cmp: add -i, --ignore-initial, an alternative to skip1/skip2 args

This is compatible with GNU cmp.

Reviewed by:	markj
Sponsored by:	Klara, Inc.
Differential Revision:	https://reviews.freebsd.org/D32073

usr.bin/cmp/cmp.1

@@ -41,6 +41,7 @@
 .Nm
 .Op Fl l | s | x
 .Op Fl hz
+.Op Fl -ignore-initial Ns Cm = Ns Ar num1 Ns Op :num2
 .Op Fl -bytes Ns Cm = Ns Ar num
 .Ar file1 file2
 .Op Ar skip1 Op Ar skip2
@@ -60,6 +61,23 @@ The following options are available:
 .Bl -tag -width indent
 .It Fl h
 Do not follow symbolic links.
+.It Fl i Ar num1 Ns Oo :num2 Oc , Fl -ignore-initial= Ns Ar num1 Ns Op :num2
+Skip
+.Ar num1
+bytes from
+.Ar file1 ,
+and optionally skip
+.Ar num2
+bytes from
+.Ar file2 .
+If
+.Ar num2
+is not specified, then
+.Ar num1
+is applied for both
+.Ar file1
+and
+.Ar file2 .
 .It Fl l , Fl -verbose
 Print the byte number (decimal) and the differing
 byte values (octal) for each difference.
@@ -170,6 +188,7 @@ utility is expected to be
 compatible.
 The
 .Fl h ,
+.Fl i ,
 .Fl n ,
 .Fl x ,
 and

usr.bin/cmp/cmp.c

@@ -66,6 +66,7 @@ bool	lflag, sflag, xflag, zflag;
 
 static const struct option long_opts[] =
 {
+	{"ignore-initial", required_argument,	NULL, 'i'},
 	{"verbose",	no_argument,		NULL, 'l'},
 	{"bytes",	required_argument,	NULL, 'n'},
 	{"silent",	no_argument,		NULL, 's'},
@@ -75,6 +76,25 @@ static const struct option long_opts[] =
 
 static void usage(void);
 
+static bool
+parse_iskipspec(char *spec, off_t *skip1, off_t *skip2)
+{
+	char *colon;
+
+	colon = strchr(spec, ':');
+	if (colon != NULL)
+		*colon++ = '\0';
+
+	if (expand_number(spec, skip1) < 0)
+		return (false);
+
+	if (colon != NULL)
+		return (expand_number(colon, skip2) == 0);
+
+	*skip2 = *skip1;
+	return (true);
+}
+
 int
 main(int argc, char *argv[])
 {
@@ -86,11 +106,19 @@ main(int argc, char *argv[])
 
 	skip1 = skip2 = 0;
 	oflag = O_RDONLY;
-	while ((ch = getopt_long(argc, argv, "+hln:sxz", long_opts, NULL)) != -1)
+	while ((ch = getopt_long(argc, argv, "+hi:ln:sxz", long_opts, NULL)) != -1)
 		switch (ch) {
 		case 'h':		/* Don't follow symlinks */
 			oflag |= O_NOFOLLOW;
 			break;
+		case 'i':
+			if (!parse_iskipspec(optarg, &skip1, &skip2)) {
+				fprintf(stderr,
+				    "Invalid --ignore-initial: %s\n",
+				    optarg);
+				usage();
+			}
+			break;
 		case 'l':		/* print all differences */
 			lflag = true;
 			break;

usr.bin/cmp/tests/cmp_test2.sh

@@ -71,8 +71,13 @@ pr252542_body()
 {
 	echo -n '1234567890' > a
 	echo -n 'abc567890' > b
+	echo -n 'xbc567890' > c
 	atf_check -s exit:0 cmp -s a b 4 3
+	atf_check -s exit:0 cmp -i 4:3 -s a b
+	atf_check -s exit:0 cmp -i 1 -s b c
 	atf_check -s exit:1 -o ignore cmp -z a b 4 3
+	atf_check -s exit:1 -o ignore cmp -i 4:3 -z a b
+	atf_check -s exit:1 -o ignore cmp -i 1 -z a b
 }
 
 atf_test_case skipsuff