Added arithmetic operators

manuscript/chapter12.txt

@@ -1,13 +1,42 @@
-# PERFORMANCE
+# TRAPS
 
-## Disable Unicode
+Traps allow a script to execute code on various signals. In [pxltrm](https://github.com/dylanaraps/pxltrm) (*a pixel art editor written in bash*)  traps are used to redraw the user interface on window resize. Another use case is cleaning up temporary files on script exit.
 
-If unicode is not required, it can be disabled for a performance increase. Results may vary however there have been noticeable improvements in [neofetch](https://github.com/dylanaraps/neofetch) and other programs.
+Traps should be added near the start of scripts so any early errors are also caught.
+
+**NOTE:** For a full list of signals, see `trap -l`.
+
+
+## Do something on script exit
 
 ```shell
-# Disable unicode.
-LC_ALL=C
-LANG=C
+# Clear screen on script exit.
+trap 'printf \\e[2J\\e[H\\e[m' EXIT
+```
+
+## Ignore terminal interrupt (CTRL+C, SIGINT)
+
+```shell
+trap '' INT
+```
+
+## React to window resize.
+
+```shell
+# Call a function on window resize.
+trap 'code_here' SIGWINCH
+```
+
+## Do something before every command.
+
+```shell
+trap 'code_here' DEBUG
+```
+
+## Do something when a shell function or a sourced file finishes executing
+
+```shell
+trap 'code_here' RETURN
 ```
 
 <!-- CHAPTER END -->