Log File Navigator
Charly's Column – lnav
During a long trek through the verbose syslog, really important warnings and errors are scattered along the path. Sometimes a missing message can be the decisive event. Sys admin columnist Charly has now hired a tracker to help him search for clues: Log File Navigator.
Searching in logfiles is the sys admin's bread and butter. Finding a specific piece of information often requires long cascades of grep
commands. What makes this even more difficult is if a log message that I expect every five minutes is delayed. Of course, this is a warning signal, but I can't use grep
to figure this out. What can draw my attention to the fact that warning
messages are piling up? These difficulties prompted me to onboard Log File Navigator (lnav
, [1] ).
If you launch lnav
without any options, it opens /var/log/syslog
(Figure 1). Using:
lnav /var/log/syslog*

makes more sense, because it then includes older syslog files – whether compressed or not. lnav
bears the name "Navigator," because it makes it easy to walk through the logfiles in small steps or giant leaps. For example, Shift+D beams you back 24 hours into the past, and pressing D without Shift takes you back to the present. Shift+1 lets you jump back to 10 minutes after the last full hour, while Shift+2 jumps back to 20 minutes after the last full hour, and so on. Shift+G always takes you to the end of the log.
Searching is easy, too. You simply type /
followed by a search term. Besides strings, lnav
also accepts regular expressions, which makes complex and fuzzy searches possible. N and Shift+N let you jump between the hits. A search function using SQL syntax is currently still experimental.
W and Shift+W jump to the next/previous warning, while E and Shift+E jump to errors. Great stuff: S and Shift+S navigate to events that are out of sync – such as delayed events.
lnav
keeps statistics in the background. The History view (Figure 2) proves to be practical. It displays a graph showing the number of messages received and the proportion of warnings and errors. In the screenshot, the entries are totaled in 10-minute blocks. Z and Shift+Z let you zoom in and out of the time periods.

Once you have familiarized yourself with the keyboard shortcuts, working with lnav
will be as easy as pie for you. I only mentioned what are the most important shortcuts for me here; the complete list is available under "Hotkey Reference" on [2]. If I could wish for something in a future version, it would be more color schemes. I like to work with dark screens, but some color-highlighted areas in the log are not easy to read.
Infos
- lnav: https://lnav.org
- lnav documentation: https://lnav.readthedocs.io/en/latest/
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