Expose state information via environment variables to child processes

Close #3582

man/man1/fzf.1

@@ -592,8 +592,6 @@ Also,
 * \fB{n}\fR is replaced to the zero-based ordinal index of the current item.
   Use \fB{+n}\fR if you want all index numbers when multiple lines are selected.
 .br
-* \fB{fzf:action}\fR is replaced to to the name of the last action performed
-* \fB{fzf:prompt}\fR is replaced to to the prompt string
 
 Note that you can escape a placeholder pattern by prepending a backslash.
 
@@ -901,6 +899,39 @@ of field index expressions.
 .BR .. "     All the fields"
 .br
 
+.SH ENVIRONMENT VARIABLES EXPORTED TO CHILD PROCESSES
+
+fzf exports the following environment variables to its child processes.
+
+.BR FZF_LINES "           Number of lines fzf takes up excluding padding and margin"
+.br
+.BR FZF_COLUMNS "         Number of columns fzf takes up excluding padding and margin"
+.br
+.BR FZF_TOTAL_COUNT "     Total number of items"
+.br
+.BR FZF_MATCH_COUNT "     Number of matched items"
+.br
+.BR FZF_SELECT_COUNT "    Number of selected items"
+.br
+.BR FZF_QUERY "           Current query string"
+.br
+.BR FZF_PROMPT "          Prompt string"
+.br
+.BR FZF_ACTION "          The name of the last action performed"
+.br
+.BR FZF_PORT "            Port number when --listen option is used"
+.br
+
+The following variables are additionally exported to the preview commands.
+
+.BR FZF_PREVIEW_TOP "     Top position of the preview window"
+.br
+.BR FZF_PREVIEW_LEFT "    Left position of the preview window"
+.br
+.BR FZF_PREVIEW_LINES "   Number of lines in the preview window"
+.br
+.BR FZF_PREVIEW_COLUMNS " Number of columns in the preview window"
+
 .SH EXTENDED SEARCH MODE
 
 Unless specified otherwise, fzf will start in "extended-search mode". In this