C system() Function | Execute Shell Commands

Introduction

The system() function in C is a standard library function that executes a system command. It is part of the C standard library (stdlib.h). This function is useful for running shell commands from within a C program, allowing the program to interact with the operating system.

system() Function Syntax

The syntax for the system() function is as follows:

int system(const char *command);

Parameters:

• command: A C string that contains the command to be executed. If command is NULL, the system() function checks if a command processor is available.

Returns:

• The function returns an implementation-defined value, typically the exit status of the command executed. If the command is NULL and a command processor is available, it returns a non-zero value; otherwise, it returns 0.

Understanding system() Function

The system() function passes the command string to the host environment to be executed by the command processor. The behavior and return value of this function can vary depending on the system and command executed. It is important to check the return value to determine if the command was executed successfully.

Examples

Executing a Simple Command

To demonstrate how to use system() to execute a simple command, we will write a program that lists the contents of the current directory.

Example

#include <stdio.h>
#include <stdlib.h>

int main() {
    // Execute the "ls" command (or "dir" on Windows)
    int result = system("ls");

    // Check the return value
    if (result == -1) {
        printf("Error: Command processor not available.\n");
        return 1;
    }

    // Print the result of the command
    printf("Command executed with return value: %d\n", result);

    return 0;
}

Output:

[contents of the current directory]
Command executed with return value: 0

Checking the Return Status of a Command

This example shows how to check the return status of a command executed by system().

Example

#include <stdio.h>
#include <stdlib.h>

int main() {
    // Execute a command that is expected to fail
    int result = system("nonexistent_command");

    // Check the return value
    if (result == -1) {
        printf("Error: Command processor not available.\n");
        return 1;
    } else if (result != 0) {
        printf("Command failed with return value: %d\n", result);
    } else {
        printf("Command executed successfully.\n");
    }

    return 0;
}

Output:

sh: nonexistent_command: command not found
Command failed with return value: 32512

Real-World Use Case

Running a Backup Script

In real-world applications, the system() function can be used to run scripts or commands that automate system tasks, such as backing up files.

Example: Running a Backup Script

#include <stdio.h>
#include <stdlib.h>

int main() {
    // Command to run a backup script
    const char *backup_command = "/path/to/backup_script.sh";

    // Execute the backup script
    int result = system(backup_command);

    // Check the return value
    if (result == -1) {
        printf("Error: Command processor not available.\n");
        return 1;
    } else if (result != 0) {
        printf("Backup script failed with return value: %d\n", result);
    } else {
        printf("Backup script executed successfully.\n");
    }

    return 0;
}

Output:

[output of the backup script]
Backup script executed successfully.

Conclusion

The system() function allows a C program to execute a shell command as if it were typed at the command line. This function can be used to perform various system-related tasks, such as file operations, invoking other programs, or managing processes.