Mimic the Linux adduser command in C

The adduser command in Linux is used to add new user accounts on Unix-like operating systems. System administrators frequently use it to create new users with predetermined usernames, passwords, and other user-related information. This article demonstrates how to mimic this functionality using C programming and system calls.

System Calls Overview

System calls allow software to communicate with the operating system kernel, which manages system resources and provides services to user-level programs. In C programming, system calls provide access to OS features including file I/O, process management, and network connectivity.

Essential System Calls

open Opens or creates a file and returns a file descriptor for other file operations.

int fd = open("filename.txt", O_RDWR);
if (fd == -1) {
    perror("open");
}

read Reads data from a file descriptor into a buffer.

char buffer[1024];
int num_read = read(fd, buffer, sizeof(buffer));
if (num_read == -1) {
    perror("read");
}

write Writes data from a buffer to a file descriptor.

char data[] = "Hello World";
int num_written = write(fd, data, strlen(data));
if (num_written == -1) {
    perror("write");
}

close Closes a file descriptor.

close(fd);

fork Creates a duplicate process, enabling concurrent execution of multiple tasks.

pid_t pid = fork();
if (pid == -1) {
    perror("fork");
} else if (pid == 0) {
    // child process code
} else {
    // parent process code
}

Steps to Implement Linux adduser in C

To mimic the Linux adduser command, a C program must perform operations similar to those when adding new user accounts. Here are the key steps:

Step 1: Accept User Input

Use C's input/output functions to collect new user details including username and password.

char username[50];
char password[50];
printf("Enter username: ");
scanf("%s", username);
printf("Enter password: ");
scanf("%s", password);

Step 2: Validate Input

Perform input validation to ensure data accuracy and compliance with system requirements.

if (strlen(password) < 8) {
    printf("Password should be at least 8 characters long.<br>");
    exit(1);
}
if (strlen(username) == 0) {
    printf("Username cannot be empty.<br>");
    exit(1);
}

Step 3: Create User Account

Execute system commands to create the new user account using the useradd command.

char command[200];
sprintf(command, "useradd -m -s /bin/bash %s", username);
int status = system(command);
if (status == -1) {
    printf("Error creating user account.<br>");
    exit(1);
}

Step 4: Set User Password

Set the password for the newly created user using the chpasswd command.

sprintf(command, "echo '%s:%s' | chpasswd", username, password);
status = system(command);
if (status != 0) {
    printf("Error setting password.<br>");
    exit(1);
}

Step 5: Configure User Permissions

Set appropriate permissions for the user's home directory using system calls like chown and chmod.

char home_path[100];
sprintf(home_path, "/home/%s", username);
chmod(home_path, 0755);

Step 6: Additional Configuration

Configure additional user settings such as group membership or shell preferences.

sprintf(command, "usermod -aG users %s", username);
system(command);

Step 7: Display Results

Provide feedback about the success or failure of the user creation process.

printf("User '%s' created successfully.<br>", username);
printf("Home directory: /home/%s<br>", username);

Complete Implementation Example

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/stat.h>
#include <unistd.h>

int main() {
    char username[50];
    char password[50];
    char command[200];
    int status;
    
    // Get user input
    printf("Enter username: ");
    scanf("%49s", username);
    printf("Enter password: ");
    scanf("%49s", password);
    
    // Validate input
    if (strlen(password) < 8) {
        printf("Password must be at least 8 characters long.<br>");
        return 1;
    }
    
    // Create user account
    sprintf(command, "useradd -m -s /bin/bash %s", username);
    status = system(command);
    if (status != 0) {
        printf("Error creating user account.<br>");
        return 1;
    }
    
    // Set password
    sprintf(command, "echo '%s:%s' | chpasswd", username, password);
    status = system(command);
    if (status != 0) {
        printf("Error setting password.<br>");
        return 1;
    }
    
    printf("User '%s' created successfully!<br>", username);
    return 0;
}

Key Considerations

When implementing this functionality, consider portability by using standard C libraries and avoiding platform-specific code. Handle errors carefully and avoid hardcoding file paths. Use configuration files or environment variables for dynamic directory selection. Always validate user input to prevent security vulnerabilities.

Note that this program requires root privileges to execute system commands like useradd and chpasswd. In production environments, implement proper authentication and authorization mechanisms.

Conclusion

Mimicking the Linux adduser command in C involves using system calls and library functions to create user accounts, set passwords, and manage user information. This approach demonstrates how C programs can interact with the operating system to perform administrative tasks while maintaining security and error handling best practices.