diff --git a/NOTES.md b/NOTES.md
index fb4c645..0ca6a61 100644
--- a/NOTES.md
+++ b/NOTES.md
@@ -34,6 +34,8 @@ PRYSK12345 2 0
 
 [The Architecture of Open Source Applications (Volume 1) The Bourne-Again Shell](https://aosabook.org/en/v1/bash.html)
 
+[IEEE Open Group Base Specification Issue 7: Shell Command Language](https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_01)
+
 ## Shell Operation
 
 cf. 3.1.1 Shell Operation