Log Formats

Log files loaded into lnav are parsed based on formats defined in configuration files. Many formats are already built in to the lnav binary and you can define your own using a JSON file. When loading files, each format is checked to see if it can parse the first few lines in the file. Once a match is found, that format will be considered that files format and used to parse the remaining lines in the file. If no match is found, the file is considered to be plain text and can be viewed in the “text” view that is accessed with the t key.

The following log formats are built into lnav:

Name Table Name Description
Common Access Log access_log The default web access log format for servers like Apache.
VMware vSphere Auto Deploy log format autodeploy_log The log format for the VMware Auto Deploy service
Generic Block block_log A generic format for logs, like cron, that have a date at the start of a block.
Candlepin log format candlepin_log Log format used by Candlepin registration system
Yum choose_repo Log choose_repo_log The log format for the yum choose_repo tool.
CUPS log format cups_log Log format used by the Common Unix Printing System
Dpkg Log dpkg_log The debian dpkg log.
Amazon ELB log elb_log Log format for Amazon Elastic Load Balancers
engine log engine_log The log format for the engine.log files from RHEV/oVirt
Common Error Log error_log The default web error log format for servers like Apache.
Fsck_hfs Log fsck_hfs_log Log for the fsck_hfs tool on Mac OS X.
Glog glog_log The google glog format.
Java log format java_log Log format used by log4j and output by most java programs
Katello log format katello_log Log format used by katello and foreman as used in Satellite 6.
OpenAM Log openam_log The OpenAM identity provider.
OpenAM Debug Log openamdb_log Debug logs for the OpenAM identity provider.
OpenStack log format openstack_log The log format for the OpenStack log files
CUPS Page Log page_log The CUPS server log of printed pages.
Papertrail Service papertrail_log Log format for the papertrail log management service
SnapLogic Server Log snaplogic_log The SnapLogic server log format.
SSSD log format sssd_log Log format used by the System Security Services Daemon
Strace strace_log The strace output format.
sudo sudo_log The sudo privilege management tool.
Syslog syslog_log The system logger format found on most posix systems.
TCF Log tcf_log Target Communication Framework log
TCSH History tcsh_history The tcsh history file format.
Uwsgi Log uwsgi_log The uwsgi log format.
Vdsm Logs vdsm_log The Open Virtual Datacenter VDSM log format
VMKernel Logs vmk_log The VMKernel’s log format
VMware Logs vmw_log One of the log formats used in VMware’s ESXi and vCenter software.
RHN server XMLRPC log format xmlrpc_log Generated by Satellite’s XMLRPC component

Defining a New Format

New log formats can be defined by placing JSON configuration files in subdirectories of the ~/.lnav/formats/ directory. The directories and files can be named anything you like, but the files must have the ‘.json’ suffix. A sample file containing the builtin configuration will be written to this directory when lnav starts up. You can consult that file when writing your own formats or if you need to modify existing ones. Format directories can also contain ‘.sql’ and ‘.lnav’ script files that can be used automate log file analysis.

The contents of the format configuration should be a JSON object with a field for each format defined by the file. Each field name should be the symbolic name of the format. This value will also be used as the SQL table name for the log. The value for each field should be another object with the following fields:


The short and human-readable name for the format.


A longer description of the format.


A URL to the definition of the format.


A regular expression used to match log file paths. Typically, every file format will be tried during the detection process. This field can be used to limit which files a format is applied to in case there is a potential for conflicts.


This object contains sub-objects that describe the message formats to match in a plain log file. Log files that contain JSON messages should not specify this field.

pattern:The regular expression that should be used to match log messages. The PCRE library is used by lnav to do all regular expression matching.
module-format:If true, this regex will only be used to parse message bodies for formats that can act as containers, such as syslog. Default: false.

True if each log line is JSON-encoded.


An array that specifies the text format for JSON-encoded log messages. Log files that are JSON-encoded will have each message converted from the raw JSON encoding into this format. Each element is either an object that defines which fields should be inserted into the final message string and or a string constant that should be inserted. For example, the following configuration will tranform each log message object into a string that contains the timestamp, followed by a space, and then the message body:

[ { "field": "ts" }, " ", { "field": "msg" } ]

The name of the message field that should be inserted at this point in the message.


The minimum width for the field. If the value for the field in a given log message is shorter, padding will be added as needed to meet the minimum-width requirement. (v0.8.2+)


The maximum width for the field. If the value for the field in a given log message is longer, the overflow algorithm will be applied to try and shorten the field. (v0.8.2+)


Specifies the alignment for the field, either “left” or “right”. If “left”, padding to meet the minimum-width will be added on the right. If “right”, padding will be added on the left. (v0.8.2+)


The algorithm used to shorten a field that is longer than “max-width”. The following algorithms are supported:

abbrev:Removes all but the first letter in dotted text. For example, “com.example.foo” would be shortened to “c.e.foo”.
truncate:Truncates any text past the maximum width.
dot-dot:Cuts out the middle of the text and replaces it with two dots (i.e. ‘..’).



The timestamp format to use when displaying the time for this log message. (v0.8.2+)


The default value to use if the field could not be found in the current log message. The built-in default is “-”.


The name of the field that contains the log message timestamp. Defaults to “timestamp”.


An array of timestamp formats using a subset of the strftime conversion specification. The following conversions are supported: %a, %b, %L, %M, %H, %I, %d, %e, %k, %l, %m, %p, %y, %Y, %S, %s, %Z, %z. In addition, you can also use ‘%i’ for milliseconds from the epoch.


The name of the regex capture group that contains the log message level. Defaults to “level”.


The name of the field that contains the main body of the message. Defaults to “body”.


The name of the field that contains the “operation ID” of the message. An “operation ID” establishes a thread of messages that might correspond to a particular operation/request/transaction. The user can press the ‘o’ or ‘Shift+O’ hotkeys to move forward/backward through the list of messages that have the same operation ID. Note: For JSON-encoded logs, the opid field can be a path (e.g. “foo/bar/opid”) if the field is nested in an object and it MUST be included in the “line-format” for the ‘o’ hotkeys to work.


The name of the field that contains the module identifier that distinguishes messages from one log source from another. This field should be used if this message format can act as a container for other types of log messages. For example, an Apache access log can be sent to syslog instead of written to a file. In this case, lnav will parse the syslog message and then separately parse the body of the message to determine the “sub” format. This module identifier is used to help lnav quickly identify the format to use when parsing message bodies.


A boolean for JSON logs that indicates whether fields not present in the line-format should be displayed on their own lines.


A mapping of error levels to regular expressions. During scanning the contents of the capture group specified by level-field will be checked against each of these regexes. Once a match is found, the log message level will set to the corresponding level. The available levels, in order of severity, are: fatal, critical, error, warning, stats, info, debug, debug2-5, trace.


If false, lnav will consider any log lines that do not match one of the message patterns to be in error when checking files with the ‘-C’ option. This flag will not affect normal viewing operation. Default: true.


This object contains the definitions for the values captured by the regexes.


The type of data that was captured string, integer, float, json, quoted.


The collation function for this value.


A boolean that indicates whether or not this field represents an identifier and should be syntax colored.


A boolean that indicates that this field is a key and should not be graphed. This should only need to be set for integer fields.


A boolean for JSON log fields that indicates whether they should be displayed if they are not present in the line-format.


A command to rewrite this field when pretty-printing log messages containing this value. The command must start with ‘:’, ‘;’, or ‘|’ to signify whether it is a regular command, SQL query, or a script to be executed. The other fields in the line are accessible in SQL by using the ‘:’ prefix. The text value of this field will then be replaced with the result of the command when pretty-printing. For example, the HTTP access log format will rewrite the status code field to include the textual version (e.g. 200 (OK)) using the following SQL query:

;SELECT :sc_status || ' (' || (SELECT message FROM http_status_codes WHERE status = :sc_status) || ') '

A list of objects that contain sample log messages. All formats must include at least one sample and it must be matched by one of the included regexes. Each object must contain the following field:

line:The sample message.

Example format:

    "example_log" : {
        "title" : "Example Log Format",
        "description" : "Log format used in the documentation example.",
        "url" : "http://example.com/log-format.html",
        "regex" : {
            "basic" : {
                "pattern" : "^(?<timestamp>\\d{4}-\\d{2}-\\d{2}T\\d{2}:\\d{2}:\\d{2}\\.\\d{3}Z)>>(?<level>\\w+)>>(?<component>\\w+)>>(?<body>.*)$"
        "level-field" : "level",
        "level" : {
            "error" : "ERROR",
            "warning" : "WARNING"
        "value" : {
            "component" : {
                "kind" : "string",
                "identifier" : true
        "sample" : [
                "line" : "2011-04-01T15:14:34.203Z>>ERROR>>core>>Shit's on fire yo!"

Modifying an Existing Format

When loading log formats from files, lnav will overlay any new data over previously loaded data. This feature allows you to override existing value or append new ones to the format configurations. For example, you can separately add a new regex to the example log format given above by creating another file with the following contents:

    "example_log" : {
        "regex" : {
            "custom1" : {
                "pattern" : "^(?<timestamp>\\d{4}-\\d{2}-\\d{2}T\\d{2}:\\d{2}:\\d{2}\\.\\d{3}Z)<<(?<level>\\w+)--(?<component>\\w+)>>(?<body>.*)$"
        "sample" : [
                "line" : "2011-04-01T15:14:34.203Z<<ERROR--core>>Shit's on fire yo!"


Format directories may also contain ‘.sql’ and ‘.lnav’ files to help automate log file analysis. The SQL files are executed on startup to create any helper tables or views and the ‘.lnav’ script files can be executed using the pipe hotkey (|). For example, lnav includes a “partition-by-boot” script that partitions the log view based on boot messages from the Linux kernel. A script can have a mix of SQL and lnav commands, as well as include other scripts. The type of statement to execute is determined by the leading character on a line: a semi-colon begins a SQL statement; a colon starts an lnav command; and a pipe (|) denotes another script to be executed. Lines beginning with a hash are treated as comments. Any arguments passed to a script can be referenced using ‘$N’ where ‘N’ is the index of the argument. Remember that you need to use the ‘:eval’ command (see Miscellaneous) when referencing variables in most lnav commands. Scripts can provide help text to be displayed during interactive usage by adding the following tags in a comment header:


The synopsis should contain the name of the script and any parameters to be passed. For example:

# @synopsis: hello-world <name1> [<name2> ... <nameN>]

A one-line description of what the script does. For example:

# @description: Say hello to the given names.

Installing Formats

File formats are loaded from subdirectories in /etc/lnav/formats and ~/.lnav/formats/. You can manually create these subdirectories and copy the format files into there. Or, you can pass the ‘-i’ option to lnav to automatically install formats from the command-line. For example:

$ lnav -i myformat.json
info: installed: /home/example/.lnav/formats/installed/myformat_log.json

Format files installed using this method will be placed in the installed subdirectory and named based on the first format name found in the file.

You can also install formats from git repositories by passing the repository’s clone URL. A standard set of repositories is maintained at (https://github.com/tstack/lnav-config) and can be installed by passing ‘extra’ on the command line, like so:

$ lnav -i extra

These repositories can be updated by running lnav with the ‘-u’ flag.

Format files can also be made executable by adding a shebang (#!) line to the top of the file, like so:

#! /usr/bin/env lnav -i
    "myformat_log" : ...

Executing the format file should then install it automatically:

$ chmod ugo+rx myformat.json
$ ./myformat.json
info: installed: /home/example/.lnav/formats/installed/myformat_log.json

Format Order When Scanning a File

When lnav loads a file, it tries each log format against the first ~1000 lines of the file trying to find a match. When a match is found, that log format will be locked in and used for the rest of the lines in that file. Since there may be overlap between formats, lnav performs a test on startup to determine which formats match each others sample lines. Using this information it will create an ordering of the formats so that the more specific formats are tried before the more generic ones. For example, a format that matches certain syslog messages will match its own sample lines, but not the ones in the syslog samples. On the other hand, the syslog format will match its own samples and those in the more specific format. You can see the order of the format by enabling debugging and checking the lnav log file for the “Format order” message:

$ lnav -d /tmp/lnav.log