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The following was released under the licenses of BSD and Caldera.

Here are extracts from the 3BSD (03/'80) variant of the Bourne shell, usr/src/cmd/sh/service.c.

LOCAL STRING	execs(ap,t)
	STRING		ap;
	REG STRING	t[];
{
	[...]
	execve(p, &t[0] ,xecenv);
	SWITCH errno IN
	    case ENOEXEC:
		[...]

		/* band aid to get csh... 2/26/79 */
		{
			char c;
			if (!isatty(input)) {
				read(input, &c, 1);
				if (c == '#')
					gocsh(t, xecenv);
				lseek(input, (long) 0, 0);
			}
		}
[...]
}

gocsh(t, xecenv)
	register char **t, **xecenv;
{
	char **newt[1000];
	register char **p;
	register int i;

	for (i = 0; t[i]; i++)
		newt[i+1] = t[i];
	newt[i+1] = 0;
	newt[0] = "/bin/csh";
	execve("/bin/csh", newt, xecenv);
}