UNIX Shell

Please code in C++ and the starter code provided, the main method cannot be void.

This project consists of designing a C program to serve as a shell interface that accepts user commands and then executes each command in a separate process. Your implementation will support input and output redirection, as well as pipes 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 and can be completed on any Linux, UNIX, or macOS system.

I. Overview

A shell interface gives the user a prompt, after which the next command is entered. The example below illustrates the prompt osh> and the users next command: cat prog.c. (This command displays the file prog.c on the terminal using the UNIX cat command.)

 osh > cat prog.c 

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

 osh > cat prog.c & 

the parent and child processes will run concurrently. The separate child process is created using the fork() system call, and the users command is executed using one of the system calls in the exec() family (as described in Section 3.3.1).

A C program that provides the general operations of a command-line shell is supplied in Figure 3.36. The main() function presents the prompt osh-> and outlines the steps to be taken after input from the user has been read. The main() function continually loops as long as should run equals 1; when the user enters exit at the prompt, your program will set should run to 0 and terminate.

#include  #include  #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("osh>"); 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; }

Figure 3.36 Outline of simple shell.

1. Creating the child process and executing the command in the child 2. Providing a history feature 3. Adding support of input and output redirection 4. Allowing the parent and child processes to communicate via a pipe 5. This project is organized into several parts:

Project 2 - Unix Shell

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II. Executing Command in a Child Process

The first task is to modify the main() function in Figure 3.36 so that a child process is forked and executes the command specified by the user. This will require parsing what the user has entered into separate tokens and storing the tokens in an array of character strings (args in Figure 3.36). For example, if the user enters the command

ps -ael at the osh > prompt, the values stored in the args array are:

 args[0] = "ps" args[1] = "-ael" args[2] = NULL 

This args array will be passed to the execvp() function, which has the following prototype: execvp(char *command, char *params[])

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

III. Creating a History Feature

The next task is to modify the shell interface program so that it provides a history feature to allow a user to execute the most recent command by entering !!.For example, if a user enters the command ls -l, she can then execute that command again by entering !! at the prompt. Any command executed in this fashion should be echoed on the users screen, and the command should also be placed in the history buffer as the next command.

Your program should also manage basic error handling. If there is no recent command in the history, entering!!should result in a message No commands in history.

IV. Redirecting Input and Output

Your shell should then be modified to support the > and < redirection operators, where > redirects the output of a command to a file and < redirects the input to a command from a file. For example, if a user enters

osh > ls > out.txt the output from the ls command will be redirected to the file out.txt. Similarly, input can be redirected as well. For

example, if the user enters

 osh > 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, which duplicates an existing file descriptor to another file descriptor. For example, if fd is a file descriptor to the file out.txt, the call

dup2(fd, STDOUT FILENO); duplicates fd to standard output (the terminal). This means that any writes to standard output will in fact be sent to the

out.txt file. You can assume that commands will contain either one input or one output redirection and will not contain both. In other

words, you do not have to be concerned with command sequences such as sort < in.txt > out.txt.

Starter Code:

/** * Assignment 2: Simple UNIX Shell * @file pcbtable.h * @author * @brief This is the main function of a simple UNIX Shell. You may add additional functions in this file for your implementation * @version 0.1 */ // You must complete the all parts marked as "TODO". Delete "TODO" after you are done. // Remember to add sufficient and clear comments to your code

#include #include #include #include #include #include #include

using namespace std;

#define MAX_LINE 80 // The maximum length command

/** * @brief parse out the command and arguments from the input command separated by spaces * * @param command * @param args * @return int */ int parse_command(char command[], char *args[]) { // TODO: implement this function }

// TODO: Add additional functions if you need

/** * @brief The main function of a simple UNIX Shell. You may add additional functions in this file for your implementation * @param argc The number of arguments * @param argv The array of arguments * @return The exit status of the program */ int main(int argc, char *argv[]) { char command[MAX_LINE]; // the command that was entered char *args[MAX_LINE / 2 + 1]; // parsed out command line arguments int should_run = 1; /* flag to determine when to exit program */

// TODO: Add additional variables for the implementation.

while (should_run) { printf("osh>"); fflush(stdout); // Read the input command fgets(command, MAX_LINE, stdin); // Parse the input command int num_args = parse_command(command, args);

// TODO: Add your code for the implementation /** * 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; }