C - minimal shell using fork and execvp

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <limits.h>     /* for PATH_MAX */
#include <signal.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>

enum {BASE = 10, ARGSIZE = 20, BUF_SIZE = 1024};    /* constants */

void execute (char **args);
void cd (char *directory);
int killpid (char *pitstr, int sig);

int main (void) {

    char line[BUF_SIZE] = "",
        *args[ARGSIZE],
        *delim = " \n",
        *token;
    int argIndex;

    while (1) {

        argIndex = 0;                   /* reinitialize variables */
        for (int i = 0; i < ARGSIZE; i++)
            args[i] = NULL;

        printf ("shell> ");             /* prompt */

        if (!fgets (line, BUF_SIZE, stdin)) {
            fprintf (stderr, "Input canceled - EOF received.\n");
            return 0;
        }
        if (*line == '\n')              /* Enter alone - empty line */
            continue;

        for (token = strtok (line, delim); token; token = strtok (NULL, delim))
            args[argIndex++] = token;

        if (!argIndex) continue;        /* validate at least 1 arg */

        if (strcmp (args[0], "quit") == 0 || strcmp (args[0], "exit") == 0)
            break;

        /* handle 'cd' or 'kill' separately */
        if ((strcmp (args[0], "cd") == 0))
            cd (args[1]);
        else if ((strcmp (args[0], "kill") == 0)) {
            if (args[1]) killpid (args[1], SIGTERM);
        }
        else
            execute (args);

    }
    return 0;
}

void execute (char **args)
{
    pid_t pid, status;
    pid = fork ();

    if (pid < 0) {
        perror ("fork");
        return;
    }
    else if (pid > 0) {
        while (wait (&status) != pid)
            continue;
    }
    else if (pid == 0) {
        if (execvp (args[0], args) == -1) {
            perror ("execvp");
        }
        _exit (EXIT_FAILURE);   /* must _exit after execvp return */
    }
}

void cd (char *directory)
{
    char dir[PATH_MAX] = "";

    if (!directory) {           /* handle 'cd' alone */
        directory = getenv ("HOME");
        if (chdir (directory))
            fprintf (stderr, "error: chdir (%s)\n", directory);
        else
            printf ("%s\n", directory);
        return;
    }

    if (*directory == '~') {    /* handle cd ~/stuff */
        strcpy (dir, getenv ("HOME"));
        strcat (dir, "/");
        strcat (dir, directory + 2);
        if (chdir (dir))
            fprintf (stderr, "error: chdir (%s)\n", dir);
        else
            printf ("%s\n", dir);
        return;
    }

    if (chdir (directory))      /* handle given directory */
        fprintf (stderr, "Failed to enter directory: %s\n", directory);
    else
        printf ("%s\n", directory);
}

int killpid (char *pidstr, int sig)
{
    errno = 0;  /* set errno to validate strtol conversion */
    pid_t pid = (pid_t)strtol (pidstr, NULL, BASE);

    if (errno) {
        perror ("strtol");
        return -1;
    }
    else if (pid < 1) { /* validate pid */
        fprintf (stderr, "warning: requested pid < 1, ignoring\n");
        return (int)pid;
    }

    /* temporary print of what would be done */
    printf (" killing pid '%d' with signal '%d'\n", (int)pid, sig);
    // return kill (pid, sig);  /* uncomment for actual use */

    return 0;
}