diff --git a/build.sh b/build.sh
index afaf7782..d7481d24 100755
--- a/build.sh
+++ b/build.sh
@@ -7,15 +7,13 @@ COMMIT_HASH=""
 BUILD_UNIXTIME=""
 
 echo_red() {
-    printf "\033[31m$1\033[0m\n"
+    printf '\033[31m%s\033[0m\n' "$1"
 }
 
 check_dependency() {
     for cmd in $1
     do
-        which "$cmd" > /dev/null
-
-        if [ $? != 0 ]; then
+        if ! which "$cmd" > /dev/null; then
             echo_red "Error: \"$cmd\" is required."
             exit 127
         fi
@@ -41,7 +39,7 @@ EOF
 }
 
 parse_args() {
-    if [ "$1" == "backend" ] || [ "$1" == "frontend" ] || [ "$1" == "docker" ]; then
+    if [ "$1" = "backend" ] || [ "$1" = "frontend" ] || [ "$1" = "docker" ]; then
         TYPE="$1"
         shift 1
     fi
@@ -67,7 +65,7 @@ parse_args() {
 }
 
 check_type_dependencies() {
-    if [ "$TYPE" == "" ]; then
+    if [ "$TYPE" = "" ]; then
         echo_red "No specified type"
         show_help
         exit 2
@@ -75,59 +73,59 @@ check_type_dependencies() {
 
     check_dependency "git"
 
-    if [ "$TYPE" == "backend" ]; then
+    if [ "$TYPE" = "backend" ]; then
         check_dependency "go"
-    elif [ "$TYPE" == "frontend" ]; then
+    elif [ "$TYPE" = "frontend" ]; then
         check_dependency "node npm"
-    elif [ "$TYPE" == "docker" ]; then
+    elif [ "$TYPE" = "docker" ]; then
         check_dependency "docker"
     fi
 }
 
 set_build_parameters() {
-    VERSION="`grep '"version": ' package.json | awk -F ':' '{print $2}' | tr -d ' ' | tr -d ',' | tr -d '"'`"
+    VERSION="$(grep '"version": ' package.json | awk -F ':' '{print $2}' | tr -d ' ' | tr -d ',' | tr -d '"')"
     COMMIT_HASH="$(git rev-parse --short HEAD)"
     BUILD_UNIXTIME="$(date '+%s')"
 }
 
 build_backend() {
-    local extra_arguments="-X github.com/mayswind/ezbookkeeping/pkg/version.Version=$VERSION"
-    extra_arguments="$extra_arguments -X github.com/mayswind/ezbookkeeping/pkg/version.CommitHash=$COMMIT_HASH"
+    backend_extra_arguments="-X github.com/mayswind/ezbookkeeping/pkg/version.Version=$VERSION"
+    backend_extra_arguments="$backend_extra_arguments -X github.com/mayswind/ezbookkeeping/pkg/version.CommitHash=$COMMIT_HASH"
 
-    if [ "$RELEASE" == "0" ]; then
-        extra_arguments="$extra_arguments -X github.com/mayswind/ezbookkeeping/pkg/version.BuildUnixTime=$BUILD_UNIXTIME"
+    if [ "$RELEASE" = "0" ]; then
+        backend_extra_arguments="$backend_extra_arguments -X github.com/mayswind/ezbookkeeping/pkg/version.BuildUnixTime=$BUILD_UNIXTIME"
     fi
 
     echo "Building backend binary file..."
 
-    CGO_ENABLED=1 go build -a -v -trimpath -ldflags "-w -s -linkmode external -extldflags '-static' $extra_arguments" -o ezbookkeeping ezbookkeeping.go
+    CGO_ENABLED=1 go build -a -v -trimpath -ldflags "-w -s -linkmode external -extldflags '-static' $backend_extra_arguments" -o ezbookkeeping ezbookkeeping.go
     chmod +x ezbookkeeping
 }
 
 build_frontend() {
-    local build_arguments="--";
+    frontend_build_arguments="--";
 
-    if [ "$RELEASE" == "0" ]; then
-        build_arguments="$build_arguments --buildUnixTime=$BUILD_UNIXTIME"
+    if [ "$RELEASE" = "0" ]; then
+        frontend_build_arguments="$frontend_build_arguments --buildUnixTime=$BUILD_UNIXTIME"
     fi
 
     echo "Pulling frontend dependencies..."
     npm install
 
     echo "Building frontend files..."
-    npm run build $build_arguments
+    npm run build "$frontend_build_arguments"
 }
 
 build_docker() {
-    local docker_tag="$VERSION"
+    docker_tag="$VERSION"
 
-    if [ "$RELEASE" == "0" ]; then
+    if [ "$RELEASE" = "0" ]; then
         docker_tag="SNAPSHOT-$(date '+%Y%m%d')";
     fi
 
     echo "Building docker image \"ezbookkeeping:$docker_tag\"..."
 
-    docker build . -t ezbookkeeping:$docker_tag
+    docker build . -t "ezbookkeeping:$docker_tag"
 }
 
 main() {
@@ -140,11 +138,11 @@ main() {
     check_type_dependencies "$TYPE"
     set_build_parameters
 
-    if [ "$TYPE" == "backend" ]; then
+    if [ "$TYPE" = "backend" ]; then
         build_backend
-    elif [ "$TYPE" == "frontend" ]; then
+    elif [ "$TYPE" = "frontend" ]; then
         build_frontend
-    elif [ "$TYPE" == "docker" ]; then
+    elif [ "$TYPE" = "docker" ]; then
         build_docker
     fi
 }