ネタ元: https://twitter.com/yoh2_sdj/status/1111284169978466304

cdsh.c

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

const char *get_dir(int argc, char *argv[])
{
	if(argc <= 1)
	{
		const char *home = getenv("HOME");
		return (home == NULL) ? "/" : home;
	}
	else
	{
		return argv[1];
	}
}

const char *get_sh(void)
{
	const char *sh = getenv("SHELL");
	return (sh == NULL) ? "/bin/sh" : sh;
}

int main(int argc, char *argv[])
{
	const char *dir = get_dir(argc, argv);
	if(chdir(dir) != 0)
	{
		perror(dir);
		return 1;
	}

	const char *sh = get_sh();
	if(execlp(sh, sh, (const char *)NULL) != 0)
	{
		perror(sh);
		return 1;
	}
}