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let's build a linux shell part ii

CyberChef answered on January 2, 2024 Popularity 2/10 Helpfulness 8/10

answer let's build a linux shell part ii

let's build a linux shell part ii

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To continue building a Linux shell 
(assuming you've already started with the basics in Part I)

we'll focus on adding more advanced features. 

**1. Handling Built-in Commands:**
First, let's add support for built-in shell commands like `cd`, `exit`, and `help`.

```c
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>

void shellLoop();
char *readLine();
char **splitLine(char *);
int executeCommand(char **);
int shell_cd(char **);
int shell_help(char **);
int shell_exit(char **);

// List of builtin commands, followed by their corresponding functions.
char *builtin_str[] = {
  "cd",
  "help",
  "exit"
};

int (*builtin_func[]) (char **) = {
  &shell_cd,
  &shell_help,
  &shell_exit
};

int num_builtins() {
  return sizeof(builtin_str) / sizeof(char *);
}

// Implementations of builtin functions.
int shell_cd(char **args) {
  if (args[1] == NULL) {
    fprintf(stderr, "Expected argument to \"cd\"\n");
  } else {
    if (chdir(args[1]) != 0) {
      perror("shell");
    }
  }
  return 1;
}

int shell_help(char **args) {
  int i;
  printf("Your shell implementation\n");
  printf("Type program names and arguments, and hit enter.\n");
  printf("The following are built in:\n");

  for (i = 0; i < num_builtins(); i++) {
    printf("  %s\n", builtin_str[i]);
  }

  return 1;
}

int shell_exit(char **args) {
  return 0;
}
```

**2. Command Execution:**
Modify the `executeCommand` function to handle the built-in commands.

```c
int executeCommand(char **args) {
  int i;

  if (args[0] == NULL) {
    // An empty command was entered.
    return 1;
  }

  for (i = 0; i < num_builtins(); i++) {
    if (strcmp(args[0], builtin_str[i]) == 0) {
      return (*builtin_func[i])(args);
    }
  }

  // Code for launching programs...
}

// ... Rest of your shell implementation
```

**3. Redirection and Pipes:**
Handling redirection (`>`, `<`) and pipes (`|`) is more complex. You'll need to parse the command line for these symbols and adjust the file descriptors accordingly before executing a command.

This is a broad topic and may require a thorough understanding of file descriptors, `fork()`, `exec()`, `dup2()`, and pipe mechanisms in Unix/Linux.

**4. Background Processing:**
To allow a command to run in the background, you need to look for an `&` at the end of the command line. If it's present, you should run this command in a child process without waiting for it to complete.

Here is a simplified version of how you might implement this:

```c
// In your command execution function:
pid_t pid, wpid;
int status;

pid = fork();
if (pid == 0) {
  // Child process
  if (execvp(args[0], args) == -1) {
    perror("shell");
  }
  exit(EXIT_FAILURE);
} else if (pid < 0) {
  // Error forking
  perror("shell");
} else {
  // Parent process
  if (background_process) {
    printf("Process running in background: %d\n", pid);
  } else {
    do {
      wpid = waitpid(pid, &status, WUNTRACED);
    } while (!WIFEXITED(status) && !WIFSIGNALED(status));
  }
}
```

Please note, this is a very high-level overview. Building a shell involves many intricate details, particularly around process management and file IO. For a full implementation, you would need to handle various edge cases, potential security issues, and more complex features like job control.

Popularity 2/10 Helpfulness 8/10 Language shell

Source: Grepper

Tags: build linux shell

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Contributed on Jan 02 2024

CyberChef

0 Answers Avg Quality 2/10

let's build a linux shell part ii

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