Use a variable hls_path in nginx - nginx

Question

Right now I am saving all the files and folders created by hls in a single directory like this
application live {
live on; # Allows live input
# Once receive stream, transcode for adaptive streaming
# This single ffmpeg command takes the input and transforms
# the source into 4 different streams with different bitrate
# and quality. P.S. The scaling done here respects the aspect
# ratio of the input.
exec ffmpeg -i rtmp://127.0.0.1/$app/$name -async 1 -vsync -1
-c:v libx264 -c:a aac -strict -2 -b:v 256k -b:a 32k -vf "scale=480:trunc(ow/a/2)*2" -tune zerolatency -preset veryfast$
-c:v libx264 -c:a aac -strict -2 -b:v 768k -b:a 96k -vf "scale=720:trunc(ow/a/2)*2" -tune zerolatency -preset veryfast$
-c:v libx264 -c:a aac -strict -2 -b:v 1024k -b:a 128k -vf "scale=960:trunc(ow/a/2)*2" -tune zerolatency -preset veryfa$
-c:v libx264 -c:a aac -strict -2 -b:v 1920k -b:a 128k -vf "scale=1280:trunc(ow/a/2)*2" -tune zerolatency -preset veryf$
-c copy -f flv rtmp://127.0.0.1/show/$name_src;
}
# This application is for splitting the stream into HLS fragments
application show {
live on; # Allows live input from above
hls on; # Enable HTTP Live Streaming
hls_cleanup off;
hls_nested on;
# Pointing this to an SSD is better as this involves lots of IO
#exec mkdir /mnt/HLS/;
hls_path /mnt/HLS/;
# Instruct clients to adjust resolution according to bandwidth
hls_variant _low BANDWIDTH=288000; # Low bitrate, sub-SD resolution
hls_variant _mid BANDWIDTH=448000; # Medium bitrate, SD resolution
hls_variant _high BANDWIDTH=1152000; # High bitrate, higher-than-SD resolution
hls_variant _hd720 BANDWIDTH=2048000; # High bitrate, HD 720p resolution
}
How do I make a new directory for every hls streams according to the name of streams.