5

I want my bash script to detect, if an AWS RDS instance with a specific name exists already.

This is what I tried:

#!/usr/bin/env bash

DBINSTANCEIDENTIFIER=greatdb

EXISTINGINSTANCE=$(aws rds describe-db-instances \
    --db-instance-identifier="$DBINSTANCEIDENTIFIER" \
    --output text\
    )

I would expect an empty list, or a zero as a result - but I receive an error message:

An error occurred (DBInstanceNotFound) when calling the DescribeDBInstances operation: DBInstance greatdb not found.

Does anyone know how to properly find out if an instance exists or not, without an error message?

7

Notice the constraint in the docs when using --db-instance-indentifier:

--db-instance-identifier (string)

  The user-supplied instance identifier. If this parameter is specified, 
  information from only the specific DB instance is returned. This parameter 
  isn't case-sensitive.

Constraints:   
  - If supplied, must match the identifier of an existing DBInstance

So you can only use this option if you know the DB in fact exists.

Using queries

To search for a DB that may or may not exist you'll have to use the --query option:

$ aws rds describe-db-instances \
    --query 'DBInstances[*].[DBName,DBInstanceIdentifier]' --output text

The DBINstances JSON structure is accessible in the awscli help:

$ aws rds describe-db-instances help --output text
...
...
       {
          "DBInstances": [
              {
                  "PubliclyAccessible": false,
                  "MasterUsername": "mymasteruser",
                  "MonitoringInterval": 0,
                  "LicenseModel": "general-public-license",
                  ...
                  ...
                  "DBName": "sample",
                  ...
                  ...
                  "DBInstanceStatus": "stopped",
                  "EngineVersion": "5.6.27",
                  "AvailabilityZone": "us-east-1e",
                  "StorageType": "standard",
                  "StorageEncrypted": false,
                  "DBInstanceClass": "db.t2.micro",
                  "DbInstancePort": 0,
                  "DBInstanceIdentifier": "mydbinstance-1"
              }
          ]
      }
...
...

Using Filters

Another simple solution for the initial question is, to use the --filters parameter. The query will return either the instance identifier (if the instance exists), or an empty string (if it does not exist):

#!/usr/bin/env bash

DBINSTANCEIDENTIFIER="greatdb"
EXISTINGINSTANCE=$(aws rds describe-db-instances \
    --query 'DBInstances[*].[DBInstanceIdentifier]' \
    --filters Name=db-instance-id,Values=$DBINSTANCEIDENTIFIER \
    --output text \
    )

if [ -z $EXISTINGINSTANCE ]
then
    echo "instance $DBINSTANCEIDENTIFIER does not exist!"
else
    echo "instance $DBINSTANCEIDENTIFIER exists!"
fi

References

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