cmd args, environment variables, getting pid and ppid, simple fork

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

int main(int argc, char **argv)
{
    printf("argc = %d\n\n", argc);
    for (int i = 0; i < argc; i++)
        printf("argv[%d]: %s\n", i, argv[i]);
    return 0;
}

#include <stdio.h>

int main(int argc, char **argv, char **env){
    while (*env != 0)
        printf("%s\n", *env++);
    return 0;
}

#include <stdlib.h>
#include <stdio.h>
#define NAME "PATH"

int main(){
    char *s;
    s = getenv(NAME);
    (s == 0) ? printf("Variable not defined!\n") : printf("%s = %s\n", NAME, s);
    return 0;
}


#include <stdio.h>

extern char **environ;

int main(){
    for (int i = 0; environ[i] != 0; i++)
        printf("%s\n", environ[i]);
    return 0;
}

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

int main(){
    pid_t pid, ppid;
    pid = getpid();
    ppid = getppid();
    printf("PID = %d, PPID = %d\n", pid, ppid);
    return 0;
}

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

int main(){
    pid_t pid;
    printf("Parent\n");
    pid = fork();
    if (pid == 0)
        printf("Hello, child %d\n", getpid());
    else
        printf("Hello, parent %d\n", getpid());
    return 0;
}