bin/brew: fix corner cases in prefix computation

If `bin/brew` happens to be symlinked to `/brew`, `/bin/brew`, or some
similar location or (worse yet) Homebrew is installed to `/`, then
computation of the prefix and/or repository path could break down and
result in an invalid or empty path.

Closes Homebrew/homebrew-core#2430.

1 files changed, 5 insertions, 2 deletions

diff --git a/bin/brew b/bin/brew
index b49b961e1..77611c153 100755
--- a/bin/brew
+++ b/bin/brew
@@ -5,10 +5,13 @@ quiet_cd() {
   cd "$@" >/dev/null
 }
 
-BREW_FILE_DIRECTORY="$(quiet_cd "${0%/*}" && pwd -P)"
-HOMEBREW_BREW_FILE="$BREW_FILE_DIRECTORY/${0##*/}"
+BREW_FILE_DIRECTORY="$(quiet_cd "${0%/*}/" && pwd -P)"
+HOMEBREW_BREW_FILE="${BREW_FILE_DIRECTORY%/}/${0##*/}"
 
 HOMEBREW_PREFIX="${HOMEBREW_BREW_FILE%/*/*}"
+[[ -n "$HOMEBREW_PREFIX" && "$HOMEBREW_PREFIX" != "$HOMEBREW_BREW_FILE" ]] \
+  || HOMEBREW_PREFIX="/"
+
 HOMEBREW_REPOSITORY="$HOMEBREW_PREFIX"
 
 if [[ -L "$HOMEBREW_BREW_FILE" ]]