[Solved] EECS3221-Mini Proj1 Shell

1. To dust off your C programming skills.2. To design and implement a simple shell.

This project consists of designing a C program to serve as a shell interface that accepts user commands and thenexecutes each command in a separate process. Your implementation will support input and output redirection, as well aspipes as a form of IPC between a pair of commands. Completing this project will involve using the UNIX `fork()`, `exec()`,`wait()`, `dup2()`, and `pipe()` system calls.

### overviewA shell interface gives the user a prompt, after which the next command is entered. The example below illustrates theprompt `mysh:~$ ` and the users next command: `cat prog.c`. (This command displays the file `prog.c` on the terminal using theUNIX `cat` command.)

`mysh:~$ cat prog.c`

One technique for implementing a shell interface is to have the parent process first read what the user enters on thecommand line (in this case, `cat prog.c`) and then create a separate child process that performs the command. Unlessotherwise specified, the parent process waits for the child to exit before continuing. This is similar infunctionality to the new process creation illustrated in Figure 3.9. However, UNIX shells typically also allow thechild process to run in the background, or concurrently. To accomplish this, we add an ampersand `&` at the end ofthe command. Thus, if we rewrite the above command as

`mysh:~$ cat prog.c &`

the parent and child processes will run concurrently. The separate child process is created using the `fork()` systemcall, and the users command is executed using one of the system calls in the `exec()` family (as described in Section3.3.1). A C program that provides the general operations of a command-line shellis supplied in the code block bellow (Figure 3.36 in the book). The `main()` function presents the prompt `mysh:~$ ` andoutlines the steps to be taken after input from the user has been read. The `main()` function continually loops as longas `should_run` equals 1; when the user enters exit at the prompt, your program will set `should_run` to 0 and terminate.

`c#include <stdio.h>#include <unistd.h>#define MAX LINE 80 /* The maximum length command */

int main(void) {char *args[MAX LINE/2 + 1]; /* command line arguments */int should_run = 1; /* flag to determine when to exit program */

while (should_run){printf(mysh:~$ );fflush(stdout);

/** After reading user input, the steps are:* (1) fork a child process using fork()* (2) the child process will invoke execvp()* (3) parent will invoke wait() unless command included &*/}

return 0;}`This project is organized into several parts:1. Creating the child process and executing the command in the child2. Providing a history feature3. Adding support of input and output redirection4. Allowing the parent and child processes to communicate via a pipe

### Executing Command in a Child Process

The first task is to modify the `main()` function in code above so that a child process is forked and executes thecommand specified by the user. This will require parsing what the user has entered into separate tokens and storingthe tokens in an array of character strings (args in the code). For example, if the user enters the command `ps -ael`at the `mysh:~$ ` prompt, the values stored in the args array are:`args[0] = psargs[1] = -aelargs[2] = NULL`This args array will be passed to the `execvp()` function, which has the following prototype:

`cexecvp(char *command, char *params[]);`Here, command represents the command to be performed and params stores the parameters to this command. For thisProject, the `execvp()` function should be invoked as execvp(args[0], args). Be sure to check whether the userincluded `&` to determine whether or not the parent process is to wait for the child to exit.

### Creating a History FeatureThe next task is to modify the shell interface program so that it provides a **history** feature to allow a user toexecute the most recent command by entering `!!`. For example, if a user enters the command `ls -l`, she can then executethat command again by entering `!!` at the prompt. Any command executed in this fashion should be *echoed* on the usersscreen, and the command should also be placed in the history buffer as the next command. Your program should alsomanage basic error handling. If there is no recent command in the history, entering `!!` should result in a messageNo commands in history.

### Redirecting Input and OutputYour shell should then be modified to support the `>` and `<` redirection operators, where `>` redirects the outputof a command to a file and `<` redirects the input to a command from a file. For example, if a user enters`mysh:~$ ls > out.txt` the output from the ls command will be redirected to the file `out.txt`. Similarly, input can beredirected as well. For example, if the user enters `mysh:~$ sort < in.txt` the file `in.txt` will serve as input to the`sort` command. Managing the redirection of both input and output will involve using the `dup2()` function, whichduplicates an existing file descriptor to another file descriptor. For example, if fd is a file descriptor to thefile `out.txt`, the call `dup2(fd, STDOUT FILENO);` duplicates fd to standard output (the terminal). This means thatany writes to standard output will in fact be sent to the `out.txt` file. You can assume that commands will containeither one input or one output redirection and will not contain both. In other words, you do **not** have to be concernedwith command sequences such as `sort < in.txt > out.txt.`

### Communication via a PipeThe final modification to your shell is to allow the output of one command to serve as input to another using a`pipe`. For example, the following command sequence `mysh:~$ ls -l | less` has the output of the command `ls -l`serve as the input to the less command. Both the `ls` and `less` commands will run as separate processes and willcommunicate using the UNIX `pipe()` function described in Section 3.7.4. Perhaps the easiest way to create theseseparate processes is to have the parent process create the child process (which will execute `ls -l`). This child willalso create another child process (which will execute `less`) and will establish a pipe between itself and the childprocess it creates. Implementing pipe functionality will also require using the `dup2()` function as described in theprevious section. Finally, although several commands can be chained together using multiple pipes, you can assumethat commands will contain only one pipe character and will not be combined with any redirection operators.

### EvaluationYou need to submit the C source code to your shell, and a Makefile that compiles the shell in the Moodle. The TA willuse your Makefile to compile and run your program in the course Virtual Machine (VM).[Here is a quick tutorial on makefile](http://www.cs.colby.edu/maxwell/courses/tutorials/maketutor/), in case you arenot familiar. The TAs then grade your project according to the following table:

| Item | Grade || – |:-:|| Accurate Makefile and no error in compilation | 2 || Simple shell commands | 2 || History feature | 2 || Handle redirection | 2 || Handle pipe | 2 || **Sum** | **10** |