How do I create a new shell in a new terminal using C programming (Linux terminal)?

To create a new shell using C programming in a Linux environment, you need to follow several steps that involve command input, parsing, execution, and handling various system calls. Here’s a detailed guide based on the provided sources:

Step 1: Setting Up the Basic Structure

• Create a directory for your shell project and add the necessary files. For example:

  mkdir my_shell
  cd my_shell
  touch main.c main.h

Step 2: Writing the Header File

• Include the necessary headers in your main.h file:

  #ifndef MAIN_H
  #define MAIN_H
  #include <stdio.h>
  #include <stdlib.h>
  #include <unistd.h>
  #include <sys/types.h>
  #include <sys/wait.h>
  #include <string.h>
  #endif /* MAIN_H */

Step 3: Writing the Main Program

• In your main.c file, implement the basic functionality of the shell. Here is a simplified example:

#include "main.h"

int main(int argc, char *argv[])
{
    (void)argc;
    (void)argv;

    char *buf = NULL;
    size_t count = 0;
    ssize_t nread;
    pid_t child_pid;
    int status;

    while (1) {
        // Print the shell prompt
        write(STDOUT_FILENO, "MyShell$ ", 9);

        // Read the user input
        nread = getline(&buf, &count, stdin);
        if (nread == -1) {
            perror("Exiting shell");
            exit(1);
        }

        // Parse the input (simple example, you may need more complex parsing)
        char *array[100];
        int i = 0;
        array[i] = strtok(buf, " \n");
        while (array[i] != NULL) {
            i++;
            array[i] = strtok(NULL, " \n");
        }

        // Fork a new process
        child_pid = fork();
        if (child_pid == -1) {
            perror("Failed to create.");
            exit(41);
        }

        if (child_pid == 0) {
            // Child process executes the command
            if (execve(array[0], array, NULL) == -1) {
                perror("Failed to execute");
                exit(97);
            }
        } else {
            // Parent process waits for the child to finish
            wait(&status);
        }

        free(buf);
    }

    return 0;
}

Step 4: Compiling and Running the Shell

• Compile your program using:

  gcc -Wall -Wextra -pedantic *.c -o shell

• Run your shell:

  ./shell

Additional Features

Command Parsing

• Use functions like strtok to parse the command line into individual words and arguments12.

Built-in Commands

• Handle built-in commands like cd, exit, etc., by checking if the command matches any built-in commands and executing them accordingly. For example, cd cannot be executed using execvp and needs to be handled using chdir2.

Pipe Handling

• To handle pipes, you need to separate the commands, create a pipe using pipe(), and execute each part of the command in separate child processes. Here is a simplified example:

// Check for pipes
if (strchr(buf, '|') != NULL) {
    // Separate commands
    char *cmd1 = strtok(buf, "|");
    char *cmd2 = strtok(NULL, "|");

    int pipefd2;
    pipe(pipefd);

    child_pid = fork();
    if (child_pid == -1) {
        perror("Failed to create.");
        exit(41);
    }

    if (child_pid == 0) {
        // Child 1: Execute the first command and pipe output
        close(pipefd[0]); // Close reading end
        dup2(pipefd1, STDOUT_FILENO); // Redirect stdout to pipe
        close(pipefd1); // Close writing end
        execve(cmd1, array, NULL);
    } else {
        child_pid = fork();
        if (child_pid == -1) {
            perror("Failed to create.");
            exit(41);
        }

        if (child_pid == 0) {
            // Child 2: Execute the second command and read from pipe
            close(pipefd1); // Close writing end
            dup2(pipefd[0], STDIN_FILENO); // Redirect stdin to pipe
            close(pipefd[0]); // Close reading end
            execve(cmd2, array, NULL);
        } else {
            // Parent: Wait for both children to finish
            wait(&status);
            wait(&status);
        }
    }
}

History and Autocomplete

• For advanced features like command history and autocomplete, you can use the readline library provided by GNU2.

By following these steps, you can create a basic shell in C that can execute commands, handle built-in commands, and manage pipes. For more advanced features, you would need to expand on the parsing, execution, and handling of special characters and built-in commands.