notes: add environment section

NOTES.md

@@ -439,6 +439,23 @@ execution environment.
 
 A subshell is a copy of the shell process.
 
+#### Environment
+cf. 3.7.4 Environment
+
+When a program is invoked it is given an array of strings called the
+"environment".  This is a list of name-value pairs, of the form 'name=value'.
+
+Bash provides several ways to manipulate the environment.  On invocation, the
+shell scans its own environment and creates a parameter for each name found,
+automatically marking it for 'export' to child processes.  Executed commands
+inherit the environment.  The 'export' and 'unset' builtins allow parameters to
+be added to and deleted from the environment.  If the value of a parameter in
+the environment is modified using the 'export' builtin, the new value becomes
+part of the environment, replacing the old.  The environment inherited by any
+executed command consists of the shell's initial environment, whose values may
+be modified in the shell, less any pairs removed by the 'unset' builtin, plus
+any additions via the 'export' command.
+
 ## Here Documents
 cf. Bash Reference Manual 3.6.6 Here Documents