How to display printk messages filtered by level as they come?

Question

I wrote a hello word module that performs a printk when it is loaded in level ALERT and another printk when it is released with level INFO:

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

MODULE_LICENSE("GPL");

static int init_hello(void)
{
    printk(KERN_ALERT "init called in hello\n");
    return 0;
}

static void cleanup_hello(void)
{
    printk(KERN_INFO "cleanup called in hello\n");
}

module_init(init_hello);
module_exit(cleanup_hello);

Then I was trying to filter the kernel messages by its level, I did that using dmesg -l , but I want to see the messages as they appear, so I was using

tail -f /var/log/kern.log

Using this command I can see the messages, but how can I filter the messages by level?

I am using Ubuntu 14.04 with kernel 3.16.4

I then tried to print the messages to the console using:

echo 7 7 7 7 > /proc/sys/kernel/printk

But it doesn't print any message to the console. I tried to set the console-level using dmesg -n alert, but it doesn't work either, thus I am not sure what setting the console-level by dmesg or /proc/sys/kernel/printk does.

How can I use these levels efficiently?

Thanks for your help

Answer 1

On Ubuntu 16.04, util-linux 2.27.1, you can do:

dmesg --level err,warn --raw -w

where:

• -w follows the messages as they come
• --level restricts which levels will show
• --raw shows the level at the beginning of the message e.g. as <3>