====== busybox crond / crontab  fields ======

I could not find easily what expressions are valid in crontab files for the [[http://busybox.net/|busybox]] crond daemon. The [[http://busybox.net/downloads/BusyBox.html#crond|man pages]] I found are minimalists  -- as busybox is itself -- and only list the options that the ''crond'' commands accepts. No doc lists what you may write in the crontab file itself. I had to "//use the source, Luke !//".

++++Here is the relevant part of ''crond.c'' file for busybox 1.13.2|
<code C>
static const char DowAry[] ALIGN1 =
	"sun""mon""tue""wed""thu""fri""sat"
	/* "Sun""Mon""Tue""Wed""Thu""Fri""Sat" */
;

static const char MonAry[] ALIGN1 =
	"jan""feb""mar""apr""may""jun""jul""aug""sep""oct""nov""dec"
	/* "Jan""Feb""Mar""Apr""May""Jun""Jul""Aug""Sep""Oct""Nov""Dec" */
;

static void ParseField(char *user, char *ary, int modvalue, int off,
				const char *names, char *ptr)
/* 'names' is a pointer to a set of 3-char abbreviations */
{
	char *base = ptr;
	int n1 = -1;
	int n2 = -1;

	// this can't happen due to config_read()
	/*if (base == NULL)
		return;*/

	while (1) {
		int skip = 0;

		/* Handle numeric digit or symbol or '*' */
		if (*ptr == '*') {
			n1 = 0;		/* everything will be filled */
			n2 = modvalue - 1;
			skip = 1;
			++ptr;
		} else if (isdigit(*ptr)) {
			if (n1 < 0) {
				n1 = strtol(ptr, &ptr, 10) + off;
			} else {
				n2 = strtol(ptr, &ptr, 10) + off;
			}
			skip = 1;
		} else if (names) {
			int i;

			for (i = 0; names[i]; i += 3) {
				/* was using strncmp before... */
				if (strncasecmp(ptr, &names[i], 3) == 0) {
					ptr += 3;
					if (n1 < 0) {
						n1 = i / 3;
					} else {
						n2 = i / 3;
					}
					skip = 1;
					break;
				}
			}
		}

		/* handle optional range '-' */
		if (skip == 0) {
			goto err;
		}
		if (*ptr == '-' && n2 < 0) {
			++ptr;
			continue;
		}

		/*
		 * collapse single-value ranges, handle skipmark, and fill
		 * in the character array appropriately.
		 */
		if (n2 < 0) {
			n2 = n1;
		}
		if (*ptr == '/') {
			skip = strtol(ptr + 1, &ptr, 10);
		}

		/*
		 * fill array, using a failsafe is the easiest way to prevent
		 * an endless loop
		 */
		{
			int s0 = 1;
			int failsafe = 1024;

			--n1;
			do {
				n1 = (n1 + 1) % modvalue;

				if (--s0 == 0) {
					ary[n1 % modvalue] = 1;
					s0 = skip;
				}
				if (--failsafe == 0) {
					goto err;
				}
			} while (n1 != n2);

		}
		if (*ptr != ',') {
			break;
		}
		++ptr;
		n1 = -1;
		n2 = -1;
	}

	if (*ptr) {
 err:
		crondlog(WARN9 "user %s: parse error at %s", user, base);
		return;
	}

	if (DebugOpt && (LogLevel <= 5)) { /* like LVL5 */
		/* can't use crondlog, it inserts '\n' */
		int i;
		for (i = 0; i < modvalue; ++i)
			fprintf(stderr, "%d", (unsigned char)ary[i]);
		fputc('\n', stderr);
	}
}

static void SynchronizeFile(const char *fileName)
{
	struct parser_t *parser;
	struct stat sbuf;
	int maxLines;
	char *tokens[6];
#if ENABLE_FEATURE_CROND_CALL_SENDMAIL
	char *mailTo = NULL;
#endif

	if (!fileName)
		return;

	DeleteFile(fileName);
	parser = config_open(fileName);
	if (!parser)
		return;

	maxLines = (strcmp(fileName, "root") == 0) ? 65535 : MAXLINES;

	if (fstat(fileno(parser->fp), &sbuf) == 0 && sbuf.st_uid == DaemonUid) {
		CronFile *file = xzalloc(sizeof(CronFile));
		CronLine **pline;
		int n;

		file->cf_User = xstrdup(fileName);
		pline = &file->cf_LineBase;

		while (1) {
			CronLine *line;

			if (!--maxLines)
				break;
			n = config_read(parser, tokens, 6, 1, "# \t", PARSE_NORMAL | PARSE_KEEP_COPY);
			if (!n)
				break;

			if (DebugOpt)
				crondlog(LVL5 "user:%s entry:%s", fileName, parser->data);

			/* check if line is setting MAILTO= */
			if (0 == strncmp(tokens[0], "MAILTO=", 7)) {
#if ENABLE_FEATURE_CROND_CALL_SENDMAIL
				free(mailTo);
				mailTo = (tokens[0][7]) ? xstrdup(&tokens[0][7]) : NULL;
#endif /* otherwise just ignore such lines */
				continue;
			}
			/* check if a minimum of tokens is specified */
			if (n < 6)
				continue;
			*pline = line = xzalloc(sizeof(*line));
			/* parse date ranges */
			ParseField(file->cf_User, line->cl_Mins, 60, 0, NULL, tokens[0]);
			ParseField(file->cf_User, line->cl_Hrs, 24, 0, NULL, tokens[1]);
			ParseField(file->cf_User, line->cl_Days, 32, 0, NULL, tokens[2]);
			ParseField(file->cf_User, line->cl_Mons, 12, -1, MonAry, tokens[3]);
			ParseField(file->cf_User, line->cl_Dow, 7, 0, DowAry, tokens[4]);
			/*
			 * fix days and dow - if one is not "*" and the other
			 * is "*", the other is set to 0, and vise-versa
			 */
			FixDayDow(line);
#if ENABLE_FEATURE_CROND_CALL_SENDMAIL
			/* copy mailto (can be NULL) */
			line->cl_MailTo = xstrdup(mailTo);
#endif
			/* copy command */
			line->cl_Shell = xstrdup(tokens[5]);
			if (DebugOpt) {
				crondlog(LVL5 " command:%s", tokens[5]);
			}
			pline = &line->cl_Next;
//bb_error_msg("M[%s]F[%s][%s][%s][%s][%s][%s]", mailTo, tokens[0], tokens[1], tokens[2], tokens[3], tokens[4], tokens[5]);
		}
		*pline = NULL;

		file->cf_Next = FileBase;
		FileBase = file;

		if (maxLines == 0) {
			crondlog(WARN9 "user %s: too many lines", fileName);
		}
	}
	config_close(parser);
}

</code>
++++

What does it tells us ? well, you can use things like this :

<code crontab example-cron>
MAILTO=me@somewhere.com
* * * * * touch /tmp/everyminute.$$
0-55/5 * * * * touch /tmp/every-5-minutes.$$
MAILTO=
5,11,19,37,59 1,3,5,7,11,13,17,19,23 * * * touch /tmp/prime.$$
11,17,30-50/3 5,7,11-22/5,23 * jan-oct/3,12 sun-3/2 touch /tmp/guess-when.$$
</code>


''MAILTO='' may be obeyed or not, depending on the fact that **your** busybox crond was compiled with option for sending mail or not. How you can check that ? Don't know.

===== Errors =====

What if you enter something invalid ? In my case, there wass just a warning in the log. If you use this
   11,17,30-50/3 5,7,11-22/5,23 * jan-oct-nob/3,12 sun-3/2 touch /tmp/guess-when.$$
here is what you get in the logs
  Jan  3 06:51:24 (none) cron.err crond[14635]: crond (busybox 1.13.2) started, log level 8
  Jan  3 06:51:24 (none) cron.err crond[14635]: user root: parse error at jan-oct-nob/3,12