Expose state information via environment variables to child processes

Close #3582

CHANGELOG.md

@@ -4,6 +4,33 @@ CHANGELOG
 0.46.0 (WIP)
 ------
 - Added `result` event that is triggered when the filtering for the current query is complete and the result list is ready.
+- fzf now exports the following environment variables to the child processes
+  | Variable           | Description                                                 |
+  | ---                | ---                                                         |
+  | `FZF_LINES`        | Number of lines fzf takes up excluding padding and margin   |
+  | `FZF_COLUMNS`      | Number of columns fzf takes up excluding padding and margin |
+  | `FZF_TOTAL_COUNT`  | Total number of items                                       |
+  | `FZF_MATCH_COUNT`  | Number of matched items                                     |
+  | `FZF_SELECT_COUNT` | Number of selected items                                    |
+  | `FZF_QUERY`        | Current query string                                        |
+  | `FZF_PROMPT`       | Prompt string                                               |
+  | `FZF_ACTION`       | The name of the last action performed                       |
+  - This allows you to write sophisticated transformations like so
+    ```sh
+    # Dynamically resize preview window
+    seq 10000 | fzf --bind 'result:transform:
+      # 1 line for info, another for prompt, and 2 more lines for preview window border
+      lines=$(( FZF_LINES - FZF_MATCH_COUNT - 4 ))
+      if [[ $FZF_MATCH_COUNT -eq 0 ]]; then
+        echo "change-preview-window:hidden"
+      elif [[ $lines -gt 3 ]]; then
+        echo "change-preview-window:$lines"
+      elif [[ $FZF_PREVIEW_LINES -ne 3 ]]; then
+        echo "change-preview-window:3"
+      fi
+    ' --preview 'seq {} 10000' --preview-window up
+    ```
+  - And we're phasing out `{fzf:prompt}` and `{fzf:action}`
 - Bug fixes
 
 0.45.0