systemd/Journal
systemd has its own logging system called the journal; running a separate logging daemon is not required. To read the log, use journalctl(1) .
In Arch Linux, the directory /var/log/journal/ is a part of the systemd package, and the journal (when Storage= is set to auto in /etc/systemd/journald.conf ) will write to /var/log/journal/ . If that directory is deleted, systemd will not recreate it automatically and instead will write its logs to /run/systemd/journal in a nonpersistent way. However, the directory will be recreated if Storage=persistent is added to journald.conf and systemd-journald.service is restarted (or the system is rebooted).
Systemd journal classifies messages by Priority level and Facility. Logging classification corresponds to classic Syslog protocol (RFC 5424).
Priority level
A syslog severity code (in systemd called priority) is used to mark the importance of a message RFC 5424 6.2.1.
| Value | Severity | Keyword | Description | Examples |
|---|---|---|---|---|
| 0 | Emergency | emerg | System is unusable | Severe Kernel BUG, systemd dumped core. This level should not be used by applications. |
| 1 | Alert | alert | Should be corrected immediately | Vital subsystem goes out of work. Data loss. kernel: BUG: unable to handle kernel paging request at ffffc90403238ffc . |
| 2 | Critical | crit | Critical conditions | Crashes, coredumps. Like familiar flash: systemd-coredump[25319]: Process 25310 (plugin-containe) of user 1000 dumped core Failure in the system primary application, like X11. |
| 3 | Error | err | Error conditions | Non-fatal error reported: kernel: usb 1-3: 3:1: cannot get freq at ep 0x84 , systemd[1]: Failed unmounting /var. , libvirtd[1720]: internal error: Failed to initialize a valid firewall backend |
| 4 | Warning | warning | May indicate that an error will occur if action is not taken | A non-root file system has only 1GB free. org.freedesktop. Notifications[1860]: (process:5999): Gtk-WARNING **: Locale not supported by C library. Using the fallback ‘C’ locale |
| 5 | Notice | notice | Events that are unusual, but not error conditions | systemd[1]: var.mount: Directory /var to mount over is not empty, mounting anyway , gcr-prompter[4997]: Gtk: GtkDialog mapped without a transient parent. This is discouraged |
| 6 | Informational | info | Normal operational messages that require no action | lvm[585]: 7 logical volume(s) in volume group «archvg» now active |
| 7 | Debug | debug | Messages which may need to be enabled first, only useful for debugging | kdeinit5[1900]: powerdevil: Scheduling inhibition from «:1.14» «firefox» with cookie 13 and reason «screen» |
These rules are recommendations, and the priority level of a given error is at the application developer’s discretion. It is always possible that the error will be at a higher or lower level than expected.
Facility
A syslog facility code is used to specify the type of program that is logging the message RFC 5424 6.2.1.
| Facility code | Keyword | Description | Info |
|---|---|---|---|
| 0 | kern | Kernel messages | |
| 1 | user | User-level messages | |
| 2 | Mail system | Archaic POSIX still supported and sometimes used (for more mail(1) ) | |
| 3 | daemon | System daemons | All daemons, including systemd and its subsystems |
| 4 | auth | Security/authorization messages | Also watch for different facility 10 |
| 5 | syslog | Messages generated internally by syslogd | For syslogd implementations (not used by systemd, see facility 3) |
| 6 | lpr | Line printer subsystem (archaic subsystem) | |
| 7 | news | Network news subsystem (archaic subsystem) | |
| 8 | uucp | UUCP subsystem (archaic subsystem) | |
| 9 | Clock daemon | systemd-timesyncd | |
| 10 | authpriv | Security/authorization messages | Also watch for different facility 4 |
| 11 | ftp | FTP daemon | |
| 12 | — | NTP subsystem | |
| 13 | — | Log audit | |
| 14 | — | Log alert | |
| 15 | cron | Scheduling daemon | |
| 16 | local0 | Local use 0 (local0) | |
| 17 | local1 | Local use 1 (local1) | |
| 18 | local2 | Local use 2 (local2) | |
| 19 | local3 | Local use 3 (local3) | |
| 20 | local4 | Local use 4 (local4) | |
| 21 | local5 | Local use 5 (local5) | |
| 22 | local6 | Local use 6 (local6) | |
| 23 | local7 | Local use 7 (local7) |
Useful facilities to watch: 0, 1, 3, 4, 9, 10, 15.
Filtering output
journalctl allows for the filtering of output by specific fields. If there are many messages to display, or if the filtering of large time spans has to be done, the output of this command can be extensively delayed.
- Show all messages matching PATTERN :
# journalctl --grep=PATTERN
# journalctl --since="2012-10-30 18:17:16"
# journalctl --since "20 min ago"
DESCRIPTION
Note: Probably, you are looking for the C library function syslog(), which talks to syslogd(8); see syslog(3) for details.
This page describes the kernel syslog() system call, which is used to control the kernel printk() buffer; the glibc wrapper function for the system call is called klogctl().
The kernel log buffer
The kernel has a cyclic buffer of length LOG_BUF_LEN in which messages given as arguments to the kernel function printk() are stored (regardless of their log level). In early kernels, LOG_BUF_LEN had the value 4096; from Linux 1.3.54, it was 8192; from Linux 2.1.113, it was 16384; since Linux 2.4.23/2.6, the value is a kernel configuration option (CONFIG_LOG_BUF_SHIFT, default value dependent on the architecture). Since Linux 2.6.6, the size can be queried with command type 10 (see below).
Commands
The type argument determines the action taken by this function. The list below specifies the values for type. The symbolic names are defined in the kernel source, but are not exported to user space; you will either need to use the numbers, or define the names yourself.
SYSLOG_ACTION_CLOSE (0) Close the log. Currently a NOP. SYSLOG_ACTION_OPEN (1) Open the log. Currently a NOP. SYSLOG_ACTION_READ (2) Read from the log. The call waits until the kernel log buffer is nonempty, and then reads at most len bytes into the buffer pointed to by bufp. The call returns the number of bytes read. Bytes read from the log disappear from the log buffer: the information can be read only once. This is the function executed by the kernel when a user program reads /proc/kmsg. SYSLOG_ACTION_READ_ALL (3) Read all messages remaining in the ring buffer, placing them in the buffer pointed to by bufp. The call reads the last len bytes from the log buffer (nondestructively), but will not read more than was written into the buffer since the last «clear ring buffer» command (see command 5 below)). The call returns the number of bytes read. SYSLOG_ACTION_READ_CLEAR (4) Read and clear all messages remaining in the ring buffer. The call does precisely the same as for a type of 3, but also executes the «clear ring buffer» command. SYSLOG_ACTION_CLEAR (5) The call executes just the «clear ring buffer» command. The bufp and len arguments are ignored. This command does not really clear the ring buffer. Rather, it sets a kernel bookkeeping variable that determines the results returned by commands 3 (SYSLOG_ACTION_READ_ALL) and 4 (SYSLOG_ACTION_READ_CLEAR). This command has no effect on commands 2 (SYSLOG_ACTION_READ) and 9 (SYSLOG_ACTION_SIZE_UNREAD). SYSLOG_ACTION_CONSOLE_OFF (6) The command saves the current value of console_loglevel and then sets console_loglevel to minimum_console_loglevel, so that no messages are printed to the console. Before Linux 2.6.32, the command simply sets console_loglevel to minimum_console_loglevel. See the discussion of /proc/sys/kernel/printk, below. The bufp and len arguments are ignored. SYSLOG_ACTION_CONSOLE_ON (7) If a previous SYSLOG_ACTION_CONSOLE_OFF command has been performed, this command restores console_loglevel to the value that was saved by that command. Before Linux 2.6.32, this command simply sets console_loglevel to default_console_loglevel. See the discussion of /proc/sys/kernel/printk, below. The bufp and len arguments are ignored. SYSLOG_ACTION_CONSOLE_LEVEL (8) The call sets console_loglevel to the value given in len, which must be an integer between 1 and 8 (inclusive). The kernel silently enforces a minimum value of minimum_console_loglevel for len. See the log level section for details. The bufp argument is ignored. SYSLOG_ACTION_SIZE_UNREAD (9) (since Linux 2.4.10) The call returns the number of bytes currently available to be read from the kernel log buffer via command 2 (SYSLOG_ACTION_READ). The bufp and len arguments are ignored. SYSLOG_ACTION_SIZE_BUFFER (10) (since Linux 2.6.6) This command returns the total size of the kernel log buffer. The bufp and len arguments are ignored.
All commands except 3 and 10 require privilege. In Linux kernels before Linux 2.6.37, command types 3 and 10 are allowed to unprivileged processes; since Linux 2.6.37, these commands are allowed to unprivileged processes only if /proc/sys/kernel/dmesg_restrict has the value 0. Before Linux 2.6.37, «privileged» means that the caller has the CAP_SYS_ADMIN capability. Since Linux 2.6.37, «privileged» means that the caller has either the CAP_SYS_ADMIN capability (now deprecated for this purpose) or the (new) CAP_SYSLOG capability.
/proc/sys/kernel/printk
/proc/sys/kernel/printk is a writable file containing four integer values that influence kernel printk() behavior when printing or logging error messages. The four values are:
console_loglevel Only messages with a log level lower than this value will be printed to the console. The default value for this field is DEFAULT_CONSOLE_LOGLEVEL (7), but it is set to 4 if the kernel command line contains the word «quiet», 10 if the kernel command line contains the word «debug», and to 15 in case of a kernel fault (the 10 and 15 are just silly, and equivalent to 8). The value of console_loglevel can be set (to a value in the range 1–8) by a syslog() call with a type of 8. default_message_loglevel This value will be used as the log level for printk() messages that do not have an explicit level. Up to and including Linux 2.6.38, the hard-coded default value for this field was 4 (KERN_WARNING); since Linux 2.6.39, the default value is defined by the kernel configuration option CONFIG_DEFAULT_MESSAGE_LOGLEVEL, which defaults to 4. minimum_console_loglevel The value in this field is the minimum value to which console_loglevel can be set. default_console_loglevel This is the default value for console_loglevel.
The log level
Every printk() message has its own log level. If the log level is not explicitly specified as part of the message, it defaults to default_message_loglevel. The conventional meaning of the log level is as follows:
| Kernel constant | Level value | Meaning |
| KERN_EMERG | 0 | System is unusable |
| KERN_ALERT | 1 | Action must be taken immediately |
| KERN_CRIT | 2 | Critical conditions |
| KERN_ERR | 3 | Error conditions |
| KERN_WARNING | 4 | Warning conditions |
| KERN_NOTICE | 5 | Normal but significant condition |
| KERN_INFO | 6 | Informational |
| KERN_DEBUG | 7 | Debug-level messages |
The kernel printk() routine will print a message on the console only if it has a log level less than the value of console_loglevel.
RETURN VALUE
For type equal to 2, 3, or 4, a successful call to syslog() returns the number of bytes read. For type 9, syslog() returns the number of bytes currently available to be read on the kernel log buffer. For type 10, syslog() returns the total size of the kernel log buffer. For other values of type, 0 is returned on success.
In case of error, -1 is returned, and errno is set to indicate the error.
ERRORS
EINVAL Bad arguments (e.g., bad type; or for type 2, 3, or 4, buf is NULL, or len is less than zero; or for type 8, the level is outside the range 1 to 8). ENOSYS This syslog() system call is not available, because the kernel was compiled with the CONFIG_PRINTK kernel-configuration option disabled. EPERM An attempt was made to change console_loglevel or clear the kernel message ring buffer by a process without sufficient privilege (more precisely: without the CAP_SYS_ADMIN or CAP_SYSLOG capability). ERESTARTSYS System call was interrupted by a signal; nothing was read. (This can be seen only during a trace.)
STANDARDS
HISTORY
From the very start, people noted that it is unfortunate that a system call and a library routine of the same name are entirely different animals.
SEE ALSO
Package name: core/man-pages Version: 6.04-1 Upstream: https://www.kernel.org/doc/man-pages/ Licenses: GPL, custom Manuals: /listing/core/man-pages/ Table of contents
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