diff --git a/src/sum.sh b/src/sum.sh
index 7169c99..ecb7322 100644
--- a/src/sum.sh
+++ b/src/sum.sh
@@ -1,9 +1,18 @@
 # /etc/profile.d/sum.sh
 
-# Ensure the script is sourced
-if [[ "${BASH_SOURCE[0]}" == "${0}" || "${(%):-%N}" == "$0" ]]; then
-  echo "This script should be sourced, not executed."
-  exit 1
+# Ensure the script is sourced, not executed
+if [ -n "$ZSH_VERSION" ]; then
+  # For zsh
+  if [[ "${(%):-%x}" == "$0" ]]; then
+    echo "This script should be sourced, not executed."
+    return 1
+  fi
+else
+  # For bash
+  if [[ "${BASH_SOURCE[0]}" == "${0}" ]]; then
+    echo "This script should be sourced, not executed."
+    return 1
+  fi
 fi
 
 # Function to generate SHA-256 hashes for .sh files in a given directory
@@ -16,15 +25,10 @@ generate_sha256_hashes() {
     return 1
   fi
 
-  # Determine the command to use for calculating SHA-256
-  local hash_cmd
+  # Use shasum by default, and fallback to sha256sum if available
+  local hash_cmd="shasum -a 256"
   if command -v sha256sum >/dev/null 2>&1; then
     hash_cmd="sha256sum"
-  elif command -v shasum >/dev/null 2>&1; then
-    hash_cmd="shasum -a 256"
-  else
-    echo "Neither sha256sum nor shasum command is available."
-    return 1
   fi
 
   # Loop through all .sh files in the directory