diff --git a/bin/compton-trans b/bin/compton-trans
index f813433..d81a8ab 100755
--- a/bin/compton-trans
+++ b/bin/compton-trans
@@ -37,43 +37,44 @@ while getopts "scn:w:o:" option; do
     c)
       active=$(xprop -root -notype "_NET_ACTIVE_WINDOW" \
         | sed 's/^.*\(0x\S*\).*$/\1/')
-      wprefix='-id '; window="$active"
+      wprefix='-id'; window=$active
     ;;
-    n) wprefix='-name '; window="$OPTARG" ;;
-    w) wprefix='-id '; window="$OPTARG" ;;
-    o) opacity="$OPTARG" ;;
+    n) wprefix='-name'; window=$OPTARG ;;
+    w) wprefix='-id'; window=$OPTARG ;;
+    o) opacity=$OPTARG ;;
     \?) exit 1;;
   esac
 done
 
 # Read positional arguments
 shift $(($OPTIND - 1))
-[ -n "$1" ] && opacity="$1"
+test -n "$1" && opacity=$1
 
 # Validate opacity value
-if [ -z "$opacity" ]; then
+if test -z "$opacity"; then
   echo "No opacity specified."
   exit 1
 fi
 
-opacity="$(echo "$opacity" \
-  | sed -rn 's/^[[:space:]]*([+-]?[[:digit:]]+)[[:space:]]*$/\1/p')"
+opacity=$(echo "$opacity" \
+  | sed 's/%//g' \
+  | sed -rn 's/^[[:space:]]*([+-]?[[:digit:]]+)[[:space:]]*$/\1/p')
 
-if [ -z "$opacity" ]; then
+if test -z "$opacity"; then
   echo "Invalid opacity value."
   exit 1
 fi
 
 # Get ID of the target window
-if [ -z "$wprefix" ]; then
+if test -z "$wprefix"; then
   treeout=$(xwininfo -children -frame)
 else
-  treeout=$(xwininfo -children $wprefix"$window")
+  treeout=$(xwininfo -children $wprefix "$window")
 fi
 
 wid=$(echo "$treeout" | sed -n 's/^xwininfo:.*: \(0x[[:xdigit:]]*\).*$/\1/p')
 
-if [ -z "$wid" ]; then
+if test -z "$wid"; then
   echo "Failed to find window."
   exit 1
 fi
@@ -86,29 +87,31 @@ fi
 
 # If it's already the topmost window
 if echo "$treeout" | grep "Parent window id: 0x[[:xdigit:]]* (the root window)" > /dev/null; then
-  topmost="$wid"
+  topmost=$wid
 else
   # Get the whole window tree
-  treeout="$(xwininfo -root -tree)"
+  treeout=$(xwininfo -root -tree)
 
-  if [ -z "$treeout" ]; then
+  if test -z "$treeout"; then
     echo "Failed to get root window tree."
     exit 1
   fi
 
   # Find the line number of the target window in the window tree
-  lineno="$(echo -n "$treeout" | grep -nw "$wid" | head -n1 | cut -d ':' -f 1)"
+  lineno=$(echo -n "$treeout" | grep -nw "$wid" | head -n1 | cut -d ':' -f 1)
 
-  if [ -z "$lineno" ]; then
+  if test -z "$lineno"; then
     echo "Failed to find window in window tree."
     exit 1
   fi
 
   # Find the highest ancestor of the target window below
-  topmost=$(echo -n "$treeout" | head -n $(($lineno + 1)) | sed -n 's/^     \(0x[[:xdigit:]]*\).*/\1/p' | tail -n1)
+  topmost=$(echo -n "$treeout" \
+    | head -n $(($lineno + 1)) \
+    | sed -n 's/^     \(0x[[:xdigit:]]*\).*/\1/p' | tail -n1)
 fi
 
-if [ -z "$topmost" ]; then
+if test -z "$topmost"; then
   echo "Failed to find the highest parent window below root of the" \
     "selected window."
   exit 1
@@ -116,19 +119,17 @@ fi
 
 # Calculate the desired opacity
 if echo "$opacity" | grep '^[+-]' > /dev/null; then
-  sign=$(echo "$opacity" | cut -b1)
   cur=$(xprop -id "$topmost" -notype "_NET_WM_WINDOW_OPACITY" \
     | sed 's/^.*\b\([0-9]\+\).*$\|^.*$/\1/')
-  [ -z "$cur" ] && cur=0xffffffff
+  test -z "$cur" && cur=0xffffffff
   cur=$((cur * 100 / 0xffffffff))
-  opacity="$(echo "$opacity" | sed 's/^[+-]//')"
-    opacity=$(($cur $sign $opacity))
+  opacity=$(($cur + $opacity))
 fi
 
-[ $opacity -lt 0 ] && opacity=0
-[ $opacity -gt 100 ] && opacity=100
+test $opacity -lt 0 && opacity=0
+test $opacity -gt 100 && opacity=100
 
 # Set opacity
-opacity=$(($opacity * 0xffffffff / 100))
+opacity=$((opacity * 0xffffffff / 100))
 xprop -id "$topmost" -f _NET_WM_WINDOW_OPACITY 32c \
   -set _NET_WM_WINDOW_OPACITY "$opacity"