Enforce part of line in a file

Enforcing part of a line, to ensure that some parameter is set, or setting some left hand side, allows to roll out changes in files you can’t control completely.

Managing the values of key→value lines is the easiest use case, by using the generic methods file_key_value_present, file_key_value_present_option or even more complex use cases file_key_value_parameter_present_in_list.

For more complex changes, using the more powerful File content technique is necessary.

For instance, to change statements in rsyslog.conf file to ensure that logs from local3.none go to /var/log/messages, it is necessary to edit file with "Line replacement regular expressions", to match lines ending by /var/log/messages and replacing them by the desired content.

Please note that the regular expressions are PCRE expressions, and the replacement needs to be convergent: the regular expression must not match the replaced string.

So the regular expression must not match lines having local3.none as a source, and /var/log/messages as a destination, which translates as ^[A-Za-z0-9;.,](?!local3\.none)\s+/var/log/messages where:

^ is the start of the line (regex are not anchored)
[A-Za-z0-9;.,*] are all the valid characters for source
(?!local3\.none) to prevent matching line containing local3.none.

Captured sequence can be reused with the $(match.X) notation, where X represents the matched elements: $(match.0) is the whole string, $(match.1) is the first matched element, etc

The replacement should be:

$(match.1);local3.none        /var/log/messages

to add ;local3.none at the end of the line.

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