til/unix/apply-successive-filters-to-lines-in-less.md

Apply Successive Filters To Lines In Less

Let's say I've opened a large Rails log file with less:

$ less logs/development.log

I have an idea of what I'm looking for, but there is way more noise than signal. I can start to filter out some of the noise. The & command starts a filter prompt. If I start to filter by something like INSERT INTO, then a ton of lines disappear leaving just those matching that pattern.

Scrolling through the current set of lines, I start to have a better idea of what I'm looking for, but there is still too much noise. I can apply an additional successive filter on the remaining lines by hitting & again and entering in another pattern -- e.g. GoodJob.

Now I only see lines that contain both INSERT INTO and GoodJob somewhere in them.

As less puts it:

Multiple & commands may be entered, in which case only lines which match all of the patterns will be displayed.

If I want to undo all the filtering, I just need to enter an empty & filter prompt and it will reset things back to displaying all lines.

If pattern is empty (if you type & immediately followed by ENTER), any filtering is turned off, and all lines are displayed.

See man less for more details.