Do not symlink a symlink in mkrc

We have a bug when calling mkrc(1) on a symlink:

```sh
mkrc ~/.vimrc # links ~/.vimrc to ~/.dotfiles/vimrc
mkrc ~/.vimrc # deletes ~/.dotfiles/vimrc
```

This catches that case ahead of time, preventing the user from running
mkrc(1) on a symlink.

Fix #144.

2 files changed, 23 insertions, 4 deletions

diff --git a/bin/mkrc.in b/bin/mkrc.in
index 5d6af7e..d246c33 100755
--- a/bin/mkrc.in
+++ b/bin/mkrc.in
@@ -20,6 +20,28 @@ destination() {
   fi
 }
 
+exit_if_dangerous() {
+  local file="$1"
+
+  if [ -L "$file" ]; then
+      $ERROR 1 "'$file' is a symlink. Cannot process file."
+  elif is_nested "$file"; then
+    # Remove DEST_DIR in case $HOME is under a symlink
+    saved_ifs="$IFS"
+    IFS=/
+    set -- $(dirname "$file" | sed "s|$DEST_DIR/||")
+    IFS="$saved_ifs"
+
+    built_dir="$DEST_DIR"
+    for dir in $@; do
+      built_dir="$built_dir/$dir"
+      if [ -L "$built_dir" ]; then
+        $ERROR 1 "'$file' path contains a symlink ($dir). Cannot process file."
+      fi
+    done
+  fi
+}
+
 show_help() {
   local exit_code=${1:-0}
 
@@ -84,6 +106,7 @@ fi
 
 files=""
 for i; do
+  exit_if_dangerous "$i" 
   files="$(printf "$files\n$i")"
 done
 
diff --git a/bin/rcup.in b/bin/rcup.in
index 910e534..78f5faa 100755
--- a/bin/rcup.in
+++ b/bin/rcup.in
@@ -121,10 +121,6 @@ replace_file() {
   link_file "$src" "$dest" "$sigil"
 }
 
-is_nested() {
-  echo "$1" | sed "s:$DEST_DIR/::" | grep '/' >/dev/null
-}
-
 is_identical() {
   diff -c "$1" "$2" > /dev/null 2>&1
 }