Add an option to print hovered files to a FIFO (#548)

* Add an option to print hovered files to a FIFO

This adds an env variable, `NNN_FIFO`, that can be set to a path that
`nnn` will open/create as a FIFO, and where every hovered file's path is
printed. This allows creating external perview/quick open plugins, ...

It can be compiled out with the make variable `O_NOFIFO`.

* Check filename ptr instead of full path (for FIFO)

* Add documentation to use NNN_FIFO in plugins

* Fix path sent to FIFO in empty dirs

1 files changed, 7 insertions, 0 deletions

diff --git a/nnn.1 b/nnn.1
index 5048bb0..95e93a6 100644
--- a/nnn.1
+++ b/nnn.1
@@ -423,6 +423,13 @@ separated by \fI;\fR:
 .Pp
 \fBNNN_SEL:\fR absolute path to custom selection file.
 .Pp
+\fBNNN_FIFO:\fR path of a named pipe to write current file path:
+.Bd -literal
+    export NNN_FIFO='/tmp/nnn.fifo'
+
+    NOTE: If the FIFO file doesn't exist it will be created, but it will never be removed.
+.Ed
+.Pp
 \fBnnn:\fR this is a special variable set to the hovered entry before executing
 a command from the command prompt or spawning a shell.
 .Pp