Updated Concepts (markdown)

Concepts.md

@@ -256,6 +256,32 @@ The following special variables can be accessed from the command prompt or a spa
 
 E.g. to copy the hovered file in context 1 to the directory open in context 3, run `cp "$f1" "$d3"`.
 
+Selection can be accessed from the command prompt:
+
+| Variable | Substitution |
+|:---:| --- |
+| `%j` | repeat the command with `%j` substituted by each selected file path (all occurrances of`%j` are substituted) |
+| `%J` | run the command only once with the last `%J` substituted by all selected file paths |
+
+E.g.
+
+Look up a string in each file in selection:
+
+```sh
+printf "Searching for string in %s\n" "%j"; grep "string" "%j"
+```
+
+Archive all files in selection:
+
+```sh
+tar uvf archive.tar %J
+```
+
+Notes:
+
+1. You can only use either `%j` or `%J` inside a single command. If both are encountered the prompt will close. No command will be executed.
+2. When using `%J` `nnn` won't quote the file paths from selection. Quoting `%J` will just place quotes around the entire selection, it won't quote each item individually.
+
 ## Design
 
 `nnn` (the core C utility) is, generally speaking, _feature-restricted_. It includes features which you _really_ need so it can remain light enough to finish your workflow accurately before your train of thought is lost.