minicom.sh should check for a char device, not a file.
diff --git a/scripts/minicom.sh b/scripts/minicom.sh
index b22e105..58685c6 100755
--- a/scripts/minicom.sh
+++ b/scripts/minicom.sh
@@ -2,7 +2,7 @@
 
 # If you want to use toybox netcat to talk to a serial port, use this.
 
-if [ ! -f "$1" ]
+if [ ! -c "$1" ]
 then
   echo "Usage: minicom.sh /dev/ttyS0"
   exit 1