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Unix – System Logging

Unix systems have a very flexible and powerful logging system, which enables you to record almost anything you can imagine and then manipulate the logs to retrieve the information you require.

Many versions of UNIX provide a general-purpose logging facility called syslog. Individual programs that need to have information logged send the information to syslog.

Unix syslog is a host-configurable, uniform system logging facility. The system uses a centralized system logging process that runs the program /etc/syslogd or /etc/syslog.

The operation of the system logger is quite straightforward. Programs send their log entries to syslogd, which consults the configuration file /etc/syslogd.conf or /etc/syslog and, when a match is found, writes the log message to the desired log file.

There are four basic syslog terms that you should understand:
FacilityThe identifier used to describe the application or process that submitted the log message. Examples are mail, kernel, and ftp.
PriorityAn indicator of the importance of the message. Levels are defined within syslog as guidelines, from debugging information to critical events.
SelectorA combination of one or more facilities and levels. When an incoming event matches a selector, an action is performed.
ActionWhat happens to an incoming message that matches a selector. Actions can write the message to a log file, echo the message to a console or other device, write the message to a logged in user, or send the message along to another syslog server.
Syslog Facilities:

Here are the available facilities for the selector. Not all facilities are present on all versions of UNIX.
authActivity related to requesting name and password (getty, su, login)
authprivSame as auth but logged to a file that can only be read by selected users
consoleUsed to capture messages that would generally be directed to the system console
cronMessages from the cron system scheduler
daemonSystem daemon catch-all
ftpMessages relating to the ftp daemon
kernKernel messages
local0.local7Local facilities defined per site
lprMessages from the line printing system
mailMessages relating to the mail system
markPseudo event used to generate timestamps in log files
newsMessages relating to network news protocol (nntp)
ntpMessages relating to network time protocol
userRegular user processes
uucpUUCP subsystem
Syslog Priorities:

The syslog priorities are summarized in the following table:
EmergEmergency condition, such as an imminent system crash, usually broadcast to all users
AlertCondition that should be corrected immediately, such as a corrupted system database
CritCritical condition, such as a hardware error
ErrOrdinary error
NoticeCondition that is not an error, but possibly should be handled in a special way
InfoInformational message
DebugMessages that are used when debugging programs
NonePseudo level used to specify not to log messages.
The combination of facilities and levels enables you to be discerning about what is logged and where that information goes.

As each program sends its messages dutifully to the system logger, the logger makes decisions on what to keep track of and what to discard based on the levels defined in the selector.

When you specify a level, the system will keep track of everything at that level and higher.

The /etc/syslog.conf file:

The /etc/syslog.conf file controls where messages are logged. A typical syslog.conf file might look like this:

Each line of the file contains two parts:
  • A message selector that specifies which kind of messages to log. For example, all error messages or all debugging messages from the kernel.
  • An action field that says what should be done with the message. For example, put it in a file or send the message to a user's terminal.
Following are the notable points for the above configuration:
  • Message selectors have two parts: a facility and a priority. For example, kern.debug selects all debug messages (the priority) generated by the kernel (the facility).
  • Message selectetor kern.debug selects all priorities that are greater than debug.
  • An asterisk in place of either the facility or the priority indicates "all." For example, *.debug means all debug messages, while kern.* means all messages generated by the kernel.
  • You can also use commas to specify multiple facilities. Two or more selectors can be grouped together by using a semicolon.
Logging Actions:

The action field specifies one of five actions:

1. Log message to a file or a device. For example, /var/log/lpr.log or /dev/console.

2. Send a message to a user. You can specify multiple usernames by separating them with commas (e.g., root, amrood).

3. Send a message to all users. In this case, the action field consists of an asterisk (e.g., *).

4. Pipe the message to a program. In this case, the program is specified after the UNIX pipe symbol (|).

5. Send the message to the syslog on another host. In this case, the action field consists of a hostname, preceded by an at sign (e.g.,

The logger Command:

UNIX provides the logger command, which is an extremely useful command to deal with system logging. The logger command sends logging messages to the syslogd daemon, and consequently provokes system logging.

This means we can check from the command line at any time the syslogd daemon and its configuration. The logger command provides a method for adding one-line entries to the system log file from the command line.

The format of the command is:

logger [-i] [-f file] [-p priority] [-t tag] [message]...

Here is the detail of the parameters:
-f filenameUse the contents of file filename as the message to log.
-iLog the process ID of the logger process with each line.
-p priorityEnter the message with the specified priority (specified selector entry); the message priority can be specified numerically, or as a facility.priority pair. The default priority is user.notice.
-t tagMark each line added to the log with the specified tag.
messageThe string arguments whose contents are concatenated together in the specified order, separated by the space
You can use Manpage Help to check complete syntax for this command.

Log Rotation:

Log files have the propensity to grow very fast and consume large amounts of disk space. To enable log rotations, most distributions use tools such as newsyslog or logrotate.

These tools should be called on a frequent time interval using the cron daemon. Check the man pages for newsyslog or logrotate for more details.

Important Log Locations

All the system applications create their log files in /var/log and its sub-directories. Here are few important applications and their coressponding log directories:
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