Add a -w warning flag to ln(1). When the -w option is enabled,

ln(1) checks to see if the source of a symlink, i.e. the file it
should point to actually exists.  The default is the old ln
behavior, that does not check, to avoid surprising people who may
be using ln(1) in scripts or other non-interactive places.

PR:		bin/7265
Submitted by:	Joel Ray Holveck, detlev!joelh at mail.camalott.com
MFC after:	2 weeks

2 files changed, 47 insertions, 7 deletions

diff --git a/bin/ln/ln.1 b/bin/ln/ln.1
index e6ac24a..7ae4ea8 100644
--- a/bin/ln/ln.1
+++ b/bin/ln/ln.1
@@ -32,7 +32,7 @@
 .\"	@(#)ln.1	8.2 (Berkeley) 12/30/93
 .\" $FreeBSD$
 .\"
-.Dd February 14, 2006
+.Dd June 6, 2008
 .Dt LN 1
 .Os
 .Sh NAME
@@ -42,13 +42,13 @@
 .Sh SYNOPSIS
 .Nm
 .Op Fl s Op Fl F
-.Op Fl f | i
+.Op Fl f | iw
 .Op Fl hnv
 .Ar source_file
 .Op Ar target_file
 .Nm
 .Op Fl s Op Fl F
-.Op Fl f | i
+.Op Fl f | iw
 .Op Fl hnv
 .Ar source_file ...
 .Ar target_dir
@@ -79,6 +79,8 @@ then unlink it so that the link may occur.
 .Fl f
 option overrides any previous
 .Fl i
+and
+.Fl w
 options.)
 .It Fl F
 If the target file already exists and is a directory, then remove it
@@ -134,6 +136,8 @@ Create a symbolic link.
 Cause
 .Nm
 to be verbose, showing files as they are processed.
+.It Fl w
+Warn if the source of a symbolic link does not currently exist.
 .El
 .Pp
 By default,
@@ -194,9 +198,10 @@ operation using the two passed arguments.
 The
 .Fl h ,
 .Fl i ,
-.Fl n
-and
+.Fl n ,
 .Fl v
+and
+.Fl w
 options are non-standard and their use in scripts is not recommended.
 They are provided solely for compatibility with other
 .Nm
diff --git a/bin/ln/ln.c b/bin/ln/ln.c
index d164c76..45a7cb3 100644
--- a/bin/ln/ln.c
+++ b/bin/ln/ln.c
@@ -58,6 +58,8 @@ int	hflag;				/* Check new name for symlink first. */
 int	iflag;				/* Interactive mode. */
 int	sflag;				/* Symbolic, not hard, link. */
 int	vflag;				/* Verbose output. */
+int	wflag;				/* Warn if symlink target does not
+					 * exist, and -f is not enabled. */
 					/* System link call. */
 int (*linkf)(const char *, const char *);
 char	linkch;
@@ -92,7 +94,7 @@ main(int argc, char *argv[])
 		exit(linkit(argv[0], argv[1], 0));
 	}
 
-	while ((ch = getopt(argc, argv, "Ffhinsv")) != -1)
+	while ((ch = getopt(argc, argv, "Ffhinsvw")) != -1)
 		switch (ch) {
 		case 'F':
 			Fflag = 1;
@@ -100,6 +102,7 @@ main(int argc, char *argv[])
 		case 'f':
 			fflag = 1;
 			iflag = 0;
+			wflag = 0;
 			break;
 		case 'h':
 		case 'n':
@@ -115,6 +118,9 @@ main(int argc, char *argv[])
 		case 'v':
 			vflag = 1;
 			break;
+		case 'w':
+			wflag = 1;
+			break;
 		case '?':
 		default:
 			usage();
@@ -127,8 +133,10 @@ main(int argc, char *argv[])
 	linkch = sflag ? '-' : '=';
 	if (sflag == 0)
 		Fflag = 0;
-	if (Fflag == 1 && iflag == 0)
+	if (Fflag == 1 && iflag == 0) {
 		fflag = 1;
+		wflag = 0;		/* Implied when fflag != 0 */
+	}
 
 	switch(argc) {
 	case 0:
@@ -167,6 +175,7 @@ linkit(const char *source, const char *target, int isdir)
 	const char *p;
 	int ch, exists, first;
 	char path[PATH_MAX];
+	char wbuf[PATH_MAX];
 
 	if (!sflag) {
 		/* If source doesn't exist, quit now. */
@@ -204,6 +213,32 @@ linkit(const char *source, const char *target, int isdir)
 
 	exists = !lstat(target, &sb);
 	/*
+	 * If the link source doesn't exist, and a symbolic link was
+	 * requested, and -w was specified, give a warning.
+	 */
+	if (sflag && wflag) {
+		if (*source == '/') {
+			/* Absolute link source. */
+			if (stat(source, &sb) != 0)
+				 warn("warning: %s inaccessible", source);
+		} else {
+			/*
+			 * Relative symlink source.  Try to construct the
+			 * absolute path of the source, by appending `source'
+			 * to the parent directory of the target.
+			 */
+			p = strrchr(target, '/');
+			if (p != NULL)
+				p++;
+			else
+				p = target;
+			(void)snprintf(wbuf, sizeof(wbuf), "%.*s%s",
+			    (p - target), target, source);
+			if (stat(wbuf, &sb) != 0)
+				warn("warning: %s", source);
+		}
+	}
+	/*
 	 * If the file exists, then unlink it forcibly if -f was specified
 	 * and interactively if -i was specified.
 	 */