Command Shell

project is to write a simple shell - myshell - that has the following properties: 1. The shell must support the following internal commands: i. cd - change the current default directory to . If the argument is not present, report the current directory. If the directory does not exist an appropriate error should be reported. This command should also change the PWD environment variable. ii. clr - clear the screen. iii. dir - list the contents of directory iv. echo - display on the display followed by a new line (multiple spaces/tabs may be reduced to a single space) v. help - display the user manual using the more filter vi. pause - pause operation of the shell until 'Enter' is pressed vii. quit - quit the shell 2. The shell must be able to take its command line input from a file. i.e. if the shell is invoked with a command line argument: myshell batchfile then batchfile is assumed to contain a set of command lines for the shell to process. When the end-of -file is reached, the shell should exit. Obviously, if the shell is invoked without a command line argument it solicits input from the user via a prompt on the display. 3. The shell must support i/o-redirection on either or both stdin and/or stdout. i.e. the command line: programname arg1 arg2 outputfile will execute the program programname with arguments arg1 and arg2, the stdin FILE stream replaced by inputfile and the stdout FILE stream replaced by outputfile. stdout redirection should also be possible for the internal commands: dir, environ, echo, & help. With output redirection, if the redirection character is > then the outputfile is created if it does not exist and truncated if it does. If the redirection token is >> then outputfile is created if it does not exist and appended to if it does. 4. The shell must support background execution of programs. An ampersand (&) at the end of the command line indicates that the shell should return to the command line prompt immediately after launching that program. 5. The command line prompt must contain the pathname of the current directory.