In this assignment, you will write a shell $($a command line interpreter$).$ The shell should operate in this basic way: when you type in a command $($in response to its prompt$),$ the shell creates a child process that executes the command you entered and then prompts for more user input when it has finished. The shells you implement will be similar to$,$ but much simpler than, the one you run every day in Unix. You can find out which shell you are running by typing echo $SHELL at a prompt. You may then wish to look at the man pages for sh or the shell you are running $($more likely tcsh or bash$)$ to learn more about all of the functionality that can be present. For this project, you do not need to implement much functionality; but you will need to be able to handle running multiple commands simultaneously. Your shell can be run in two ways: interactive and batch. In interactive mode, you will display a prompt $($any string of your choosing$)$ and the user of the shell will type in a command at the prompt. In batch mode, your shell is started by specifying a batch file on its command line; the batch file contains the list of commands that should be executed. In batch mode, you should not display a prompt. In batch mode you should echo each line you read from the batch file back to the user before executing it; this will help you when you debug your shells $($and us when we test your programs$).$ In both interactive and batch mode, your shell stops accepting new commands when it sees the quit command on a line or reaches the end of the input stream $($i$.$e$.,$ the end of the batch file or the user types $\text{'}$Ctrl$-$D$\text{'}).$ The shell should then exit after all running processes have terminated. Each line $($of the batch file or typed at the prompt$)$ may contain multiple commands separated with the ; character. Each of the commands separated by a ; should be run simultaneously, or concurrently. $($Note that this is different behavior than standard Linux shells which run these commands one at a time, in order.$)$ The shell should not print the next prompt or take more input until all of these commands have finished executing $($the wait$()$ and$/$or waitpid$()$ system calls may be useful here$).$ For example, the following lines are all valid and have reasonable commands specified:

prompt$>$

prompt$>$ ls

prompt$>/$bin$/$ls

prompt$>$ ls $-$l

prompt$>$ ls $-$l ; cat file

prompt$>$ ls $-$l ; cat file ; grep foo file$2$

For example, on the last line, the commands ls $-$l $,$ cat file and grep foo file$2$ should all be running at the same time; as a result, you may see that their output is intermixed. To exit the shell, the user can type quit. This should just exit the shell and be done with it $($the exit$()$ system call will be useful here$).$ Note that quit is a built$-$in shell command; it is not to be executed like other programs the user types in$.$ If the "quit" command is on the same line with other commands, you should ensure that the other commands execute $($and finish$)$ before you exit your shell. These are all valid examples for quitting the shell.

prompt$>$ quit

prompt$>$ quit ; cat file

prompt$>$ cat file ; quit

This project is not as hard as it may seem at first reading $($or perhaps it doesn't seem that hard at all, which is good!$)$; in fact, the code you write will be much smaller than this specification. Writing your shell in a simple manner is a matter of finding the relevant library routines and calling them properly. Your finished programs will probably be under $200$ lines, including comments. If you find that you are writing a lot of code, it probably means that you are doing something wrong and should take a break from hacking and instead think about what you are trying to do$.$

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