#3298 - SSL monitoring script for icinga

check_ssl_cert.sh

 #!/bin/sh
 
 # Icinga plugin that checks how many days are left
 # until SSL certificate expires.
 # Usage:
 # $PLUGIN_DIR/check_ssl_cert.sh -H $HOST -P $PORT -c $CRITICAL -w $WARNING
 
 get_ssl_expiry_date() {
   openssl s_client -connect "$1:$2" </dev/null 2>/dev/null \
   | openssl x509 -noout -enddate 2>/dev/null
 }
 
 CURRENT_DATE=`date +%y%m%d`
 HOST="$2"
 PORT="$4"
 CRITICAL="$6"
 WARNING="$8"
 STATE_OK=0
 STATE_WARNING=1
 STATE_CRITICAL=2
 STATE_UNKNOWN=3
 OUTPUT=`get_ssl_expiry_date "$HOST" "$PORT"`
 
 if [ -z "$OUTPUT" ]
 then
   echo "UNKNOWN - Could not connect to $HOST via port $PORT"
   exit "$STATE_UNKNOWN"
 fi
 
-DAY=`echo "$OUTPUT" | awk '{print $2}'`
+DAY=`echo "$OUTPUT" | awk '{printf "02d", $2}'`
 MONTH=`echo "$OUTPUT" | awk '{print $1}' | cut -c 10-`
 YEAR=`echo "$OUTPUT" | awk '{print $4}'`
-
-if [ "$DAY" -lt 10 ]
-then
-  DAY="0$DAY"
-fi

[Fred, 2018/07/04 14:24:10]

That should work.
But you could also use awk's printf function instead:
In line 30 do this:
DAY=`echo "$OUTPUT" | awk '{printf "%02d", $2}'`

 
 case "$MONTH" in
 
   "Jan")
     MONTH="01"
     ;;
   "Feb")
     MONTH="02"
     ;;
   "Mar")
@@ skipping 45 matching lines @@
   echo "WARNING - $HOST: SSL certificate will be expired in $DIFFERENCE days!"
   exit "$STATE_WARNING"
 elif [ "$DIFFERENCE" -gt "$WARNING" ]
 then
   echo "OK - $HOST: SSL certificate will be expired in $DIFFERENCE days"
   exit "$STATE_OK"
 else
   echo "UNKNOWN - $HOST: Could not retrieve data"
   exit "$STATE_UNKNOWN"
 fi