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it's quite easy to drift off in certain unwanted direction like reading a comment in the codebase about a tool and then reading the whole manpage of that tool, or joining the irc channel for that tool to discuss certian term you couldn't understand from the man page. but it wasn't about the tool in the first place.
this is called 'getting distracted or sidetracked', and it's a big hurdle preventing you from finishing the tasks. one has to narrow down their focus to things which help make progress on the task. having an agenda before starting the work helps, and there after you can just check whether what you are doing aligns with the agenda or not.
the agenda should be specific, it's your plate for the day. put no more than what you can eat and digest. be consistent, and that's how you eat an elephant, one bite at a time.