred, prefixed by the string s.