diff --git a/Library/Homebrew/brew.sh b/Library/Homebrew/brew.sh
index 166b816e41..727119d9bd 100644
--- a/Library/Homebrew/brew.sh
+++ b/Library/Homebrew/brew.sh
@@ -104,10 +104,9 @@ then
     HOMEBREW_FORCE_BREWED_GIT="1"
   fi
 
-  if [[ -z "$HOMEBREW_CACHE" ]]
-  then
-    HOMEBREW_CACHE="$HOME/Library/Caches/Homebrew"
-  fi
+  HOMEBREW_CACHE="${HOMEBREW_CACHE:-${HOME}/Library/Caches/Homebrew}"
+
+  HOMEBREW_TEMP="${HOMEBREW_TEMP:-/private/tmp}"
 else
   HOMEBREW_PROCESSOR="$(uname -m)"
   HOMEBREW_PRODUCT="${HOMEBREW_SYSTEM}brew"
@@ -115,15 +114,10 @@ else
   : "${HOMEBREW_OS_VERSION:=$(uname -r)}"
   HOMEBREW_OS_USER_AGENT_VERSION="$HOMEBREW_OS_VERSION"
 
-  if [[ -z "$HOMEBREW_CACHE" ]]
-  then
-    if [[ -n "$XDG_CACHE_HOME" ]]
-    then
-      HOMEBREW_CACHE="$XDG_CACHE_HOME/Homebrew"
-    else
-      HOMEBREW_CACHE="$HOME/.cache/Homebrew"
-    fi
-  fi
+  CACHE_HOME="${XDG_CACHE_HOME:-${HOME}/.cache}"
+  HOMEBREW_CACHE="${HOMEBREW_CACHE:-${CACHE_HOME}/Homebrew}"
+
+  HOMEBREW_TEMP="${HOMEBREW_TEMP:-/tmp}"
 fi
 
 if [[ -n "$HOMEBREW_FORCE_BREWED_CURL" &&
@@ -153,6 +147,7 @@ export HOMEBREW_BREW_FILE
 export HOMEBREW_PREFIX
 export HOMEBREW_REPOSITORY
 export HOMEBREW_LIBRARY
+export HOMEBREW_TEMP
 
 # Declared in brew.sh
 export HOMEBREW_VERSION
@@ -309,6 +304,21 @@ EOS
 }
 check-run-command-as-root
 
+check-prefix-is-not-tmpdir() {
+  [[ -z "${HOMEBREW_MACOS}" ]] && return
+
+  if [[ "${HOMEBREW_PREFIX}" = "${HOMEBREW_TEMP}"* ]]
+  then
+    odie <<EOS
+Your HOMEBREW_PREFIX is in the Homebrew temporary directory, which Homebrew
+uses to store downloads and builds. You can resolve this by installing Homebrew to
+either the standard prefix (/usr/local) or to a non-standard prefix that is not
+in the Homebrew temporary directory.
+EOS
+  fi
+}
+check-prefix-is-not-tmpdir
+
 if [[ "$HOMEBREW_PREFIX" = "/usr/local" &&
       "$HOMEBREW_PREFIX" != "$HOMEBREW_REPOSITORY" &&
       "$HOMEBREW_CELLAR" = "$HOMEBREW_REPOSITORY/Cellar" ]]
diff --git a/Library/Homebrew/config.rb b/Library/Homebrew/config.rb
index 753616a8c8..6247060fb6 100644
--- a/Library/Homebrew/config.rb
+++ b/Library/Homebrew/config.rb
@@ -39,7 +39,7 @@ HOMEBREW_CACHE_FORMULA = HOMEBREW_CACHE/"Formula"
 HOMEBREW_LOGS = Pathname.new(ENV["HOMEBREW_LOGS"] || "~/Library/Logs/Homebrew/").expand_path
 
 # Must use /tmp instead of $TMPDIR because long paths break Unix domain sockets
-HOMEBREW_TEMP = Pathname.new(ENV.fetch("HOMEBREW_TEMP", "/tmp"))
+HOMEBREW_TEMP = Pathname.new(ENV["HOMEBREW_TEMP"]).realpath
 
 unless defined? HOMEBREW_LIBRARY_PATH
   # Root of the Homebrew code base