Unix / Linux – System Logging

In this topic we will discuss about Unix / Linux – 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 −

Sr.No.Term & Description
1FacilityThe identifier used to describe the application or process that submitted the log message. For example, mail, kernel, and ftp.
2PriorityAn indicator of the importance of the message. Levels are defined within syslog as guidelines, from debugging information to critical events.
3SelectorA combination of one or more facilities and levels. When an incoming event matches a selector, an action is performed.
4ActionWhat 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

We will now understand about the syslog facilities. Here are the available facilities for the selector. Not all facilities are present on all versions of Unix.

1authActivity related to requesting name and password (getty, su, login)
2authprivSame as auth but logged to a file that can only be read by selected users
3consoleUsed to capture messages that are generally directed to the system console
4cronMessages from the cron system scheduler
5daemonSystem daemon catch-all
6ftpMessages relating to the ftp daemon
7kernKernel messages
8local0.local7Local facilities defined per site
9lprMessages from the line printing system
10mailMessages relating to the mail system
11markPseudo-event used to generate timestamps in log files
12newsMessages relating to network news protocol (nntp)
13ntpMessages relating to network time protocol
14userRegular user processes
15uucpUUCP subsystem

Syslog Priorities

The syslog priorities are summarized in the following table −

Sr.No.Priority & Description
1emergEmergency condition, such as an imminent system crash, usually broadcast to all users
2alertCondition that should be corrected immediately, such as a corrupted system database
3critCritical condition, such as a hardware error
4errOrdinary error
6noticeCondition that is not an error, but possibly should be handled in a special way
7infoInformational message
8debugMessages that are used when debugging programs
9nonePseudo 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 −

*.err;kern.debug;auth.notice /dev/console
daemon,auth.notice           /var/log/messages
lpr.info                     /var/log/lpr.log
mail.*                       /var/log/mail.log
ftp.*                        /var/log/ftp.log
auth.*                       @prep.ai.mit.edu
auth.*                       root,amrood
netinfo.err                  /var/log/netinfo.log
install.*                    /var/log/install.log
*.emerg                      *
*.alert                      |program_name
mark.*                       /dev/console

Each line of the file contains two parts −

  • 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 selector 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 −

  • Log message to a file or a device. For example, /var/log/lpr.log or /dev/console.
  • Send a message to a user. You can specify multiple usernames by separating them with commas; for example, root, amrood.
  • Send a message to all users. In this case, the action field consists of an asterisk; for example, *.
  • Pipe the message to a program. In this case, the program is specified after the Unix pipe symbol (|).
  • Send the message to the syslog on another host. In this case, the action field consists of a hostname, preceded by an at sign;

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 −

Sr.No.Option & Description
1-f filenameUses the contents of file filename as the message to log.
2-iLogs the process ID of the logger process with each line.
3-p priorityEnters 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.
4-t tagMarks each line added to the log with the specified tag.
5messageThe string arguments whose contents are concatenated together in the specified order, separated by the space.

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 corresponding log directories −


In this guide we learnt about the Unix / Linux – System Logging.

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