Add ln -r and relative_path() to lib, plus test.

And a few small cleanups while I was there.

lib/lib.c

@@ -216,6 +216,7 @@ int mkpath(char *dir)
 }
 
 // Split a path into linked list of components, tracking head and tail of list.
+// Assigns head of list to *list, returns address of ->next entry to extend list
 // Filters out // entries with no contents.
 struct string_list **splitpath(char *path, struct string_list **list)
 {
@@ -1028,6 +1029,34 @@ char *fileunderdir(char *file, char *dir)
   return rc ? s2 : 0;
 }
 
+// return (malloced) relative path to get from "from" to "to"
+char *relative_path(char *from, char *to)
+{
+  char *s, *ret = 0;
+  int i, j, k;
+
+  if (!(from = xabspath(from, -1))) return 0;
+  if (!(to = xabspath(to, -1))) goto error;
+
+  // skip common directories from root
+  for (i = j = 0; from[i] && from[i] == to[i]; i++) if (to[i] == '/') j = i+1;
+
+  // count remaining destination directories
+  for (i = j, k = 0; from[i]; i++) if (from[i] == '/') k++;
+
+  if (!k) ret = xstrdup(to+j);
+  else {
+    s = ret = xmprintf("%*c%s", 3*k, ' ', to+j);
+    while (k--) memcpy(s+3*k, "../", 3);
+  }
+
+error:
+  free(from);
+  free(to);
+
+  return ret;
+}
+
 // Execute a callback for each PID that matches a process name from a list.
 void names_to_pid(char **names, int (*callback)(pid_t pid, char *name),
     int scripts)

lib/lib.h

@@ -402,6 +402,7 @@ mode_t string_to_mode(char *mode_str, mode_t base);
 void mode_to_string(mode_t mode, char *buf);
 char *getbasename(char *name);
 char *fileunderdir(char *file, char *dir);
+char *relative_path(char *from, char *to);
 void names_to_pid(char **names, int (*callback)(pid_t pid, char *name),
     int scripts);
 

tests/ln.test

@@ -55,6 +55,7 @@ rm -rf file dir slink
 
 testing "-t" "ln -st . one/two three && readlink two three" "one/two\nthree\n" \
   "" ""
+rm -f two three
 
 touch file1 file2 && mkdir dir
 testing "create_multiple_hardlinks" "ln file* dir/ &&
@@ -78,3 +79,12 @@ testing "create_hardlink_and_remove_sourcefile" "ln file hlink &&
           [ file -ef hlink ] && rm file && [ -f hlink ] && echo 'yes'" \
           "yes\n" "" ""
 rm -f file hlink
+
+mkdir -p one/two
+ln -s . circular
+mkdir -p three
+echo hello > three/four
+testing "ln -r" \
+  "ln -sr circular/three/../three/four one/two/five && cat one/two/five" \
+  "hello\n" "" ""
+rm -rf one three circular

toys/posix/ln.c

@@ -4,7 +4,7 @@
  *
  * See http://opengroup.org/onlinepubs/9699919799/utilities/ln.html
 
-USE_LN(NEWTOY(ln, "<1t:Tvnfs", TOYFLAG_BIN))
+USE_LN(NEWTOY(ln, "<1rt:Tvnfs", TOYFLAG_BIN))
 
 config LN
   bool "ln"
@@ -18,6 +18,7 @@ config LN
     -s	Create a symbolic link
     -f	Force the creation of the link, even if TO already exists
     -n	Symlink at TO treated as file
+    -r	Create relative symlink from -> to
     -t	Create links in DIR
     -T	TO always treated as file, max 2 arguments
     -v	Verbose
@@ -34,30 +35,37 @@ void ln_main(void)
 {
   char *dest = TT.t ? TT.t : toys.optargs[--toys.optc], *new;
   struct stat buf;
-  int i;
+  int i, rc;
+
+  if (FLAG(T) && toys.optc>1) help_exit("Max 2 arguments");
 
   // With one argument, create link in current directory.
   if (!toys.optc) {
     toys.optc++;
-    dest=".";
+    dest = ".";
   }
 
-  if (FLAG(T) && toys.optc>1) help_exit("Max 2 arguments");
   // Is destination a directory?
   if (!((FLAG(n)||FLAG(T)) ? lstat : stat)(dest, &buf)) {
-    i = S_ISDIR(buf.st_mode);
-
-    if ((FLAG(T) && i) || (!i && (toys.optc>1 || TT.t)))
+    if ((i = S_ISDIR(buf.st_mode)) ? FLAG(T) : (toys.optc>1 || TT.t))
       error_exit("'%s' %s a directory", dest, i ? "is" : "not");
   } else buf.st_mode = 0;
 
   for (i=0; i<toys.optc; i++) {
-    int rc;
-    char *oldnew, *try = toys.optargs[i];
+    char *oldnew = 0, *try = toys.optargs[i];
 
     if (S_ISDIR(buf.st_mode)) new = xmprintf("%s/%s", dest, basename(try));
     else new = dest;
 
+    if (FLAG(r)) {
+      try = relative_path(new, try);
+      if (!try) {
+        if (new != dest) free(new);
+        continue;
+      }
+      toys.optflags |= FLAG_s;
+    }
+
     // Force needs to unlink the existing target (if any). Do that by creating
     // a temp version and renaming it over the old one, so we can retain the
     // old file in cases we can't replace it (such as hardlink between mounts).
@@ -88,6 +96,7 @@ void ln_main(void)
                        FLAG(s) ? "symbolic" : "hard", try, new);
     else if (FLAG(v)) fprintf(stderr, "'%s' -> '%s'\n", new, try);
 
+    if (try != toys.optargs[i]) free(try);
     if (new != dest) free(new);
   }
 }