cd.c

#include <err.h>
#include <stdlib.h>
#include <unistd.h>

int main(int argc, char *argv[])
{
    char *shell;
    if (argc != 2)
        errx(1, "Usage: cd /some/path");
    if (chdir(argv[1]) == -1)
        err(1, "chdir");
    shell = getenv("SHELL");
    if (!shell || !*shell)
        shell = "/bin/sh";
    execl(shell, shell, NULL);
    err(1, "exec");
}