diff --git a/brew-common.sh b/brew-common.sh
index 653be9c..7834b64 100755
--- a/brew-common.sh
+++ b/brew-common.sh
@@ -117,13 +117,17 @@ get_fixed_length_path() {
 
   # Use the Git SHA1 of the Homebrew repository as a filler.
   if [[ -z $sha1 ]]; then
-    if [[ ! -d $path/.git ]]; then
-      fatal "Directory '$path' is not a Git repository, cannot get SHA1"
+    if [[ -f $path/GIT_SHA1 ]]; then
+      sha1=$( cat $path/GIT_SHA1 )
+    else
+      if [[ ! -d $path/.git ]]; then
+        fatal "Directory '$path' is not a Git repository, cannot get SHA1"
+      fi
+      sha1=$( cd "$path" && git rev-parse HEAD )
     fi
-    sha1=$( cd "$path" && git rev-parse HEAD )
   fi
   if [[ ! $sha1 =~ ^[0-9a-f]{40}$ ]]; then
-    fatal "Invalid Git SHA1: '$sha1'"
+    fatal "Invalid Git SHA1: '$sha1', expected 40 hex characters"
   fi
 
   fixed_length_path=\
diff --git a/post_install.template b/post_install.template
index d968239..fc17763 100644
--- a/post_install.template
+++ b/post_install.template
@@ -30,10 +30,11 @@ fi
 readonly orig_brew_home="{{ orig_brew_home }}"
 declare -i -r ORIG_LEN=${#orig_brew_home}
 
-brew_home=$PWD
+brew_home=$script_dir
+cd "$brew_home"
 LEN=${#brew_home}
 if [[ $LEN -gt $ORIG_LEN ]]; then
-  fatal "Homebrew/Linuxbrew absolute path should be no more than $ORIG_LEN bytes, but actual " \
+  fatal "Homebrew/Linuxbrew absolute path should be no more than $ORIG_LEN bytes, but actual" \
         "length is $LEN bytes: $brew_home"
 fi
 
diff --git a/test.sh b/test.sh
index 5e4b7e0..53239fd 100755
--- a/test.sh
+++ b/test.sh
@@ -45,6 +45,20 @@ cleanup() {
   exit "$exit_code"
 }
 
+find_latest_brew_dir() {
+  latest_brew_dir=$(
+    ls -td $(
+      find . -mindepth 1 -maxdepth 1 -type d -name "$YB_BREW_TYPE_LOWERCASE-*"
+    ) | head -1
+  )
+  if [[ ! -d $latest_brew_dir ]]; then
+    log "In directory $PWD, found latest brew directory: $latest_brew_dir"
+    ( set -x; ls -l )
+    fatal "Unable to find the latest Homebrew/Linuxbrew installation in $PWD"
+  fi
+  latest_brew_dir=$( cd "$latest_brew_dir" && pwd )
+}
+
 # -------------------------------------------------------------------------------------------------
 # Parsing command-line arguments
 # -------------------------------------------------------------------------------------------------
@@ -87,7 +101,7 @@ if [[ -z ${work_dir:-} ]]; then
     tmp_dir=$HOME/tmp
     mkdir -p "$tmp_dir"
   fi
-  work_dir=$tmp_dir/ybbrewtest-$$-$(date +%Y-%m-%dT%H_%M_%S)
+  work_dir=$tmp_dir/ybbrewtst-$$-$(date +%Y%m%dT%H%M)
   trap cleanup EXIT
 fi
 mkdir -p "$work_dir"
@@ -96,13 +110,26 @@ cd "$work_dir"
 export YB_BREW_BUILD_UNIT_TEST_MODE=1
 "$script_dir/brew-clone-and-build-all.sh"
 
+find_latest_brew_dir
+brew_home=$latest_brew_dir
+log "Will use the directory $brew_home for further testing"
+
+heading "Testing post_install.sh"
+
+( set -x; cd /; rm -rf "$brew_home/.git"; "$brew_home/post_install.sh" )
+
 heading "Testing brew-copy.sh"
-brew_home=$(
-  find . -mindepth 1 -maxdepth 1 -type d -name "$YB_BREW_TYPE_LOWERCASE-*" | sort | head -1
-)
+
 if [[ -z $brew_home ]]; then
   fatal "Could not find a subdirectory starting with '$YB_BREW_TYPE_LOWERCASE-' in $PWD"
 fi
 "$script_dir/brew-copy.sh" "$brew_home"
 
+heading "Testing post_install.sh after brew-copy.sh"
+find_latest_brew_dir
+if [[ $latest_brew_dir == $brew_home ]]; then
+  fatal "brew-copy.sh failed to produce a new Homebrew/Linuxbrew directory in $PWD"
+fi
+( set -x; "$latest_brew_dir/post_install.sh" )
+
 log "TEST SUCCEEDED"