NAME
dmesg – display the kernel message buffer
SYNOPSIS
dmesg [-ac] [-t [-f output_fmt]] [-M core [-N system]]
DESCRIPTION
The dmesg utility displays the contents of the kernel message buffer. If the -M option is not specified, the buffer is read from the currently running kernel via the sysctl(3) interface. Otherwise, the buffer is read from the specified core file, using the name list from the specified kernel image (or from the default image).
The options are as follows:
- -a
-
Show all data in the message buffer. This includes any syslog records and /dev/console output.
- -c
-
Clear the kernel buffer after printing.
- -t
-
Convert timestamps (see SYSCTL VARIABLES below) to an absolute date and time.
- -f
-
If -t is also specified, use the specified output_fmt for the conversion (see strftime(3) manual page).
- -M
-
Extract values associated with the name list from the specified core.
- -N
-
If -M is also specified, extract the name list from the specified system instead of the default, which is the kernel image the system has booted from.
SYSCTL VARIABLES
The following sysctl(8) variables control how the kernel timestamps entries in the message buffer: The default value is shown next to each variable.
- kern.msgbuf_show_timestamp: 0
-
If set to 0, no timestamps are added. If set to 1, then a 1-second granularity timestamp will be added to most lines in the message buffer. If set to 2, then a microsecond granularity timestamp will be added. This may also be set as a boot loader(8) tunable. The timestamps are placed at the start of most lines that the kernel generates. Some multi-line messages will have only the first line tagged with a timestamp.
FILES
- /var/run/dmesg.boot
-
usually a snapshot of the buffer contents taken soon after file systems are mounted at startup time
SEE ALSO
HISTORY
The dmesg utility appeared in 3BSD.