I would like to export selected emails from my Gmail account to a text file. How can I do this with the Gmail web interface or some other (Linux) option?
I'm not sure that you can do this directly in Gmail. However, you could either selectively forward emails to an alternative account OR download your Gmail using POP3 to the Mozilla Thunderbird email client (which is available under Linux).
If you are filtering more than a handful of emails it would be easier to download your emails en masse using POP3 and use the filtering capabilities of Thunderbird.
Thunderbird supports many add-ons. The ImportExportTools add-on for instance appears to do what you are after, allowing the export of emails to single or multiple text files (there are no doubt others).
Thanks to @Tschareck for clarifying that Gmail filters do not allow you to forward emails already received.