How can I write to dmesg from command line?

Question

I'd like to write a statement to dmesg. How can I do this?

Answer 1

Write to /dev/kmsg (not /proc/kmsg as suggested by @Nils). See linux/kernel/printk/printk.c devkmsg_writev for the kernel-side implementation and systemd/src/journal/journald-kmsg.c server_forward_kmsg for an example of usage.

Answer 2

For BSDs:

logger -p kern.notice MESSAGE

(courtesy Ian, freebsd-questions mailing list)

or other priorities.

For Linux:

su root -c 'echo MESSAGE > /dev/kmsg'

Answer 3

Assuming nobody else comes up with an official way to do this ...

You can write a kernel module that calls the printk function. There's an example here that might just do the job for you.

Answer 4

--> You may write a C program as below:

test_mod.c

#include <linux/module.h>
#include <linux/kernel.h>

int init_module(void)
{
    printk("Hello All\n This is a test init\n");
    return 0;
}

void cleanup_module(void)
{
    printk("Good Bye");
}

--> make object file:

echo "obj-m := test_mod.o" > Makefile

--> compile by running :

make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules`

--> load your module as below:

insmod ./test_mod.ko

see the output:

dmesg | tail

--> unload module:

rmmod test_mod.ko

Answer 5

echo "Add your message here" | sudo tee /dev/kmsg

Verify by running dmesg -T

Answer 6

If I understood man dmesg correctly, you should be able to write to /proc/kmsg.