# Referencing earlier messages of the context

When creating the aggregated message, or in the various parameters of the `grouping-by()` parser, you can also refer to fields and values of earlier messages of the context by adding the `@<distance-of-referenced-message-from-the-current>` suffix to the macro. For example, if there are three log messages in a context, the `${HOST}@1` expression refers to the host field of the current (third) message in the context, the `${HOST}@2` expression refers to the host field of the previous (second) message in the context, `${PID}@3` to the PID of the first message, and so on. For example, the following message can be created from SSH login/logout messages: `An SSH session for ${SSH_USERNAME}@1 from ${SSH_CLIENT_ADDRESS}@2 closed. Session lasted from ${DATE}@2 to ${DATE}`.

Warning When referencing an earlier message of the context, always enclose the field name between braces, for example, `${PID}@3`. The reference will not work if you omit the braces. 

Note To use a literal `@` character in a template, use `@@`. 

## Example: Referencing values from an earlier message

The following action can be used to log the length of an SSH session (the time difference between a login and a logout message in the context):
```
 
       aggregate(
            value('value name="MESSAGE" An SSH session for ${SSH_USERNAME}@1 from ${SSH_CLIENT_ADDRESS}@2 closed. Session lasted from ${DATE}@2 to ${DATE}')
        )
    
```

If you do not know in which message of the context contains the information you need, you can use the `grep` template function. For details, see [grep](../../../docs/axosyslog-core/chapter-manipulating-messages/customizing-message-format/reference-template-functions/index.md).

### Example: Using the grep template function

The following example selects the message of the context that has a `username` name-value pair with the `root` value, and returns the value of the `auth_method` name-value pair.
```
 
       $(grep ("${username}" == "root") ${auth_method})
    
```

To perform calculations on fields that have numerical values, see [Numerical operations](../../../docs/axosyslog-core/chapter-manipulating-messages/customizing-message-format/reference-template-functions/index.md).

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