diff --git a/asdf.sh b/asdf.sh
index d7dd4c7e..cdbb1de8 100644
--- a/asdf.sh
+++ b/asdf.sh
@@ -47,7 +47,7 @@ if [ -z "$ASDF_DIR" ]; then
 fi
 
 if [ -z "$ASDF_DIR" ]; then
-  printf "%s\n" "asdf: Error: Source directory could not be calculated. Please set it manually before sourcing this file." >&2
+  printf "%s\n" "asdf: Error: Source directory could not be calculated. Please set \$ASDF_DIR manually before sourcing this file." >&2
   return 1
 fi
 
@@ -56,22 +56,20 @@ if [ ! -d "$ASDF_DIR" ]; then
   return 1
 fi
 
-_asdf_bin="${ASDF_DIR}/bin"
+_asdf_bin="$ASDF_DIR/bin"
 _asdf_shims="${ASDF_DATA_DIR:-$HOME/.asdf}/shims"
 
-# shellcheck disable=SC3060
-if [ -n "$BASH_VERSION" ] || [ -n "$ZSH_VERSION" ]; then
-  case ":$PATH:" in
-    *":${_asdf_bin}:"*) PATH="${PATH//$_asdf_bin:/}" ;;
-  esac
-  case ":$PATH:" in
-    *":${_asdf_shims}:"*) PATH="${PATH//$_asdf_shims:/}" ;;
-  esac
-fi
+case ":$PATH:" in
+  *":$_asdf_bin:"*) : ;;
+  *) PATH="$_asdf_bin:$PATH" ;;
+esac
+case ":$PATH:" in
+  *":$_asdf_shims:"*) : ;;
+  *) PATH="$_asdf_shims:$PATH" ;;
+esac
 
-PATH="${_asdf_bin}:${_asdf_shims}:$PATH"
 unset -v _asdf_bin _asdf_shims
 
 # shellcheck source=lib/asdf.sh
-. "${ASDF_DIR}/lib/asdf.sh"
+. "$ASDF_DIR/lib/asdf.sh"