Index: usr.sbin/cron/cron/cron.h
===================================================================
--- usr.sbin/cron/cron/cron.h
+++ usr.sbin/cron/cron/cron.h
@@ -193,6 +193,7 @@
 #define	INTERVAL	0x40
 #define	DONT_LOG	0x80
 #define	MAIL_WHEN_ERR	0x100
+#define SINGLE_JOB	0x200
 	time_t	lastrun;
 } entry;
 
@@ -222,7 +223,8 @@
 		open_logfile(void),
 		sigpipe_func(void),
 		job_add(entry *, user *),
-		do_command(entry *, user *),
+		job_remove(entry *, user *),
+		job_exit(pid_t),
 		link_user(cron_db *, user *),
 		unlink_user(cron_db *, user *),
 		free_user(user *),
@@ -261,6 +263,8 @@
 
 FILE		*cron_popen(char *, char *, entry *, PID_T *);
 
+pid_t		do_command(entry *, user *);
+
 
 				/* in the C tradition, we only create
 				 * variables for the main program, just
Index: usr.sbin/cron/cron/cron.c
===================================================================
--- usr.sbin/cron/cron/cron.c
+++ usr.sbin/cron/cron/cron.c
@@ -501,6 +501,7 @@
 			return;
 		default:
 			find_interval_entry(pid);
+			job_exit(pid);
 			Debug(DPROC,
 				("[%d] sigchld...pid #%d died, stat=%d\n",
 				getpid(), pid, WEXITSTATUS(waiter)))
Index: usr.sbin/cron/cron/do_command.c
===================================================================
--- usr.sbin/cron/cron/do_command.c
+++ usr.sbin/cron/cron/do_command.c
@@ -44,7 +44,7 @@
 
 extern char	*environ;
 
-void
+pid_t
 do_command(e, u)
 	entry	*e;
 	user	*u;
@@ -79,9 +79,12 @@
 			e->lastexit = 0;
 			e->child = pid;
 		}
+		if ((e->flags & SINGLE_JOB) == 0)
+			pid = -1;
 		break;
 	}
 	Debug(DPROC, ("[%d] main process returning to work\n", getpid()))
+	return (pid);
 }
 
 
Index: usr.sbin/cron/cron/job.c
===================================================================
--- usr.sbin/cron/cron/job.c
+++ usr.sbin/cron/cron/job.c
@@ -21,18 +21,19 @@
 #endif
 
 
+#include <sys/queue.h>
 #include "cron.h"
 
 
 typedef	struct _job {
-	struct _job	*next;
-	entry		*e;
-	user		*u;
+	STAILQ_ENTRY(_job)	next;
+	entry			*e;
+	user			*u;
+	pid_t			pid;
 } job;
 
 
-static job	*jhead = NULL, *jtail = NULL;
-
+static STAILQ_HEAD(job_queue, _job) jobs = STAILQ_HEAD_INITIALIZER(jobs);
 
 void
 job_add(e, u)
@@ -42,35 +43,84 @@
 	register job *j;
 
 	/* if already on queue, keep going */
-	for (j=jhead; j; j=j->next)
+	STAILQ_FOREACH(j, &jobs, next) {
 		if (j->e == e && j->u == u) { return; }
+	}
 
 	/* build a job queue element */
 	if ((j = (job*)malloc(sizeof(job))) == NULL)
 		return;
-	j->next = (job*) NULL;
 	j->e = e;
 	j->u = u;
+	j->pid = -1;
 
 	/* add it to the tail */
-	if (!jhead) { jhead=j; }
-	else { jtail->next=j; }
-	jtail = j;
+	STAILQ_INSERT_TAIL(&jobs, j, next);
 }
 
+void
+job_remove(entry *e, user *u)
+{
+	job *j, *prev = NULL;
+
+	STAILQ_FOREACH(j, &jobs, next) {
+		if (j->e == e && j->u == u) {
+			if (prev == NULL)
+				STAILQ_REMOVE_HEAD(&jobs, next);
+			else
+				STAILQ_REMOVE_AFTER(&jobs, prev, next);
+			free(j);
+			break;
+		}
+		prev = j;
+	}
+}
+
+void
+job_exit(pid_t jobpid)
+{
+	job *j, *prev = NULL;
+
+	/* If a singleton exited, remove and free it. */
+	STAILQ_FOREACH(j, &jobs, next) {
+		if (jobpid == j->pid) {
+			if (prev == NULL)
+				STAILQ_REMOVE_HEAD(&jobs, next);
+			else
+				STAILQ_REMOVE_AFTER(&jobs, prev, next);
+			free(j);
+			break;
+		}
+		prev = j;
+	}
+}
 
 int
 job_runqueue()
 {
-	register job	*j, *jn;
+	register job	*j;
 	register int	run = 0;
+	struct job_queue singletons = STAILQ_HEAD_INITIALIZER(singletons);
+	char    	buf[MAX_TEMPSTR];
+
+	while ((j = STAILQ_FIRST(&jobs))) {
+		STAILQ_REMOVE_HEAD(&jobs, next);
+
+		/* Only start the job if it is not a running singleton. */
+		if (j->pid == -1) {
+			j->pid = do_command(j->e, j->u);
+			run++;
+		} else if ((j->e->flags & DONT_LOG) == 0) {
+			snprintf(buf, sizeof(buf), "%s", j->e->cmd);
+			log_it(j->u->name, getpid(), "SKIPPING", buf);
+		}
 
-	for (j=jhead; j; j=jn) {
-		do_command(j->e, j->u);
-		jn = j->next;
-		free(j);
-		run++;
+		/* Singleton jobs persist in the queue until they exit. */
+		if (j->pid != -1)
+			STAILQ_INSERT_TAIL(&singletons, j, next);
+		else
+			free(j);
 	}
-	jhead = jtail = NULL;
+	STAILQ_CONCAT(&jobs, &singletons);
 	return run;
 }
Index: usr.sbin/cron/crontab/crontab.5
===================================================================
--- usr.sbin/cron/crontab/crontab.5
+++ usr.sbin/cron/crontab/crontab.5
@@ -17,7 +17,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd March 29, 2020
+.Dd July 11, 2021
 .Dt CRONTAB 5
 .Os
 .Sh NAME
@@ -233,6 +233,13 @@
 .Xr cron 8 .
 .It Fl q
 Execution will not be logged.
+.It Fl s
+Only a single instance of
+.Ar command
+will be run concurrently.
+Additional instances of
+.Ar command
+will not be scheduled until the earlier one completes.
 .El
 .sp
 Duplicate options are not allowed.
@@ -354,6 +361,10 @@
 option does not mail on successful run.
 .Sh AUTHORS
 .An Paul Vixie Aq Mt paul@vix.com
+.Pp
+The
+.Fl s
+command option was inspired by similar functionality in OpenBSD.
 .Sh BUGS
 If you are in one of the 70-odd countries that observe Daylight
 Savings Time, jobs scheduled during the rollback or advance may be
Index: usr.sbin/cron/lib/entry.c
===================================================================
--- usr.sbin/cron/lib/entry.c
+++ usr.sbin/cron/lib/entry.c
@@ -456,6 +456,14 @@
 			}
 			e->flags |= DONT_LOG;
 			break;
+		case 's':
+			/* only allow the user to set the option once */
+			if ((e->flags & SINGLE_JOB) == SINGLE_JOB) {
+				ecode = e_option;
+				goto eof;
+			}
+			e->flags |= SINGLE_JOB;
+			break;
 		default:
 			Debug(DPARS|DEXT, ("load_entry()...invalid option '%c'\n", ch))
 			ecode = e_option;