syslog

syslog (is a service, not a command)

The `syslog` command, more accurately referred to as the syslog service (often implemented via rsyslog or systemd-journald), is a fundamental component of Linux and other Unix-like operating systems. It provides a standardized way for system processes and applications to record events, errors, and other relevant information. This central logging mechanism allows administrators to monitor system health, troubleshoot issues, and analyze security incidents.

The `syslog` service typically collects logs from various sources, categorizes them based on facility (e.g., kernel, user, mail) and severity level (e.g., debug, info, warning, error, critical), and stores them in designated log files (often located in `/var/log`). The configuration determines where and how logs are stored and processed. Modern implementations offer advanced features like filtering, forwarding logs to remote servers, and using structured data formats. While the `syslog` command itself is less frequently directly interacted with (replaced by management tools and configuration files), understanding its principles is crucial for system administration and security monitoring.

The 'syslog' command is more of a service controlled via configuration files (e.g., `/etc/rsyslog.conf`, `/etc/systemd/journald.conf`) rather than a direct command-line tool. There's no single executable named 'syslog' that you run with command-line arguments. Instead, interaction occurs indirectly through service control commands (e.g., `systemctl start rsyslog`, `systemctl stop rsyslog`).

The behavior of the syslog service is primarily controlled through configuration files. For `rsyslog`, the main configuration file is typically `/etc/rsyslog.conf`. This file specifies which facilities and severity levels should be logged, where the logs should be stored, and how they should be formatted. For systemd-journald, the configuration is in `/etc/systemd/journald.conf`.

Syslog messages are categorized by facility and severity. Facilities indicate the source of the message (e.g., `kern` for kernel messages, `user` for user-level messages). Severities indicate the importance of the message (e.g., `debug`, `info`, `warning`, `error`, `critical`, `alert`, `emerg`). These categories allow for fine-grained control over which messages are logged and how they are handled.

Originally, `syslog` was developed as part of BSD Unix. It has since become a standard component of nearly every Unix-like operating system. Over time, different implementations of the syslog service emerged, including `rsyslog`, `syslog-ng`, and systemd's journal. `rsyslog` is a very popular implementation known for its high performance and advanced features, replacing the original BSD syslog implementation in many distributions. The syslog protocol (RFC 5424) defines the standard for log message format and transmission. Usage evolved from very simple logging to nowadays be a central point in system logging and security information management.

rsyslogd(8), logger(1), journalctl(1), dmesg(1)