Content supplement

Author: jimcat8

docs/books/admin_guide/03-commands.md

@@ -538,7 +538,7 @@ total 1332
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 ```
 
-* List `/var` files larger than 1 megabyte but less than 1 gigabyte:
+* List `/var` files larger than 1 megabyte but less than 1 gigabyte, the example here uses the advanced use of the `grep` command - regular expressions. Novices don't have to struggle too much, there will be a special tutorial to introduce these regular expressions in the future.
 
 ```bash
 $ ls -lhR /var/ | grep ^\- | grep -E "[1-9]*\.[0-9]*M"