diff --git a/shadowsocks-all.sh b/shadowsocks-all.sh
index c999ee7..e52c561 100644
--- a/shadowsocks-all.sh
+++ b/shadowsocks-all.sh
@@ -165,44 +165,44 @@ disable_selinux() {
     fi
 }
 
-check_sys() {
+check_sys(){
     local checkType=$1
     local value=$2
 
     local release=''
     local systemPackage=''
 
-    if [ -f /etc/redhat-release ]; then
+    if [[ -f /etc/redhat-release ]]; then
         release="centos"
         systemPackage="yum"
-    elif cat /etc/issue | grep -Eqi "debian"; then
+    elif grep -Eqi "debian" /etc/issue; then
         release="debian"
         systemPackage="apt"
-    elif cat /etc/issue | grep -Eqi "ubuntu"; then
+    elif grep -Eqi "ubuntu" /etc/issue; then
         release="ubuntu"
         systemPackage="apt"
-    elif cat /etc/issue | grep -Eqi "centos|red hat|redhat"; then
+    elif grep -Eqi "centos|red hat|redhat" /etc/issue; then
         release="centos"
         systemPackage="yum"
-    elif cat /proc/version | grep -Eqi "debian"; then
+    elif grep -Eqi "debian" /proc/version; then
         release="debian"
         systemPackage="apt"
-    elif cat /proc/version | grep -Eqi "ubuntu"; then
+    elif grep -Eqi "ubuntu" /proc/version; then
         release="ubuntu"
         systemPackage="apt"
-    elif cat /proc/version | grep -Eqi "centos|red hat|redhat"; then
+    elif grep -Eqi "centos|red hat|redhat" /proc/version; then
         release="centos"
         systemPackage="yum"
     fi
 
-    if [ ${checkType} == "sysRelease" ]; then
-        if [ "$value" == "$release" ]; then
+    if [[ "${checkType}" == "sysRelease" ]]; then
+        if [ "${value}" == "${release}" ]; then
             return 0
         else
             return 1
         fi
-    elif [ ${checkType} == "packageManager" ]; then
-        if [ "$value" == "$systemPackage" ]; then
+    elif [[ "${checkType}" == "packageManager" ]]; then
+        if [ "${value}" == "${systemPackage}" ]; then
             return 0
         else
             return 1
diff --git a/shadowsocks-go.sh b/shadowsocks-go.sh
index e8a873f..5b76214 100644
--- a/shadowsocks-go.sh
+++ b/shadowsocks-go.sh
@@ -54,34 +54,34 @@ check_sys(){
     if [[ -f /etc/redhat-release ]]; then
         release="centos"
         systemPackage="yum"
-    elif cat /etc/issue | grep -Eqi "debian"; then
+    elif grep -Eqi "debian" /etc/issue; then
         release="debian"
         systemPackage="apt"
-    elif cat /etc/issue | grep -Eqi "ubuntu"; then
+    elif grep -Eqi "ubuntu" /etc/issue; then
         release="ubuntu"
         systemPackage="apt"
-    elif cat /etc/issue | grep -Eqi "centos|red hat|redhat"; then
+    elif grep -Eqi "centos|red hat|redhat" /etc/issue; then
         release="centos"
         systemPackage="yum"
-    elif cat /proc/version | grep -Eqi "debian"; then
+    elif grep -Eqi "debian" /proc/version; then
         release="debian"
         systemPackage="apt"
-    elif cat /proc/version | grep -Eqi "ubuntu"; then
+    elif grep -Eqi "ubuntu" /proc/version; then
         release="ubuntu"
         systemPackage="apt"
-    elif cat /proc/version | grep -Eqi "centos|red hat|redhat"; then
+    elif grep -Eqi "centos|red hat|redhat" /proc/version; then
         release="centos"
         systemPackage="yum"
     fi
 
-    if [[ ${checkType} == "sysRelease" ]]; then
-        if [ "$value" == "$release" ]; then
+    if [[ "${checkType}" == "sysRelease" ]]; then
+        if [ "${value}" == "${release}" ]; then
             return 0
         else
             return 1
         fi
-    elif [[ ${checkType} == "packageManager" ]]; then
-        if [ "$value" == "$systemPackage" ]; then
+    elif [[ "${checkType}" == "packageManager" ]]; then
+        if [ "${value}" == "${systemPackage}" ]; then
             return 0
         else
             return 1
diff --git a/shadowsocks-libev-debian.sh b/shadowsocks-libev-debian.sh
index 72fa700..15d2d05 100644
--- a/shadowsocks-libev-debian.sh
+++ b/shadowsocks-libev-debian.sh
@@ -142,34 +142,34 @@ check_sys(){
     if [[ -f /etc/redhat-release ]]; then
         release="centos"
         systemPackage="yum"
-    elif cat /etc/issue | grep -Eqi "debian"; then
+    elif grep -Eqi "debian" /etc/issue; then
         release="debian"
         systemPackage="apt"
-    elif cat /etc/issue | grep -Eqi "ubuntu"; then
+    elif grep -Eqi "ubuntu" /etc/issue; then
         release="ubuntu"
         systemPackage="apt"
-    elif cat /etc/issue | grep -Eqi "centos|red hat|redhat"; then
+    elif grep -Eqi "centos|red hat|redhat" /etc/issue; then
         release="centos"
         systemPackage="yum"
-    elif cat /proc/version | grep -Eqi "debian"; then
+    elif grep -Eqi "debian" /proc/version; then
         release="debian"
         systemPackage="apt"
-    elif cat /proc/version | grep -Eqi "ubuntu"; then
+    elif grep -Eqi "ubuntu" /proc/version; then
         release="ubuntu"
         systemPackage="apt"
-    elif cat /proc/version | grep -Eqi "centos|red hat|redhat"; then
+    elif grep -Eqi "centos|red hat|redhat" /proc/version; then
         release="centos"
         systemPackage="yum"
     fi
 
-    if [[ ${checkType} == "sysRelease" ]]; then
-        if [ "$value" == "$release" ]; then
+    if [[ "${checkType}" == "sysRelease" ]]; then
+        if [ "${value}" == "${release}" ]; then
             return 0
         else
             return 1
         fi
-    elif [[ ${checkType} == "packageManager" ]]; then
-        if [ "$value" == "$systemPackage" ]; then
+    elif [[ "${checkType}" == "packageManager" ]]; then
+        if [ "${value}" == "${systemPackage}" ]; then
             return 0
         else
             return 1
diff --git a/shadowsocks-libev.sh b/shadowsocks-libev.sh
index 793af0a..e7d4094 100644
--- a/shadowsocks-libev.sh
+++ b/shadowsocks-libev.sh
@@ -136,34 +136,34 @@ check_sys(){
     if [[ -f /etc/redhat-release ]]; then
         release="centos"
         systemPackage="yum"
-    elif cat /etc/issue | grep -Eqi "debian"; then
+    elif grep -Eqi "debian" /etc/issue; then
         release="debian"
         systemPackage="apt"
-    elif cat /etc/issue | grep -Eqi "ubuntu"; then
+    elif grep -Eqi "ubuntu" /etc/issue; then
         release="ubuntu"
         systemPackage="apt"
-    elif cat /etc/issue | grep -Eqi "centos|red hat|redhat"; then
+    elif grep -Eqi "centos|red hat|redhat" /etc/issue; then
         release="centos"
         systemPackage="yum"
-    elif cat /proc/version | grep -Eqi "debian"; then
+    elif grep -Eqi "debian" /proc/version; then
         release="debian"
         systemPackage="apt"
-    elif cat /proc/version | grep -Eqi "ubuntu"; then
+    elif grep -Eqi "ubuntu" /proc/version; then
         release="ubuntu"
         systemPackage="apt"
-    elif cat /proc/version | grep -Eqi "centos|red hat|redhat"; then
+    elif grep -Eqi "centos|red hat|redhat" /proc/version; then
         release="centos"
         systemPackage="yum"
     fi
 
-    if [[ ${checkType} == "sysRelease" ]]; then
-        if [ "$value" == "$release" ]; then
+    if [[ "${checkType}" == "sysRelease" ]]; then
+        if [ "${value}" == "${release}" ]; then
             return 0
         else
             return 1
         fi
-    elif [[ ${checkType} == "packageManager" ]]; then
-        if [ "$value" == "$systemPackage" ]; then
+    elif [[ "${checkType}" == "packageManager" ]]; then
+        if [ "${value}" == "${systemPackage}" ]; then
             return 0
         else
             return 1
diff --git a/shadowsocks.sh b/shadowsocks.sh
index e59deb8..26f07cb 100644
--- a/shadowsocks.sh
+++ b/shadowsocks.sh
@@ -71,34 +71,34 @@ check_sys(){
     if [[ -f /etc/redhat-release ]]; then
         release="centos"
         systemPackage="yum"
-    elif cat /etc/issue | grep -Eqi "debian"; then
+    elif grep -Eqi "debian" /etc/issue; then
         release="debian"
         systemPackage="apt"
-    elif cat /etc/issue | grep -Eqi "ubuntu"; then
+    elif grep -Eqi "ubuntu" /etc/issue; then
         release="ubuntu"
         systemPackage="apt"
-    elif cat /etc/issue | grep -Eqi "centos|red hat|redhat"; then
+    elif grep -Eqi "centos|red hat|redhat" /etc/issue; then
         release="centos"
         systemPackage="yum"
-    elif cat /proc/version | grep -Eqi "debian"; then
+    elif grep -Eqi "debian" /proc/version; then
         release="debian"
         systemPackage="apt"
-    elif cat /proc/version | grep -Eqi "ubuntu"; then
+    elif grep -Eqi "ubuntu" /proc/version; then
         release="ubuntu"
         systemPackage="apt"
-    elif cat /proc/version | grep -Eqi "centos|red hat|redhat"; then
+    elif grep -Eqi "centos|red hat|redhat" /proc/version; then
         release="centos"
         systemPackage="yum"
     fi
 
-    if [[ ${checkType} == "sysRelease" ]]; then
-        if [ "$value" == "$release" ]; then
+    if [[ "${checkType}" == "sysRelease" ]]; then
+        if [ "${value}" == "${release}" ]; then
             return 0
         else
             return 1
         fi
-    elif [[ ${checkType} == "packageManager" ]]; then
-        if [ "$value" == "$systemPackage" ]; then
+    elif [[ "${checkType}" == "packageManager" ]]; then
+        if [ "${value}" == "${systemPackage}" ]; then
             return 0
         else
             return 1
diff --git a/shadowsocksR.sh b/shadowsocksR.sh
index 1e6b307..5f022d5 100644
--- a/shadowsocksR.sh
+++ b/shadowsocksR.sh
@@ -103,34 +103,34 @@ check_sys(){
     if [[ -f /etc/redhat-release ]]; then
         release="centos"
         systemPackage="yum"
-    elif cat /etc/issue | grep -Eqi "debian"; then
+    elif grep -Eqi "debian" /etc/issue; then
         release="debian"
         systemPackage="apt"
-    elif cat /etc/issue | grep -Eqi "ubuntu"; then
+    elif grep -Eqi "ubuntu" /etc/issue; then
         release="ubuntu"
         systemPackage="apt"
-    elif cat /etc/issue | grep -Eqi "centos|red hat|redhat"; then
+    elif grep -Eqi "centos|red hat|redhat" /etc/issue; then
         release="centos"
         systemPackage="yum"
-    elif cat /proc/version | grep -Eqi "debian"; then
+    elif grep -Eqi "debian" /proc/version; then
         release="debian"
         systemPackage="apt"
-    elif cat /proc/version | grep -Eqi "ubuntu"; then
+    elif grep -Eqi "ubuntu" /proc/version; then
         release="ubuntu"
         systemPackage="apt"
-    elif cat /proc/version | grep -Eqi "centos|red hat|redhat"; then
+    elif grep -Eqi "centos|red hat|redhat" /proc/version; then
         release="centos"
         systemPackage="yum"
     fi
 
-    if [[ ${checkType} == "sysRelease" ]]; then
-        if [ "$value" == "$release" ]; then
+    if [[ "${checkType}" == "sysRelease" ]]; then
+        if [ "${value}" == "${release}" ]; then
             return 0
         else
             return 1
         fi
-    elif [[ ${checkType} == "packageManager" ]]; then
-        if [ "$value" == "$systemPackage" ]; then
+    elif [[ "${checkType}" == "packageManager" ]]; then
+        if [ "${value}" == "${systemPackage}" ]; then
             return 0
         else
             return 1