diff --git a/usr.bin/uniq/tests/uniq_test.sh b/usr.bin/uniq/tests/uniq_test.sh
--- a/usr.bin/uniq/tests/uniq_test.sh
+++ b/usr.bin/uniq/tests/uniq_test.sh
@@ -122,6 +122,30 @@
 	atf_check_uniq --unique
 }
 
+atf_test_case interactive
+interactive_head() {
+	atf_set descr "test interactive use"
+}
+interactive_body() {
+	sh -c 'yes | stdbuf -oL uniq >actual' &
+	pid=$!
+	sleep 1
+	kill $!
+	atf_check -o inline:"y\n" cat actual
+}
+
+atf_test_case interactive_repeated
+interactive_repeated_head() {
+	atf_set descr "test interactive use with -d"
+}
+interactive_repeated_body() {
+	sh -c 'yes | stdbuf -oL uniq -d >actual' &
+	pid=$!
+	sleep 1
+	kill $!
+	atf_check -o inline:"y\n" cat actual
+}
+
 atf_init_test_cases()
 {
 	atf_add_test_case basic
@@ -134,4 +158,6 @@
 	atf_add_test_case ignore_case
 	atf_add_test_case skip_chars
 	atf_add_test_case unique
+	atf_add_test_case interactive
+	atf_add_test_case interactive_repeated
 }
diff --git a/usr.bin/uniq/uniq.1 b/usr.bin/uniq/uniq.1
--- a/usr.bin/uniq/uniq.1
+++ b/usr.bin/uniq/uniq.1
@@ -28,7 +28,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd June 7, 2020
+.Dd January 11, 2024
 .Dt UNIQ 1
 .Os
 .Sh NAME
@@ -70,8 +70,13 @@
 .It Fl c , Fl -count
 Precede each output line with the count of the number of times the line
 occurred in the input, followed by a single space.
+Ignored if
+.Fl u
+is also specified.
 .It Fl d , Fl -repeated
 Output a single copy of each line that is repeated in the input.
+Ignored if
+.Fl D is also specified.
 .It Fl D , Fl -all-repeated Op Ar septype
 Output all lines that are repeated (like
 .Fl d ,
diff --git a/usr.bin/uniq/uniq.c b/usr.bin/uniq/uniq.c
--- a/usr.bin/uniq/uniq.c
+++ b/usr.bin/uniq/uniq.c
@@ -42,6 +42,7 @@
 #include <limits.h>
 #include <locale.h>
 #include <nl_types.h>
+#include <stdbool.h>
 #include <stdint.h>
 #include <stdio.h>
 #include <stdlib.h>
@@ -51,14 +52,9 @@
 #include <wchar.h>
 #include <wctype.h>
 
-static int Dflag, cflag, dflag, uflag, iflag;
-static int numchars, numfields, repeats;
-
-/* Dflag values */
-#define	DF_NONE		0
-#define	DF_NOSEP	1
-#define	DF_PRESEP	2
-#define	DF_POSTSEP	3
+static enum { DF_NONE, DF_NOSEP, DF_PRESEP, DF_POSTSEP } Dflag;
+static bool cflag, dflag, uflag, iflag;
+static long long numchars, numfields, repeats;
 
 static const struct option long_opts[] =
 {
@@ -108,13 +104,13 @@
 				usage();
 			break;
 		case 'c':
-			cflag = 1;
+			cflag = true;
 			break;
 		case 'd':
-			dflag = 1;
+			dflag = true;
 			break;
 		case 'i':
-			iflag = 1;
+			iflag = true;
 			break;
 		case 'f':
 			numfields = strtonum(optarg, 0, INT_MAX, &errstr);
@@ -127,7 +123,7 @@
 				errx(1, "character skip value is %s: %s", errstr, optarg);
 			break;
 		case 'u':
-			uflag = 1;
+			uflag = true;
 			break;
 		case '?':
 		default:
@@ -140,6 +136,11 @@
 	if (argc > 2)
 		usage();
 
+	if (Dflag && dflag)
+		dflag = false;
+	if (uflag && cflag)
+		cflag = false;
+
 	ifp = stdin;
 	ifn = "stdin";
 	ofp = stdout;
@@ -180,6 +181,8 @@
 			err(1, "%s", ifn);
 		exit(0);
 	}
+	if (!cflag && !Dflag && !dflag && !uflag)
+		show(ofp, prevline);
 	tprev = convert(prevline);
 
 	tthis = NULL;
@@ -199,7 +202,11 @@
 			/* If different, print; set previous to new value. */
 			if (Dflag == DF_POSTSEP && repeats > 0)
 				fputc('\n', ofp);
-			if (!Dflag)
+			if (!cflag && !Dflag && !dflag && !uflag)
+				show(ofp, thisline);
+			else if (!Dflag &&
+			    (!dflag || (cflag && repeats > 0)) &&
+			    (!uflag || repeats == 0))
 				show(ofp, prevline);
 			p = prevline;
 			b1 = prevbuflen;
@@ -220,13 +227,20 @@
 					show(ofp, prevline);
 				}
 				show(ofp, thisline);
+			} else if (dflag && !cflag) {
+				if (repeats == 0)
+					show(ofp, prevline);
 			}
 			++repeats;
 		}
 	}
 	if (ferror(ifp))
 		err(1, "%s", ifn);
-	if (!Dflag)
+	if (!cflag && !Dflag && !dflag && !uflag)
+		/* already printed */ ;
+	else if (!Dflag &&
+	    (!dflag || (cflag && repeats > 0)) &&
+	    (!uflag || repeats == 0))
 		show(ofp, prevline);
 	exit(0);
 }
@@ -291,11 +305,8 @@
 static void
 show(FILE *ofp, const char *str)
 {
-
-	if ((!Dflag && dflag && repeats == 0) || (uflag && repeats > 0))
-		return;
 	if (cflag)
-		(void)fprintf(ofp, "%4d %s", repeats + 1, str);
+		(void)fprintf(ofp, "%4lld %s", repeats + 1, str);
 	else
 		(void)fprintf(ofp, "%s", str);
 }