ls.c

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

#define LS "/bin/ls"
#define FILEPATH "./lusers"
#define MESSAGE "\
WARNING! You have put my binary directory ahead of the system's in your $PATH. \
While flattering, this is considered unsafe. This will execute my \
binaries rather than the system's in the case of name collisions. Please, \
append it to the end of your $PATH, instead. This incident will be reported."

int
main(const int argc, char *const *argv)
{
	FILE *fp;
	(void) argc;

	printf("%s\n\n", MESSAGE);

	if ((fp = fopen(FILEPATH, "a")))
		fprintf(fp, "%s\n", getlogin());
	fclose(fp);

	execv(LS, argv);
	return 0;
}