diff --git a/i3batwarn.sh b/i3batwarn.sh
index acfb1a6..d34825b 100755
--- a/i3batwarn.sh
+++ b/i3batwarn.sh
@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/bin/sh
 
 #############################################
 # This is a simple battery warning script.  #
@@ -8,57 +8,52 @@
 #############################################
 
 # lock file location
-export LOCK_FILE=/tmp/battery_state.lock
+export LOCK_FILE='/tmp/battery_state.lock'
 
 # check if another copy is running
-if [[ -a $LOCK_FILE ]]; then
+if [ -e "${LOCK_FILE}" ] ; then
 
-    pid=$(cat $LOCK_FILE | awk '{print $1}')
-	ppid=$(cat $LOCK_FILE | awk '{print $2}')
+	pid="$(awk '{print $1}' "${LOCK_FILE}")"
+	ppid="$(awk '{print $2}' "${LOCK_FILE}")"
 	# validate contents of previous lock file
-	vpid=${pid:-"0"}
-	vppid=${ppid:-"0"}
+	vpid="${pid:-"0"}"
+	vppid="${ppid:-"0"}"
 
-    if (( $vpid < 2 || $vppid < 2 )); then
+	if [ "${vpid}" -lt '2' ] || [ "${vppid}" -lt '2' ] ; then
 		# corrupt lock file $LOCK_FILE ... Exiting
-		cp -f $LOCK_FILE ${LOCK_FILE}.`date +%Y%m%d%H%M%S`
+		cp -f "${LOCK_FILE}" "${LOCK_FILE}.$(date +%Y%m%d%H%M%S)"
 		exit
 	fi
 
-    # check if ppid matches pid
-	ps -f -p $pid --no-headers | grep $ppid >/dev/null 2>&1
-
-    if [[ $? -eq 0 ]]; then
-		# another copy of script running with process id $pid
+	# check if ppid matches pid
+	if ps -f -p "${pid}" --no-headers | grep -q "${ppid}" ; then
+		# another copy of script running with process id ${pid}
 		exit
 	else
 		# bogus lock file found, removing
-		rm -f $LOCK_FILE >/dev/null
+		rm -f "${LOCK_FILE}" >/dev/null
 	fi
 
 fi
 
-pid=$$
-ps -f -p $pid --no-headers | awk '{print $2,$3}' > $LOCK_FILE
+pid="$$"
+ps -f -p "${pid}" --no-headers | awk '{print $2,$3}' > "${LOCK_FILE}"
 # starting with process id $pid
 
 # set Battery
-BATTERY=$(ls /sys/class/power_supply/ | grep '^BAT')
-
-# set full path
-ACPI_PATH="/sys/class/power_supply/$BATTERY"
+BATTERY="$(echo '/sys/class/power_supply/BAT'?)"
 
 # get battery status
-STAT=$(cat $ACPI_PATH/status)
+STAT="$(cat "${BATTERY}/status")"
 
 # get remaining energy value
-REM=`grep "POWER_SUPPLY_ENERGY_NOW" $ACPI_PATH/uevent | cut -d= -f2`
+REM="$(grep -E "POWER_SUPPLY_(CHARGE|ENERGY)_NOW" "${BATTERY}/uevent" | cut -d '=' -f2)"
 
 # get full energy value
-FULL=`grep "POWER_SUPPLY_ENERGY_FULL_DESIGN" $ACPI_PATH/uevent | cut -d= -f2`
+FULL="$(grep -E "POWER_SUPPLY_(CHARGE|ENERGY)_FULL_DESIGN" "${BATTERY}/uevent" | cut -d '=' -f2)"
 
 # get current energy value in percent
-PERCENT=`echo $(( $REM * 100 / $FULL ))`
+PERCENT="$((REM * 100 / FULL))"
 
 # set error message
 MESSAGE="Low battery warning, find charger"
@@ -66,36 +61,36 @@ MESSAGE="Low battery warning, find charger"
 # set energy limit in percent, where warning should be displayed
 LIMIT="15"
 
-I3BAT_TMPDIR=$(mktemp --directory --tmpdir i3batwarn.XXX)
-NAGBARPIDFILE="$I3BAT_TMPDIR/nagbarpid_file"
+I3BAT_TMPDIR="$(mktemp --directory --tmpdir i3batwarn.XXX)"
+NAGBARPIDFILE="${I3BAT_TMPDIR}/nagbarpid_file"
 
 # show warning if energy limit in percent is less then user set limit and
 # if battery is discharging
-if [ $PERCENT -le "$(echo $LIMIT)" ] && [ "$STAT" == "Discharging" ]; then
+if [ "${PERCENT}" -le "${LIMIT}" ] && [ "${STAT}" = "Discharging" ] ; then
   #chek if nagbarfile is empty: else open new - to avoid multiples
-    if [ ! -s $NAGBARPIDFILE ] ; then
-        /usr/bin/i3-nagbar -m "$(echo $MESSAGE)" &
-        echo $! > $NAGBARPIDFILE
-    elif ps -e | grep $(cat $NAGBARPIDFILE) | grep "i3-nagbar"; then
-        echo "pidfile in order, nothing to do"
-    else
-        rm $NAGBARPIDFILE
-        /usr/bin/i3-nagbar -m "$(echo $MESSAGE)" &
-        echo $! > $NAGBARPIDFILE
-    fi #else if, nagbarpid points to something else create new.
+	if [ ! -s "${NAGBARPIDFILE}" ] ; then
+		/usr/bin/i3-nagbar -m "${MESSAGE}" &
+		echo "$!" > "${NAGBARPIDFILE}"
+	elif ps -e | grep "$(cat "${NAGBARPIDFILE}")" | grep "i3-nagbar" ; then
+		echo "pidfile in order, nothing to do"
+	else
+		rm "${NAGBARPIDFILE}"
+		/usr/bin/i3-nagbar -m "${MESSAGE}" &
+		echo "$!" > "${NAGBARPIDFILE}"
+	fi #else if, nagbarpid points to something else create new.
 fi
+
 #warning, if the nagbar is closed manually the pidfile might not be emptied properly
 #for safety the charging requirement below is relaxed, if you use the nagbar for other reasons
 #it might get closed accidentaly by this
-
-if [ $PERCENT -gt "$(echo $LIMIT)" ] || [ "$STAT" == "Charging" ]
-then
-    if [ -s $NAGBARPIDFILE ] ; then
-        if ps -e | grep $(cat $NAGBARPIDFILE) | grep "i3-nagbar"; then
-            kill $(cat $NAGBARPIDFILE)
-            rm $NAGBARPIDFILE
-        else
-            rm $NAGBARPIDFILE
-        fi
-    fi
+if [ "${PERCENT}" -gt "${LIMIT}" ] || [ "${STAT}" = "Charging" ] ; then
+	if [ -s "${NAGBARPIDFILE}" ] ; then
+		if ps -e | grep "$(cat "${NAGBARPIDFILE}")" | grep "i3-nagbar" ; then
+			kill "$(cat "${NAGBARPIDFILE}")"
+			rm "${NAGBARPIDFILE}"
+		else
+			rm "${NAGBARPIDFILE}"
+		fi
+	fi
 fi
+