wat.c in git
WAT(1)                  FreeBSD General Commands Manual                 WAT(1)

NAME
     wat – watch files

SYNOPSIS
     wat file ... command
     wat file ... -- command [argument ...]

DESCRIPTION
     wat executes the command each time a file is modified.

EXAMPLES
           wat wat.c make
           wat wat.c wake.c -- make wat wake

CAVEATS
     wat does not support Linux since it uses kqueue(2).

FreeBSD 12.0-RELEASE-p10        January 1, 2018       FreeBSD 12.0-RELEASE-p10
/* Copyright (C) 2017  C. McEnroe <june@causal.agency>
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU Affero General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU Affero General Public License for more details.
 *
 * You should have received a copy of the GNU Affero General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */

#include <sys/types.h>

#include <err.h>
#include <fcntl.h>
#include <stdlib.h>
#include <sys/event.h>
#include <sys/wait.h>
#include <sysexits.h>
#include <unistd.h>

static void watch(int kq, char *path) {
	int fd = open(path, O_CLOEXEC);
	if (fd < 0) err(EX_NOINPUT, "%s", path);

	struct kevent event;
	EV_SET(
		&event,
		fd,
		EVFILT_VNODE,
		EV_ADD | EV_CLEAR,
		NOTE_WRITE | NOTE_DELETE,
		0,
		path
	);
	int nevents = kevent(kq, &event, 1, NULL, 0, NULL);
	if (nevents < 0) err(EX_OSERR, "kevent");
}

static void exec(char *const argv[]) {
	pid_t pid = fork();
	if (pid < 0) err(EX_OSERR, "fork");

	if (!pid) {
		execvp(*argv, argv);
		err(EX_NOINPUT, "%s", *argv);
	}

	int status;
	pid = wait(&status);
	if (pid < 0) err(EX_OSERR, "wait");

	if (WIFEXITED(status)) {
		warnx("exit %d\n", WEXITSTATUS(status));
	} else if (WIFSIGNALED(status)) {
		warnx("signal %d\n", WTERMSIG(status));
	} else {
		warnx("status %d\n", status);
	}
}

int main(int argc, char *argv[]) {
	if (argc < 3) return EX_USAGE;

	int kq = kqueue();
	if (kq < 0) err(EX_OSERR, "kqueue");

	int i;
	for (i = 1; i < argc - 1; ++i) {
		if (argv[i][0] == '-') {
			i++;
			break;
		}
		watch(kq, argv[i]);
	}

	exec(&argv[i]);

	for (;;) {
		struct kevent event;
		int nevents = kevent(kq, NULL, 0, &event, 1, NULL);
		if (nevents < 0) err(EX_OSERR, "kevent");

		if (event.fflags & NOTE_DELETE) {
			close(event.ident);
			sleep(1);
			watch(kq, (char *)event.udata);
		}

		exec(&argv[i]);
	}
}