Support multiple levels of relative symlinks

dart/sdk/bin/dart2js

 #!/bin/bash
 # Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 # for details. All rights reserved. Use of this source code is governed by a
 # BSD-style license that can be found in the LICENSE file.
 
+# The GNU version of readlink which has a -f option isn't available by default
+# on Mac OS X.  So we provide our own version.  We try to avoid calling external
+# programs such as dirname and readlink in the common case (not a symlink).  The
+# reason for this is that we notice an overhead in calling external programs and
+# this affects script startup time which becomes notiable when running many
+# compilations in parallel (as it happens, for example, during testing).
 function follow_links() {
   file="$1"
   while [ -h "$file" ]; do

[kustermann, 2013/12/06 13:50:41]

I think, this doesn't work in all cases, e.g.

/home/foobar/applications/dart2js/bin -> /home/foobar/dart-sdk/bin
$ /home/foobar/applications/dart2js/bin/dart2js --version

Since dart2js itself is not a symlink, the "[ -h $file ]" will result in false.

What you need to do is to split the path into all of it's components and resolve
all of them by calling readlink until you reach the fixed point.

-    # On Mac OS, readlink -f doesn't work.
-    file="$(readlink "$file")"
+    target="$(readlink "$file")"
+    if [[ "$target" != /* ]]; then

[kustermann, 2013/12/06 13:50:41]

What does this do?

+      target="$(dirname "$file")/$target"
+    fi
+    file="$target"
   done
+  # Make sure that ${foo%/*} can be used instead of dirname.
+  if [ "${file%/*}" = "$file" ]; then
+    file="./$file"
+  fi

[kustermann, 2013/12/06 13:50:41]

Why is that necessary?

(IMHO, a function follow_links should take a path and follow all links and
return a path without links in the components. That's it).

Additionally, the argument we pass to follow_links on line 29 is supposed to be
an absolute path, right?

   echo "$file"
 }
 
 # Unlike $0, $BASH_SOURCE points to the absolute path of this file.
 PROG_NAME="$(follow_links "$BASH_SOURCE")"
 
 # Handle the case where dart-sdk/bin has been symlinked to.
 BIN_DIR="$(cd "${PROG_NAME%/*}" ; pwd -P)"
 
 SDK_DIR="$(cd "${BIN_DIR}/.." ; pwd -P)"
@@ skipping 39 matching lines @@
 if [[ $DART_VM_OPTIONS ]]; then
   read -a OPTIONS <<< "$DART_VM_OPTIONS"
   EXTRA_VM_OPTIONS+=("${OPTIONS[@]}")
 fi
 
 if test -f "$SNAPSHOT"; then
   exec "$DART" "${EXTRA_VM_OPTIONS[@]}" "$SNAPSHOT" "${EXTRA_OPTIONS[@]}" "$@"
 else
   exec "$DART" "${EXTRA_VM_OPTIONS[@]}" "$DART2JS" "${EXTRA_OPTIONS[@]}" "$@"
 fi