nginx_error: fix style issues reported by shellcheck

plugins/nginx/nginx_error

@@ -49,6 +49,7 @@ None known.
 =head1 VERSION
 
 1.1 - 2018/01/20
+      * fix shell style issues reported by shellcheck
       * improve readability of symlink configuration code
 
 1.0 - 2017/02/21
@@ -99,29 +100,34 @@ http_codes[500]='Internal Server Error'
 http_codes[502]='Bad Gateway'
 http_codes[503]='Service Unavailable'
 
-do_ () { # Fetch
+
+# parse error counts from log file
+do_ () {
+  local k values
   declare -A line_counts
-  values=`awk '{print $9}' $log | sort | uniq -c`
+  values=$(awk '{print $9}' "$log" | sort | uniq -c)
   if [ -n "$values" ]; then
     while read -r line; do
-      read -a tmp <<< "$line";
+      read -r -a tmp <<< "$line"
-      line_counts[${tmp[1]}]=${tmp[0]};
+      line_counts[${tmp[1]}]=${tmp[0]}
     done <<< "$values"
   fi
 
-  for k in ${!http_codes[@]}; do
+  for k in "${!http_codes[@]}"; do
     echo "error$k.value ${line_counts[$k]:-0}"
   done
   exit 0
 }
 
+
 do_config () {
+  local k
   echo "graph_title $(basename "$log") - Nginx errors per minute"
-  echo 'graph_vlabel pages with http error codes / ${graph_period}'
+  echo "graph_vlabel pages with http error codes / \${graph_period}"
   echo "graph_category webserver"
   echo "graph_period minute"
   echo "graph_info This graph shows nginx error amount per minute"
-  for k in ${!http_codes[@]}; do
+  for k in "${!http_codes[@]}"; do
     echo "error$k.type DERIVE"
     echo "error$k.min 0"
     echo "error$k.label $k ${http_codes[$k]}"
@@ -129,14 +135,16 @@ do_config () {
   exit 0
 }
 
+
 do_autoconf () {
   echo yes
   exit 0
 }
 
+
 case $1 in
   config|autoconf|'')
-  eval do_$1
+  eval "do_$1"
 esac
 
 exit $?