diff: support -qq to output absolutely nothing

This is similar to `cmp -s`, but can be combined with:
-B --ignore-blank-lines
-b --ignore-space-change
-w --ignore-all-space

To only report a difference if it doesn't involve whitespace

Reviewed by:	des, bapt, markj (previous version), ziaee (man page)
MFC after:	1 week
Sponsored by:	Klara, Inc.
Differential Revision:	https://reviews.freebsd.org/D46717

usr.bin/diff/diff.1

@@ -30,7 +30,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd January 7, 2025
+.Dd October 12, 2026
 .Dt DIFF 1
 .Os
 .Sh NAME
@@ -319,6 +319,9 @@ This is the form used by rcsdiff.
 .It Fl q -brief
 Just print a line when the files differ.
 Does not output a list of changes.
+Specify twice
+.Pq Fl qq
+to output nothing at all, for when you only want to examine the exit code.
 .It Fl U Ar number Fl -unified Ar number
 Like
 .Fl u

usr.bin/diff/diff.c

@@ -276,7 +276,12 @@ main(int argc, char **argv)
 		case 'q':
 			if (FORMAT_MISMATCHED(D_BRIEF))
 				conflicting_format();
-			diff_format = D_BRIEF;
+			if (diff_format == D_BRIEF) {
+				diff_format = D_QUIET;
+			/* If already D_QUIET, avoid getting more verbose. */
+			} else if (diff_format != D_QUIET) {
+				diff_format = D_BRIEF;
+			}
 			break;
 		case 'S':
 			start = optarg;
@@ -474,8 +479,10 @@ main(int argc, char **argv)
 			if (stat(argv[1], &stb2) == -1)
 				err(2, "%s", argv[1]);
 		}
-		print_status(diffreg(argv[0], argv[1], dflags, 1), argv[0],
-		    argv[1], "");
+
+		if (diff_format != D_QUIET)
+			print_status(diffreg(argv[0], argv[1], dflags, 1),
+			    argv[0], argv[1], "");
 	}
 	if (fflush(stdout) != 0)
 		err(2, "stdout");

usr.bin/diff/diff.h

@@ -50,6 +50,7 @@
 #define	D_BRIEF		6	/* Say if the files differ */
 #define	D_GFORMAT	7	/* Diff with defined changed group format */
 #define D_SIDEBYSIDE    8	/* Side by side */
+#define	D_QUIET		9	/* Don't output anything */
 
 #define	D_UNSET		-2
 

usr.bin/diff/diffreg.c

@@ -415,7 +415,8 @@ diffreg_stone(char *file1, char *file2, int flags, int capsicum)
 		goto closem;
 	}
 
-	if (diff_format == D_BRIEF && ignore_pats == NULL &&
+	if ((diff_format == D_BRIEF || diff_format == D_QUIET) &&
+	    ignore_pats == NULL &&
 	    (flags & (D_FOLDBLANKS|D_IGNOREBLANKS|D_IGNORECASE|
 	    D_SKIPBLANKLINES|D_STRIPCR)) == 0)
 	{
@@ -1107,7 +1108,8 @@ restart:
 		return;
 	}
 proceed:
-	if (*pflags & D_HEADER && diff_format != D_BRIEF) {
+	if ((*pflags & D_HEADER) != 0 &&
+	    diff_format != D_BRIEF && diff_format != D_QUIET) {
 		printf("%s %s %s\n", diffargs, file1, file2);
 		*pflags &= ~D_HEADER;
 	}
@@ -1156,6 +1158,7 @@ proceed:
 		anychange = 1;
 	switch (diff_format) {
 	case D_BRIEF:
+	case D_QUIET:
 		return;
 	case D_NORMAL:
 	case D_EDIT: