update shell

technology/applications/cli/Shell.md

@@ -51,7 +51,7 @@ rm -v *.txt
 ```
 
 ## Redirections
-### 1. **Standard Input, Output, and Error**
+### **Standard Input, Output, and Error**
 In a shell environment, there are three standard streams:
 - **Standard Input (stdin - fd 0):** Represents the input to a command.
 - **Standard Output (stdout - fd 1):** Represents the output of a command.
@@ -182,6 +182,15 @@ ${fruits[*]}
 ${fruits[@]}
 ```
 
+Use variable or `default` if the variable does not exist:
+```shell
+VR1=Value
+MYVAR=${VR1:-default} # $MYVAR is 'Value'
+
+#VR2=Value
+MYVAR=${VR2:-default} # $MYVAR is 'default'
+```
+
 ### Conditionals
 You can use conditional statements.
 ```shell