improve and fix issues shellcheck

scripts/i3blocks/iface

@@ -1,6 +1,7 @@
 #!/bin/bash
 # Copyright (C) 2014 Julien Bonjean <julien@bonjean.info>
 # Copyright (C) 2014 Alexander Keller <github@nycroth.com>
+# Copyright (C) 2019 Jesús E. <heckyel@hyperbola.info>
 
 # This program is free software: you can redistribute it and/or modify
 # it under the terms of the GNU General Public License as published by
@@ -18,10 +19,10 @@
 #------------------------------------------------------------------------
 
 # Use the provided interface, otherwise the device used for the default route.
-if [[ -n $BLOCK_INSTANCE ]]; then
-  IF=$BLOCK_INSTANCE
-else
-  IF=$(ip route | awk '/^default/ { print $5 ; exit }')
+if [[ -z $IF ]] && [[ -n $BLOCK_INSTANCE ]]; then
+    IF=$BLOCK_INSTANCE
+elif [[ -z $IF ]]; then
+    IF=$(ip route | awk '/^default/ { print $5 ; exit }')
 fi
 
 #------------------------------------------------------------------------
@@ -32,27 +33,27 @@ fi
 
 #------------------------------------------------------------------------
 
-if [[ "$(cat /sys/class/net/$IF/operstate)" = 'down' ]]; then
-  echo down # full text
-  echo down # short text
-  echo \#FF0000 # color
-  exit
+if [[ "$(cat "/sys/class/net/$IF/operstate")" = 'down' ]]; then
+    echo down # full text
+    echo down # short text
+    echo \#FF0000 # color
+    exit
 fi
 
 case $1 in
-  -4)
-    AF=inet ;;
-  -6)
-    AF=inet6 ;;
-  *)
-    AF=inet6? ;;
+    -4)
+        AF=inet ;;
+    -6)
+        AF=inet6 ;;
+    *)
+        AF=inet6? ;;
 esac
 
 # if no interface is found, use the first device with a global scope
-IPADDR=$(ip addr show $IF | perl -n -e "/$AF ([^\/]+).* scope global/ && print \$1 and exit")
+IPADDR=$(ip addr show "$IF" | perl -n -e "/$AF ([^\/]+).* scope global/ && print \$1 and exit")
 
 case $BLOCK_BUTTON in
-  3) echo -n "$IPADDR" | xclip -q -se c ;;
+    3) echo -n "$IPADDR" | xclip -q -se c ;;
 esac
 
 #------------------------------------------------------------------------