diff --git a/udev.config b/udev.config
index 8d77099ca8..3eb688a3a0 100644
--- a/udev.config
+++ b/udev.config
@@ -1,13 +1,18 @@
-# There are a number of modifiers that are allowed to be used in the NAME field.
+# There are a number of modifiers that are allowed to be used in the NAME or PROGRAM fields.
 # They provide the following subsitutions:
 # %n - the "kernel number" of the device.
 #      for example, 'sda3' has a "kernel number" of '3'
 # %M - the kernel major number for the device
 # %m - the kernel minor number for the device
 # %b - the bus id for the device
+# %c - the return value for the CALLOUT program (note, this doesn't work within
+#      the PROGRAM field for the obvious reason.)
 #
 
-# My usb camera...
+# Looking for scsi bus id 42:0:0:1
+CALLOUT, BUS="scsi", PROGRAM="/bin/echo -n test-%b", ID="test-42:0:0:1", NAME="%c"
+
+# A usb camera.
 LABEL, BUS="usb", vendor="FUJIFILM", NAME="camera%n"
 
 # USB Epson printer to be called lp_epson