diff --git a/bash_programming.md b/bash_programming.md
index 05a1de0..0bf5361 100644
--- a/bash_programming.md
+++ b/bash_programming.md
@@ -109,7 +109,7 @@ EOT
 ```
 
 Common bash test expressions
-https://www.gnu.org/software/bash/manual/html_node/Bash-Conditional-Expressions.html
+https://www.gnu.org/software/bash/manual/html_node/Bash-Conditional-Expressions.html and https://stackoverflow.com/a/21164441
 ```
 The following test operators return "True" if the file exists
 -a file :: Any file type