diff --git a/README.md b/README.md
index 1f992ae..e3502c0 100644
--- a/README.md
+++ b/README.md
@@ -1,68 +1,70 @@
-
-# PiShrink #
-PiShrink is a bash script that automatically shrink a pi image that will then resize to the max size of the SD card on boot. This will make putting the image back onto the SD card faster and the shrunk images will compress better.
-In addition the shrinked image can be compressed with gzip, pigz, xz or pxz to create an even smaller image.
-
-## Usage ##
-```
-sudo pishrink.sh [-sdirzh] imagefile.img [newimagefile.img]
-  -s         Don't expand filesystem when image is booted the first time
-  -d         Write debug messages in a debug log file
-  -r         Use advanced filesystem repair option if the normal one fails
-  -p         Remove logs, apt archives, dhcp leases and ssh hostkeys
-  -v		 Be verbose
-  -z         Compress image after shrinking
-  -i TOOL    Zip tool to use to compress image. TOOLS can be one of $ZIPTOOLS (Default: gzip)
-  -a         Use parallel compress feature of compression tool
-```
-
-If you specify the `newimagefile.img` parameter, the script will make a copy of `imagefile.img` and work off that. You will need enough space to make a full copy of the image to use that option.
-
-* `-s` will skip the autoexpanding part of the process.
-* `-d` will create a logfile `pishrink.log` which may help for problem analysis.
-* `-r` will attempt to repair the filesystem if regular repairs fail
-* `-z` will compress the image after shrinking using gzip or xz. Default is `gzip`. `.gz` or `.xz` extension will be added to the filename.
-* `-a` will use option -f9 for gzip and option -T0 for xz.
-
-Default options for parallel compression can be overwritten by defining PISHRINK_GZIP or PSHRINK_XZ environment variable.
-
-## Prerequisites ##
-If you are trying to shrink a [NOOBS](https://github.com/raspberrypi/noobs) image it will likely fail. This is due to [NOOBS partitioning](https://github.com/raspberrypi/noobs/wiki/NOOBS-partitioning-explained) being significantly different than Raspbian's. Hopefully PiShrink will be able to support NOOBS in the near future.
-
-If using Ubuntu, you will likely see an error about `e2fsck` being out of date and `metadata_csum`. The simplest fix for this is to use Ubuntu 16.10 and up, as it will save you a lot of hassle in the long run.
-
-## Installation ##
-```bash
-wget https://raw.githubusercontent.com/Drewsif/PiShrink/master/pishrink.sh
-chmod +x pishrink.sh
-sudo mv pishrink.sh /usr/local/bin
-```
-
-## Example ##
-```bash
-[user@localhost PiShrink]$ sudo pishrink.sh pi.img
-e2fsck 1.42.9 (28-Dec-2013)
-Pass 1: Checking inodes, blocks, and sizes
-Pass 2: Checking directory structure
-Pass 3: Checking directory connectivity
-Pass 4: Checking reference counts
-Pass 5: Checking group summary information
-/dev/loop1: 88262/1929536 files (0.2% non-contiguous), 842728/7717632 blocks
-resize2fs 1.42.9 (28-Dec-2013)
-resize2fs 1.42.9 (28-Dec-2013)
-Resizing the filesystem on /dev/loop1 to 773603 (4k) blocks.
-Begin pass 2 (max = 100387)
-Relocating blocks             XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
-Begin pass 3 (max = 236)
-Scanning inode table          XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
-Begin pass 4 (max = 7348)
-Updating inode references     XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
-The filesystem on /dev/loop1 is now 773603 blocks long.
-
-Shrunk pi.img from 30G to 3.1G
-```
-
-## Contributing ##
-If you find a bug please create an issue for it. If you would like a new feature added, you can create an issue for it but I can't promise that I will get to it.
-
-Pull requests for new features and bug fixes are more than welcome!
+
+# PiShrink #
+PiShrink is a bash script that automatically shrink a pi image that will then resize to the max size of the SD card on boot. This will make putting the image back onto the SD card faster and the shrunk images will compress better.
+In addition the shrinked image can be compressed with gzip and xz to create an even smaller image. Parallel compression of the image
+using multiple cores is supported.
+
+## Usage ##
+```
+Usage: $0 [-sdiarpzZvh] imagefile.img [newimagefile.img]
+
+  -s         Don't expand filesystem when image is booted the first time
+  -d         Write debug messages in a debug log file
+  -r         Use advanced filesystem repair option if the normal one fails
+  -p         Remove logs, apt archives, dhcp leases and ssh hostkeys
+  -v         Be verbose
+  -z         Compress image after shrinking with gzip
+  -Z         Compress image after shrinking with xz
+  -a         Compress image in parallel using multiple cores
+```
+
+If you specify the `newimagefile.img` parameter, the script will make a copy of `imagefile.img` and work off that. You will need enough space to make a full copy of the image to use that option.
+
+* `-s` will skip the autoexpanding part of the process.
+* `-d` will create a logfile `pishrink.log` which may help for problem analysis.
+* `-r` will attempt to repair the filesystem if regular repairs fail
+* `-z` will compress the image after shrinking using gzip or xz. Default is `gzip`. `.gz` or `.xz` extension will be added to the filename.
+* `-a` will use option -f9 for pigz and option -T0 for xz.
+
+Default options for parallel compression can be overwritten by defining PISHRINK_GZIP or PSHRINK_XZ environment variables.
+
+## Prerequisites ##
+If you are trying to shrink a [NOOBS](https://github.com/raspberrypi/noobs) image it will likely fail. This is due to [NOOBS partitioning](https://github.com/raspberrypi/noobs/wiki/NOOBS-partitioning-explained) being significantly different than Raspbian's. Hopefully PiShrink will be able to support NOOBS in the near future.
+
+If using Ubuntu, you will likely see an error about `e2fsck` being out of date and `metadata_csum`. The simplest fix for this is to use Ubuntu 16.10 and up, as it will save you a lot of hassle in the long run.
+
+## Installation ##
+```bash
+wget https://raw.githubusercontent.com/Drewsif/PiShrink/master/pishrink.sh
+chmod +x pishrink.sh
+sudo mv pishrink.sh /usr/local/bin
+```
+
+## Example ##
+```bash
+[user@localhost PiShrink]$ sudo pishrink.sh pi.img
+e2fsck 1.42.9 (28-Dec-2013)
+Pass 1: Checking inodes, blocks, and sizes
+Pass 2: Checking directory structure
+Pass 3: Checking directory connectivity
+Pass 4: Checking reference counts
+Pass 5: Checking group summary information
+/dev/loop1: 88262/1929536 files (0.2% non-contiguous), 842728/7717632 blocks
+resize2fs 1.42.9 (28-Dec-2013)
+resize2fs 1.42.9 (28-Dec-2013)
+Resizing the filesystem on /dev/loop1 to 773603 (4k) blocks.
+Begin pass 2 (max = 100387)
+Relocating blocks             XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
+Begin pass 3 (max = 236)
+Scanning inode table          XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
+Begin pass 4 (max = 7348)
+Updating inode references     XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
+The filesystem on /dev/loop1 is now 773603 blocks long.
+
+Shrunk pi.img from 30G to 3.1G
+```
+
+## Contributing ##
+If you find a bug please create an issue for it. If you would like a new feature added, you can create an issue for it but I can't promise that I will get to it.
+
+Pull requests for new features and bug fixes are more than welcome!
diff --git a/pishrink.sh b/pishrink.sh
index ba97b86..8f23190 100755
--- a/pishrink.sh
+++ b/pishrink.sh
@@ -8,6 +8,7 @@ MYNAME="${SCRIPTNAME%.*}"
 LOGFILE="${CURRENT_DIR}/${SCRIPTNAME%.*}.log"
 REQUIRED_TOOLS="parted losetup tune2fs md5sum e2fsck resize2fs"
 ZIPTOOLS=("gzip xz")
+declare -A ZIP_PARALLEL_TOOL=( [gzip]="pigz" [xz]="xz" ) # parallel zip tool to use in parallel mode
 declare -A ZIP_PARALLEL_OPTIONS=( [gzip]="-f9" [xz]="-T0" ) # options for zip tools in parallel mode
 declare -A ZIPEXTENSIONS=( [gzip]="gz" [xz]="xz" ) # extensions of zipped files
 
@@ -68,16 +69,16 @@ fi
 help() {
 	local help
 	read -r -d '' help << EOM
-Usage: $0 [-sdiarpzvh] imagefile.img [newimagefile.img]
+Usage: $0 [-sdarpzZvh] imagefile.img [newimagefile.img]
 
   -s         Don't expand filesystem when image is booted the first time
   -d         Write debug messages in a debug log file
   -r         Use advanced filesystem repair option if the normal one fails
   -p         Remove logs, apt archives, dhcp leases and ssh hostkeys
   -v			 Be verbose
-  -z         Compress image after shrinking
-  -i TOOL    Zip tool to use to compress image. TOOLS can be one of $ZIPTOOLS (Default: gzip)
-  -a         Use parallel compress feature of compression tool
+  -z         Compress image after shrinking with gzip
+  -Z         Compress image after shrinking with xz
+  -a         Compress image in parallel using multiple cores
 EOM
 	echo "$help"
 	exit -1
@@ -86,11 +87,10 @@ EOM
 should_skip_autoexpand=false
 debug=false
 repair=false
-compress=false
 parallel=false
 verbose=false
 prep=false
-ziptool="gzip"
+ziptool=""
 required_tools="$REQUIRED_TOOLS"
 
 while getopts ":adhipr:svz" opt; do
@@ -98,11 +98,11 @@ while getopts ":adhipr:svz" opt; do
     a) parallel=true;;
     d) debug=true;;
     h) help;;
-    i) ziptool="$OPTARG";;
     p) prep=true;;
     r) repair=true;;
     s) should_skip_autoexpand=true ;;
-    z) compress=true;;
+    z) ziptool="gzip";;
+    Z) ziptool="xz";;
     v) verbose=true;;
     *) help;;
   esac
@@ -137,12 +137,16 @@ if (( EUID != 0 )); then
 fi
 
 # check selected compression tool is supported and installed
-if [[ $compress == true ]]; then
+if [[ -n $ziptool ]]; then
 	if [[ ! " ${ZIPTOOLS[@]} " =~ " $ziptool " ]]; then
 		error $LINENO "$ziptool is an unsupported ziptool."
 		exit -17
 	else
-		REQUIRED_TOOLS="$REQUIRED_TOOLS $ziptool"
+		if [[ $parallel == true && ziptool == "gzip" ]]; then
+			REQUIRED_TOOLS="$REQUIRED_TOOLS pigz"
+		else
+			REQUIRED_TOOLS="$REQUIRED_TOOLS $ziptool"
+		fi
 	fi
 fi
 
@@ -340,23 +344,24 @@ if ! truncate -s "$endresult" "$img"; then
 	exit -16
 fi
 
-if [[ $compress == true ]]; then
+if [[ -n $ziptool ]]; then
 	options=""
-	envVarname="${MYNAME^^}_${ziptool^^}"
+	envVarname="${MYNAME^^}_${ziptool^^}" # PISHRINK_GZIP or PISHRINK_XZ environment variables allow to override options
+	[[ $parallel == true ]] && options="${ZIP_PARALLEL_OPTIONS[$ziptool]}"
+	[[ -v $envVarname ]] && options="${!envVarname}" # if environment variable defined use these options
+	[[ $verbose == true ]] && options="$options -v" # add verbose flag 
+	
 	if [[ $parallel == true ]]; then
-		options="${ZIP_PARALLEL_OPTIONS[$ziptool]}"
-		[[ -v $envVarname ]] && options="${!envVarname}"
-		[[ $verbose == true ]] && options="$options -v"
-		info "Using $ziptool on the shrunk image using options ${options}"
-		if ! $ziptool ${options} "$img"; then
+		parallel_tool="${ZIP_PARALLEL_TOOL[$ziptool]}"
+		info "Using $parallel_tool on the shrunk image"
+		if ! $parallel_tool ${options} "$img"; then
 			rc=$?
-			error $LINENO "$ziptool failed with rc $rc"
+			error $LINENO "$parallel_tool failed with rc $rc"
 			exit -18
 		fi
+		
 	else # sequential
 		info "Using $ziptool on the shrunk image"
-		[[ -v $envVarname ]] && options="${!envVarname}"
-		[[ $verbose == true ]] && options="$options -v"
 		if ! $ziptool ${options} $img; then
 			rc=$?
 			error $LINENO "$ziptool failed with rc $rc"