The dmesg command is used to write the kernel messages in Linux and other
Unix-like operating systems to standard output (which by default is the display screen).

A kernel is the core of an operating system. It is the first part of the operating system
hat is loaded into memory when a computer boots up (i.e., starts up), and it controls virtually
everything on a system.

The numerous messages generated by the kernel that appear on the display
screen as a computer boots up show the hardware devices that the kernel detects
and indicate whether it is able to configure them.

dmesg obtains its data by reading the kernel ring buffer. A buffer is a portion of a
computer’s memory that is set aside as a temporary holding place for data that is being sent
to or received from an external device, such as a hard disk drive (HDD), printer or keyboard.

A ring buffer is a buffer of fixed size for which any new data added to it overwrites the oldest data in it.

dmesg can be very useful when troubleshooting or just trying to obtain information about the
hardware on a system. Its basic syntax is

dmesg [options]

Invoking dmesg without any of its options (which are rarely used) causes it to write all the
kernel messages to standard output.

This usually produces far too many lines to fit into the display screen all at once,
and thus only the final messages are visible.

However, the output can be redirected to the less command through the use of a pipe
(designated by the vertical bar character),

thereby allowing the startup messages to be viewed one screenful at a time:

dmesg | less

less allows the output to be moved forward one screenful at a time by pressing the
SPACE bar, backward by pressing the b key and removed by pressing the q key.

(The more command could have been used here instead of the less command; however,
 less is newer than more and has additional functions, including the ability to return to
previous pages of the output.)

When a user encounters a problem with the system, it can be convenient to write the
output of dmesg to a file and then send that file by e-mail to a system administrator or
other knowledgeable person for assistance.

For example, the output could be redirected to a file named boot_messages using the
output redirection operator

(designated by a rightward facing angle bracket) as follows:

dmesg > boot_messages

The -i option can be used to tell grep to ignore the case (i.e., lower case or upper case) of the
letters in the string. For example, the following command :
lists all references to USB (universal serial bus) devices in the kernel messages:

dmesg | grep -i usb

And the following tells dmesg to show all serial ports (which are represented by the string tty):

dmesg | grep -i tty

The dmesg and grep combination can also be used to show how much
physical memory (i.e., RAM) is available on the system:

dmesg | grep -i memory

The following command checks to confirm that the HDD(s) is running in
DMA (direct memory access) mode:

dmesg | grep -i dma

The output of dmesg is maintained in the log file /var/log/dmesg, and it can thus also be
easily viewed by reading that file with a text editor, such as vi or gedit, or with a command
such as cat, e.g.,

cat /var/log/dmesg | less