AWSCli dynamodb update-item command syntax

am using AmazonAwsCli to write a shell script to update an attribute in an item in a dynamodb table. I want to update an attribute in a table for multiple items. I am reading the attribute value from a file and am trying to update the table by injecting the value of the shell script variable in the command. The documentation available at http://ift.tt/1VnydmQ suggests using separate json files for expression-attribute-names and expression-attribute-values. However, I do not want to create separate json files. Rather, I want to write one command to update an item for a given attribute value.

My table name = MY_TABLE_NAME

hashkey = AccountId

shell script variable holding the value of AccountId = accountId

attribute name that needs to be updated = Version

shell script variable holding the value of Version = ver

I have got something like :

aws dynamodb update-item --table-name MY_TABLE_NAME --key '{"AccountId": {"S": '$accountId'}}' --update-expression "SET Version = '{"Version": {"S": '$ver'}}'" --condition-expression "attribute_exists(Version)" --return-values UPDATED_NEW

But, the above command does not work. Can someone point me to the correct syntax.