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SysOps

How to deal with dmesg timestamps

By default dmesg command print kernel ring buffer using the timestamp for each logged message. It is easy to change this behavior and display date/time in a human-readable form using just one additional parameter. Still, sometimes it is not supported, so I will shortly touch upon this topic.

$ dmesg
[...]
[    1.028871] Linux agpgart interface v0.103
[    1.028940] agpgart-intel 0000:00:00.0: Intel Sandybridge Chipset
[    1.028999] agpgart-intel 0000:00:00.0: detected gtt size: 2097152K total, 262144K mappable
[    1.029857] agpgart-intel 0000:00:00.0: detected 65536K stolen memory
[...]

How to convert timestamps to human-readable form?

The easiest way is to use -T or --ctime parameter.

$ dmesg -T
[...]
[Sat Oct 19 12:43:26 2013] Linux agpgart interface v0.103
[Sat Oct 19 12:43:26 2013] agpgart-intel 0000:00:00.0: Intel Sandybridge Chipset
[Sat Oct 19 12:43:26 2013] agpgart-intel 0000:00:00.0: detected gtt size: 2097152K total, 262144K mappable
[Sat Oct 19 12:43:26 2013] agpgart-intel 0000:00:00.0: detected 65536K stolen memory
[...]

What if the above-mentioned parameters are not supported?

Sometimes you will encounter Linux distribution in which such conversion is not supported but can be easily implemented using the following shell script.

#!/bin/bash
# Translate dmesg timestamps to human readable format

# desired date format
date_format="%a %b %d %T %Y"

# uptime in seconds
uptime=$(cut -d " " -f 1 /proc/uptime)

# run only if timestamps are enabled
if [ "Y" = "$(cat /sys/module/printk/parameters/time)" ]; then
  dmesg | sed "s/^\[[ ]*\?\([0-9.]*\)\] \(.*\)/\\1 \\2/" | while read timestamp message; do
    printf "[%s] %s\n" "$(date --date "now - $uptime seconds + $timestamp seconds" +"${date_format}")" "$message"
  done
else
  echo "Timestamps are disabled (/sys/module/printk/parameters/time)"
fi

What if timestamps are missing?

Rarely you will find that timestamps are missing from the dmesg output.

$ dmesg
[...]
Bluetooth: RFCOMM TTY layer initialized
Bluetooth: RFCOMM socket layer initialized
Bluetooth: RFCOMM ver 1.11
[...]

Read /sys/module/printk/parameters/time file to confirm that timestamps are disabled.

$ cat /sys/module/printk/parameters/time
N

Write Y to the above-mentioned file to enable logging timestamps.

$ echo Y | sudo tee /sys/module/printk/parameters/time
This change will not be permanent, as it will last until reboot. To make it permanent, add printk.time=1 parameter to grub options.

Log test message and verify dmesg output to check out changes.

$ echo "Enabled timestamps" | sudo tee /dev/kmsg
$ dmesg
[...]
Bluetooth: RFCOMM TTY layer initialized
Bluetooth: RFCOMM socket layer initialized
Bluetooth: RFCOMM ver 1.11
[...]
[271309.434405] Enabled timestamps
[...]