C Execution methods

Summary

Method	Advantages
system Function	- Simplicity: It’s straightforward to use. - No need to manage child processes manually.
exec Family of Functions	- Control: Allows fine-grained control over the command execution. - No need for a shell.
- execl Subtype	- Specify the command and its arguments as separate parameters. - Explicit argument list.
- execv Subtype	- Pass the command and its arguments as an array. - Flexibility for variable arguments.
- execve Subtype	- Pass the command and its arguments as an array. - Also specify the env variables
- execvp Subtype	- Pass the command and its arguments as an array. - Search for program in PATH
- execvpe Subtype	- Pass the command and its arguments as an array. - Search for program in PATH, and specify the env variables - Not part of the C standard library but is available on Unix like systems
popen Function	- Bidirectional: Can capture both the output and input of the command. - Streamlined I/O.
fork and exec	- Customization: Provides control over the child process. - Suitable for complex scenarios.
posix_spawn Function	- Efficiency: More efficient for spawning processes in POSIX systems. - Fine-grained control.

System

#include <stdlib.h>

int main() {
    system("ls -l");
    return 0;
}

Exec Familly

Execl

#include <unistd.h>

int main() {
    execl("/bin/ls", "ls", "-l", NULL);
    return 0;
}

Execv

#include <unistd.h>

int main() {
    char *args[] = {"/bin/ls", "-l", NULL};
    execv(args[0], args);
    return 0;
}

Execve

#include <unistd.h>

int main() {
    char *args[] = {"/bin/ls", "-l", NULL};
    char *env[] = {"PATH=/usr/bin", "USER=john", NULL};
    execve(args[0], args, env);
    return 0;
}

Execvp

#include <stdio.h>
#include <unistd.h>

int main() {
    char *args[] = {"ls", "-l", NULL};
    execvp(args[0], args);
    return 0;
}

Execvpe

#define _GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

int main() {
    char *args[] = {"ls", "-l", NULL};
    char *envp[] = {"PATH=/bin:/usr/bin", NULL};
    execvpe(args[0], args, envp);
    return 0;
}

Popen

#include <stdio.h>

int main() {
    FILE *fp = popen("ls -l", "r");
    if (fp) {
        char buffer[256];
        while (fgets(buffer, sizeof(buffer), fp) != NULL) {
            printf("%s", buffer);
        }
        pclose(fp);
    }
    return 0;
}

Fork

#include <stdio.h>
#include <unistd.h>
#include <sys/wait.h>

int main() {
    pid_t pid = fork();
    if (pid == 0) { // Child process
        execl("/bin/ls", "ls", "-l", NULL);
    } else if (pid > 0) { // Parent process
        wait(NULL); // Wait for the child to complete
    } else {
        perror("fork");
        return 1;
    }
    return 0;
}

Posix Spawn

#define _POSIX_C_SOURCE 200809L
#include <stdio.h>
#include <spawn.h>
#include <unistd.h>
#include <sys/wait.h>

int main() {
    char *const cmd[] = {"ls", "-l", NULL};
    posix_spawn_file_actions_t actions;
    posix_spawn_file_actions_init(&actions);

    pid_t pid;
    if (posix_spawn(&pid, "/bin/ls", &actions, NULL, cmd, NULL) == 0) {
        wait(NULL); // Wait for the child to complete
    } else {
        perror("posix_spawn");
        return 1;
    }

    return 0;
}