any /bin/sh is sufficient

The code is still /bin/bash compliant, but also dash, BSD's sh and
 any bourne-compatible light shell

clustercheck

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/bin/sh
 #
 # Script to make a proxy (ie HAProxy) capable of monitoring Percona XtraDB Cluster nodes properly
 #
@@ -10,7 +10,7 @@
 # Based on the original script from Unai Rodriguez
 #
 
-if [[ $1 == '-h' || $1 == '--help' ]];then
+if [ "$1" = '-h' -o "$1" = '--help' ];then
     echo "Usage: $0 <user> <pass> <available_when_donor=0|1> <log_file> <available_when_readonly=0|1> <defaults_extra_file>"
     exit
 fi
@@ -19,12 +19,12 @@ fi
 # admins to manually remove a node from a cluster easily.
 if [ -e "/var/tmp/clustercheck.disabled" ]; then
     # Shell return-code is 1
-    echo -en "HTTP/1.1 503 Service Unavailable\r\n"
-    echo -en "Content-Type: text/plain\r\n"
-    echo -en "Connection: close\r\n"
-    echo -en "Content-Length: 51\r\n"
-    echo -en "\r\n"
-    echo -en "Percona XtraDB Cluster Node is manually disabled.\r\n"
+    printf "HTTP/1.1 503 Service Unavailable\r\n"
+    printf "Content-Type: text/plain\r\n"
+    printf "Connection: close\r\n"
+    printf "Content-Length: 51\r\n"
+    printf "\r\n"
+    printf "Percona XtraDB Cluster Node is manually disabled.\r\n"
     sleep 0.1
     exit 1
 fi
@@ -46,13 +46,13 @@ DEFAULTS_EXTRA_FILE=${DEFAULTS_EXTRA_FILE:-/etc/my.cnf}
 TIMEOUT=10
 
 EXTRA_ARGS=""
-if [[ -n "$MYSQL_USERNAME" ]]; then
+if [ -n "$MYSQL_USERNAME" ]; then
     EXTRA_ARGS="$EXTRA_ARGS --user=${MYSQL_USERNAME}"
 fi
-if [[ -n "$MYSQL_PASSWORD" ]]; then
+if [ -n "$MYSQL_PASSWORD" ]; then
     EXTRA_ARGS="$EXTRA_ARGS --password=${MYSQL_PASSWORD}"
 fi
-if [[ -r $DEFAULTS_EXTRA_FILE ]];then
+if [ -r $DEFAULTS_EXTRA_FILE ];then
     MYSQL_CMDLINE="mysql --defaults-extra-file=$DEFAULTS_EXTRA_FILE -nNE --connect-timeout=$TIMEOUT \
                     ${EXTRA_ARGS}"
 else
@@ -64,47 +64,47 @@ fi
 WSREP_STATUS=$($MYSQL_CMDLINE -e "SHOW STATUS LIKE 'wsrep_local_state';" \
     2>${ERR_FILE} | tail -1 2>>${ERR_FILE})
 
-if [[ "${WSREP_STATUS}" == "4" ]] || [[ "${WSREP_STATUS}" == "2" && ${AVAILABLE_WHEN_DONOR} == 1 ]]
+if [ "${WSREP_STATUS}" = "4" ] || [ "${WSREP_STATUS}" = "2" -a ${AVAILABLE_WHEN_DONOR} -eq 1 ]
 then
     # Check only when set to 0 to avoid latency in response.
-    if [[ $AVAILABLE_WHEN_READONLY -eq 0 ]];then
+    if [ $AVAILABLE_WHEN_READONLY -eq 0 ];then
         READ_ONLY=$($MYSQL_CMDLINE -e "SHOW GLOBAL VARIABLES LIKE 'read_only';" \
                     2>${ERR_FILE} | tail -1 2>>${ERR_FILE})
 
-        if [[ "${READ_ONLY}" == "ON" ]];then
+        if [ "${READ_ONLY}" = "ON" ];then
             # Percona XtraDB Cluster node local state is 'Synced', but it is in
             # read-only mode. The variable AVAILABLE_WHEN_READONLY is set to 0.
             # => return HTTP 503
             # Shell return-code is 1
-            echo -en "HTTP/1.1 503 Service Unavailable\r\n"
-            echo -en "Content-Type: text/plain\r\n"
-            echo -en "Connection: close\r\n"
-            echo -en "Content-Length: 43\r\n"
-            echo -en "\r\n"
-            echo -en "Percona XtraDB Cluster Node is read-only.\r\n"
+            printf "HTTP/1.1 503 Service Unavailable\r\n"
+            printf "Content-Type: text/plain\r\n"
+            printf "Connection: close\r\n"
+            printf "Content-Length: 43\r\n"
+            printf "\r\n"
+            printf "Percona XtraDB Cluster Node is read-only.\r\n"
             sleep 0.1
             exit 1
         fi
     fi
     # Percona XtraDB Cluster node local state is 'Synced' => return HTTP 200
     # Shell return-code is 0
-    echo -en "HTTP/1.1 200 OK\r\n"
-    echo -en "Content-Type: text/plain\r\n"
-    echo -en "Connection: close\r\n"
-    echo -en "Content-Length: 40\r\n"
-    echo -en "\r\n"
-    echo -en "Percona XtraDB Cluster Node is synced.\r\n"
+    printf "HTTP/1.1 200 OK\r\n"
+    printf "Content-Type: text/plain\r\n"
+    printf "Connection: close\r\n"
+    printf "Content-Length: 40\r\n"
+    printf "\r\n"
+    printf "Percona XtraDB Cluster Node is synced.\r\n"
     sleep 0.1
     exit 0
 else
     # Percona XtraDB Cluster node local state is not 'Synced' => return HTTP 503
     # Shell return-code is 1
-    echo -en "HTTP/1.1 503 Service Unavailable\r\n"
-    echo -en "Content-Type: text/plain\r\n"
-    echo -en "Connection: close\r\n"
-    echo -en "Content-Length: 44\r\n"
-    echo -en "\r\n"
-    echo -en "Percona XtraDB Cluster Node is not synced.\r\n"
+    printf "HTTP/1.1 503 Service Unavailable\r\n"
+    printf "Content-Type: text/plain\r\n"
+    printf "Connection: close\r\n"
+    printf "Content-Length: 44\r\n"
+    printf "\r\n"
+    printf "Percona XtraDB Cluster Node is not synced.\r\n"
     sleep 0.1
     exit 1
 fi