diff --git a/mkuserimg.sh b/mkuserimg.sh
index 3480a20..5cf0f3a 100755
--- a/mkuserimg.sh
+++ b/mkuserimg.sh
@@ -1,51 +1,37 @@
 #!/bin/bash
 #
+# To call this script, make sure paths of executables such as
+# genext2fs, tune2fs, e2fsck are in the environmental variable PATH.
 
 function usage() {
 cat<<EOT
 Usage:
-mkuserimg.sh MKEXT2IMG TUNE2FS E2FSCK SRC_DIR OUTPUT_FILE EXT_VARIANT [LABEL]
+mkuserimg.sh SRC_DIR OUTPUT_FILE EXT_VARIANT [LABEL]
 EOT
 }
 
-if [ $# -ne 6 -a $# -ne 7 ]; then
+echo "in mkuserimg.sh PATH=$PATH"
+
+if [ $# -ne 3 -a $# -ne 4 ]; then
   usage
   exit 1
 fi
 
-MKEXT2IMG=$1
-if [ ! -x $MKEXT2IMG -o ! -f $MKEXT2IMG ]; then
-  echo "Can not find executable $MKEXT2IMG!"
+SRC_DIR=$1
+if [ ! -d $SRC_DIR ]; then
+  echo "Can not find directory $SRC_DIR!"
   exit 2
 fi
 
-TUNE2FS=$2
-if [ ! -x $TUNE2FS -o ! -f $TUNE2FS ]; then
-  echo "Can not find executable $TUNE2FS!"
-  exit 3
-fi
-
-E2FSCK=$3
-if [ ! -x $E2FSCK -o ! -f $E2FSCK ]; then
-  echo "Can not find executable $E2FSCK!"
-  exit 4
-fi
-
-SRC_DIR=$4
-if [ ! -d $SRC_DIR ]; then
-  echo "Can not find directory $SRC_DIR!"
-  exit 5
-fi
-
-OUTPUT_FILE=$5
-EXT_VARIANT=$6
-LABEL=$7
+OUTPUT_FILE=$2
+EXT_VARIANT=$3
+LABEL=$4
 
 case $EXT_VARIANT in
   ext2) ;;
   ext3) ;;
   ext4) ;;
-  *) echo "Only ext2, ext3, ext4 are supported!"; exit 6 ;;
+  *) echo "Only ext2, ext3, ext4 are supported!"; exit 3 ;;
 esac
 
 num_blocks=`du -skL $SRC_DIR | tail -n1 | awk '{print $1;}'`
@@ -61,45 +47,45 @@
 echo "num_blocks=$num_blocks"
 echo "num_inodes=$num_inodes"
 
-echo "$MKEXT2IMG -a -d $SRC_DIR -b $num_blocks -N $num_inodes -m 0 $OUTPUT_FILE"
-$MKEXT2IMG -a -d $SRC_DIR -b $num_blocks -N $num_inodes -m 0 $OUTPUT_FILE
+echo "genext2fs -a -d $SRC_DIR -b $num_blocks -N $num_inodes -m 0 $OUTPUT_FILE"
+genext2fs -a -d $SRC_DIR -b $num_blocks -N $num_inodes -m 0 $OUTPUT_FILE
 if [ $? -ne 0 ]; then
-  exit 7
+  exit 4
 fi
 
 if [ -n $LABEL ]; then
-  echo "$TUNE2FS -L $LABEL $OUTPUT_FILE"
-  $TUNE2FS -L $LABEL $OUTPUT_FILE
+  echo "tune2fs -L $LABEL $OUTPUT_FILE"
+  tune2fs -L $LABEL $OUTPUT_FILE
   if [ $? -ne 0 ]; then
-    exit 8
+    exit 5
   fi
 fi
 
 if [ $EXT_VARIANT = "ext3" ]; then
-  echo "$TUNE2FS -j $OUTPUT_FILE"
-  $TUNE2FS -j $OUTPUT_FILE
+  echo "tune2fs -j $OUTPUT_FILE"
+  tune2fs -j $OUTPUT_FILE
   if [ $? -ne 0 ]; then
-    exit 9
+    exit 6
   fi
 elif [ $EXT_VARIANT = "ext4" ]; then
-  echo "$TUNE2FS -j -O extents,uninit_bg,dir_index $OUTPUT_FILE"
-  $TUNE2FS -j -O extents,uninit_bg,dir_index $OUTPUT_FILE
+  echo "tune2fs -j -O extents,uninit_bg,dir_index $OUTPUT_FILE"
+  tune2fs -j -O extents,uninit_bg,dir_index $OUTPUT_FILE
   if [ $? -ne 0 ]; then
-    exit 10
+    exit 7
   fi
 fi
 
-echo "$TUNE2FS -C 1 $OUTPUT_FILE"
-$TUNE2FS -C 1 $OUTPUT_FILE
+echo "tune2fs -C 1 $OUTPUT_FILE"
+tune2fs -C 1 $OUTPUT_FILE
 if [ $? -ne 0 ]; then
-  exit 11
+  exit 8
 fi
 
-echo "$E2FSCK -fy $OUTPUT_FILE"
-$E2FSCK -fy $OUTPUT_FILE
+echo "e2fsck -fy $OUTPUT_FILE"
+e2fsck -fy $OUTPUT_FILE
 if [ $? -ge 4 ]; then
-  echo "$E2FSCK returns value $?, no less than 4!"
-  exit 12
+  echo "e2fsck returns value $?, no less than 4!"
+  exit 9
 else
   exit 0
 fi