Implement a bash shell in C having the following functionalities:

1.The shell must know how to launch programs in the foreground and background. If command is followed by & it must be executed in background.

2.It should recognize the internal commands: exit, ls, pwd and cd.

[Hint: exit should use the exit() system call to terminate the shell. cd uses the chdir() system call to change to a new directory. You can use the standard C library functions chdir, getcwd, mkdir, rmdir, readdir, stat etc. to implement the calls. Note that these are built-in commands for your shell and hence no new process will be created to execute them. They neither read from the standard input stream nor write to the standard output stream. So your program should ignore file redirection with those built-in commands. Your program should still recognize invalid commands, however.]

• If the command line does not indicate any internal commands, it should be in the following form: .... Your shell should invoke the program, passing it the list of arguments in the command line. The shell must wait until the started program completes.

To allow users to pass arguments you need to parse the input line into words separated by whitespace (spaces and '\t' tab characters). You might try to use strtok_r() for parsing (check the manual page of strtok_r() and Google it for examples of using it). In case you wonder, strtok_r() is a user-level utility, not a system call. This means this function is fulfilled without the help of the operating system kernel.

The shell runs programs using two core system calls: fork() and execvp(). Read the manual pages to see how to use them. In short, fork() creates an exact copy of the currently running process, and is used by the shell to spawn a new process. The execvp() call is used to overlay the currently running program with a new program, which is how the shell turns a forked process into the program it wants to run. In addition, the shell must wait until the previously started program. This is done with the wait() system call or one of its variants (such as waitpid()). All these system calls can fail due to unforeseen reasons (see their manual pages for details). You should check their return status and report errors if they occur.

No input the user gives should cause the shell to exit (except when the user types exit or Ctrl+D). This means your shell should handle errors gracefully, no matter where they occur. Even if an error occurs in the middle of a long pipeline, it should be reported accurately and your shell should recover gracefully. You may need to use signal() and kill() system calls. In addition, your shell should not generate leaking open file descriptors.

Hint: you can monitor the current open file descriptors of the shell process through the /proc file system.

Your shell must run in one of two modes: interactive or batch. If no command-line argument is provided, your shell should interactively accept inputs from the user and execute them. If a batch file of commands is provided as command-line input to your program, then your shell must execute all commands in the batch file one after the other