lnav-zero-in-problems

Excellent Utilities: lnav – the log file navigator

In Operation

The main arguments to lnav are the files, directories, glob patterns, or URLs to be viewed. If no arguments are given, the default syslog file for your system is opened.

Here’s an image of lnav in operation.

lnav

The main part of the display shows the log lines from the files interleaved based on time-of-day. New lines are automatically loaded as they are appended to the files and, if you are viewing the bottom of the files, lnav scrolls down to show the new lines.

Errors and warnings are colored in red and yellow, respectively. Highlights are also applied to: SQL keywords, IP addresses, XML tags, file and line numbers in Java backtraces, and quoted strings.

Log file contents are merged into a single view based on message timestamps. This saves a lot of time when correlating timestamps across multiple windows.

One of the features that we frequently use is filtering. It lets you display only lines matching a specific pattern, filtering out the noise. Press colon and then, for example, enter filter-in gnome-shell to show only lines matching “gnome-shell”. Filters also apply to log lines as they are loaded.

lnav-filtering

An even more powerful feature is querying logs with SQL statements. This lets you perform queries on messages without loading the data into an SQL database. Instead, lnav using SQlite3’s virtual table feature. For all supported log file types, lnav creates tables that can be queried using the subset of SQL supported by Sqlite3. Environment variables can be used in SQL statements by prefixing the variable name with a dollar-sign. The query mode is accessed by pressing semicolon. SQL queries are checked for correctness as you type.

There’s also the ability to amalgamate data by time with the timeline view. This shows a histogram of messages over time. It’s accessed by pressing the i key. You can zoom in and out in this view.

lnav-histogram

Next page: Page 3 – Other Features

Pages in this article:
Page 1 – Introduction / Installation
Page 2 – In Operation
Page 3 – Other Features
Page 4 – Summary


Complete list of articles in this series:

Excellent Utilities
tmuxA terminal multiplexer that offers a massive boost to your workflow
lnavAdvanced log file viewer for the small-scale; great for troubleshooting
PaperworkDesigned to simplify the management of your paperwork
AbricotineMarkdown editor with inline preview functionality
mdlessFormatted and highlighted view of Markdown files
fkillKill processes quick and easy
TuskAn unofficial Evernote client with bags of potential
UlauncherSublime application launcher
McFlyNavigate through your bash shell history
LanguageToolStyle and grammar checker for 30+ languages
pecoSimple interactive filtering tool that's remarkably useful
Liquid PromptAdaptive prompt for Bash & Zsh
AnanicyShell daemon created to manage processes’ IO and CPU priorities
cheat.shCommunity driven unified cheat sheet
ripgrepRecursively search directories for a regex pattern
exaA turbo-charged alternative to the venerable ls command
OCRmyPDFAdd OCR text layer to scanned PDFs
WatsonTrack the time spent on projects
fontpreviewQuickly search and preview fonts
fdWonderful alternative to the venerable find
scrcpyDisplay and control Android devices
dufDisk usage utility with more polished presentation than the classic df
tldrSimplified and community-driven man pages
lsdLike exa, lsd is a turbo-charged alternative to ls
brootNext gen tree explorer and customizable launcher
DeskreenLive streaming your desktop to a web browser
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