diff --git a/android-release b/android-release
index cbc46bfb..9ea99e9a 100755
--- a/android-release
+++ b/android-release
@@ -32,7 +32,7 @@
 # files in the program, then also delete it here.
 
 # cd to the directory where this script exists.
-cd $(dirname $0)
+cd "$(dirname $0)"
 . ./script-helper
 
 get_version
@@ -43,8 +43,8 @@ if [ -z "$VERSION" ]; then
 fi
 
 DIST_DIR=aria2-$VERSION-aarch64-linux-android-build1
-mkdir $DIST_DIR
+mkdir "$DIST_DIR"
 cp AUTHORS COPYING ChangeLog LICENSE.OpenSSL NEWS README.html README.android \
-    android-out/aria2c $DIST_DIR
-zip -9 -r $DIST_DIR.zip $DIST_DIR
-rm -rf $DIST_DIR
+    android-out/aria2c "$DIST_DIR"
+zip -9 -r "$DIST_DIR.zip" "$DIST_DIR"
+rm -rf "$DIST_DIR"
diff --git a/build_test.sh b/build_test.sh
index e0c090da..63fded82 100755
--- a/build_test.sh
+++ b/build_test.sh
@@ -6,27 +6,27 @@ BUILDDIR=/tmp/aria2buildtest
 
 if [ ! -d "$BUILDDIR" ]; then
     mkdir "$BUILDDIR" \
-	|| { echo "Failed to create directory $BUILDDIR" && exit -1; }
+	|| { echo "Failed to create directory $BUILDDIR" && exit 1; }
 fi
 
 echo -n "Starting build test "
-echo `date`
+echo "$(date)"
 
 # build CONFIGURE_OPTS BIN_SUFFIX DESC
 build()
 {
     echo -n "new build() started at "
-    echo `date`
+    echo "$(date)"
     echo "*** configure opts=$1"
     BIN_NAME="aria2c_$2"
     if [ -f "$BUILDDIR/$BIN_NAME" ]; then
 	echo "$BIN_NAME exists, skipping"
 	return
     fi
-    ./configure $1 2>&1 | tee "$BUILDDIR/configure_$2.log"\
+    ./configure $1 2>&1 | tee "$BUILDDIR/configure_$2.log" \
 	&& cp config.log "$BUILDDIR/config.log_$2" \
 	&& LANG=C make clean \
-	&& LANG=C make -j2 check 2>&1 |tee "$BUILDDIR/aria2c_$2.log" \
+	&& LANG=C make -j2 check 2>&1 | tee "$BUILDDIR/aria2c_$2.log" \
 	&& cp src/aria2c "$BUILDDIR/aria2c_$2"
 
     if [ -f "test/aria2c.log" ]; then
@@ -36,8 +36,8 @@ build()
 
 clear()
 {
-    for file in `ls $BUILDDIR`; do
-	rm -f "$BUILDDIR/$file";
+    for file in $(ls "$BUILDDIR"); do
+	rm -f "$BUILDDIR/$file"
     done
 }
 
diff --git a/import-po b/import-po
index dea98de2..cca7b57e 100755
--- a/import-po
+++ b/import-po
@@ -33,7 +33,7 @@ echo -n "en@quot en@boldquot" > "$PO_DIR"/LINGUAS
 for file in "$WORK_DIR"/*.po; do
     # First remove useless '\r' in messages
     sed -i -e 's/\\r//' "$file"
-    bn=`basename "$file"`
+    bn=$(basename "$file")
     bn=${bn#aria2-}
     dst="$PO_DIR"/"$bn"
     # copy file to po directory
diff --git a/mingw-release b/mingw-release
index 61a07899..722a7fe5 100755
--- a/mingw-release
+++ b/mingw-release
@@ -36,7 +36,7 @@ test -z "$BUILD_VER" && BUILD_VER=1
 test -z "$LABEL" && LABEL=$HOST
 
 # cd to the directory where this script exists.
-cd $(dirname $0)
+cd "$(dirname "$0")"
 . ./script-helper
 
 get_version
@@ -48,8 +48,8 @@ fi
 
 DIST_DIR=aria2-${VERSION}-${LABEL}-build${BUILD_VER}
 
-mkdir $DIST_DIR
+mkdir "$DIST_DIR"
 cp AUTHORS COPYING ChangeLog LICENSE.OpenSSL NEWS README.html README.mingw \
-    mingw-out/aria2c.exe $DIST_DIR
-zip -9 -r $DIST_DIR.zip $DIST_DIR
-rm -rf $DIST_DIR
+    mingw-out/aria2c.exe "$DIST_DIR"
+zip -9 -r "$DIST_DIR.zip" "$DIST_DIR"
+rm -rf "$DIST_DIR"
diff --git a/script-helper b/script-helper
index 30460b66..4f8e9c30 100644
--- a/script-helper
+++ b/script-helper
@@ -29,7 +29,7 @@
 # version.  If you delete this exception statement from all source
 # files in the program, then also delete it here.
 
-get_version () {
-  VERSION=`grep AC_INIT configure.ac | sed '/AC_INIT/ s/AC_INIT(\[aria2\],\[\([^]]\+\)\],.*/\1/'`
+get_version() {
+  VERSION=$(grep AC_INIT configure.ac | sed '/AC_INIT/ s/AC_INIT(\[aria2\],\[\([^]]\+\)\],.*/\1/')
   echo "Version: $VERSION"
 }