Interpreting output of dmesg and /var/log/boot.log

Question

I found two ways for printing boot messages: one using dmesg, and another the content of file /var/log/boot.log:

The contents of the /var/log/boot.log file are:

 fsck from util-linux 2.20.1
/dev/loop0: clean, 263563/2271232 files, 7843644/18169856 blocks
 * Starting configure network device security^[[74G[ OK ]
 * Starting configure network device security^[[74G[ OK ]
 * Starting configure network device^[[74G[ OK ]
 * Starting Mount network filesystems^[[74G[ OK ]
 * Starting Failsafe Boot Delay^[[74G[ OK ]
 * Stopping Mount network filesystems^[[74G[ OK ]
 * Starting Bridge socket events into upstart^[[74G[ OK ]
 * Starting mDNS/DNS-SD daemon^[[74G[ OK ]
 * Starting Userspace bootsplash^[[74G[ OK ]
 * Starting Send an event to indicate plymouth is up^[[74G[ OK ]
 * Starting bluetooth daemon^[[74G[ OK ]
 * Starting configure network device^[[74G[ OK ]
 * Stopping Failsafe Boot Delay^[[74G[ OK ]
 * Starting System V initialisation compatibility^[[74G[ OK ]
 * Starting set sysctls from /etc/sysctl.conf^[[74G[ OK ]
 * Starting configure network device security^[[74G[ OK ]
 * Starting modem connection manager^[[74G[ OK ]
 * Stopping set sysctls from /etc/sysctl.conf^[[74G[ OK ]
 * Starting CUPS printing spooler/server^[[74G[ OK ]
 * Starting network connection manager^[[74G[ OK ]
 * Stopping cold plug devices^[[74G[ OK ]
 * Stopping log initial device creation^[[74G[ OK ]
 * Starting configure network device security^[[74G[ OK ]
 * Starting save udev log and update rules^[[74G[ OK ]
 * Stopping save udev log and update rules^[[74G[ OK ]
 * Stopping Userspace bootsplash^[[74G[ OK ]
Skipping profile in /etc/apparmor.d/disable: usr.bin.firefox
Skipping profile in /etc/apparmor.d/disable: usr.sbin.rsyslogd
 * Starting AppArmor profiles       ^[[170G
^[[164G[ OK ]
speech-dispatcher disabled; edit /etc/default/speech-dispatcher
saned disabled; edit /etc/default/saned
 * Stopping System V initialisation compatibility^[[74G[ OK ]
 * Starting System V runlevel compatibility^[[74G[ OK ]

while dmesg gives something like this:

[    0.008660] Initializing cgroup subsys blkio
[    0.008663] Initializing cgroup subsys perf_event
[    0.008667] Initializing cgroup subsys hugetlb
[    0.008713] CPU: Physical Processor ID: 0
[    0.008716] CPU: Processor Core ID: 0
[    0.008720] mce: CPU supports 4 MCE banks
[    0.008739] CPU0: Thermal monitoring enabled (TM1)
[    0.008746] process: using mwait in idle threads
[    0.008753] Last level iTLB entries: 4KB 64, 2MB 64, 4MB 64
[    0.008753] Last level dTLB entries: 4KB 64, 2MB 0, 4MB 64
[    0.008753] tlb_flushall_shift: 6
[    0.008895] Freeing SMP alternatives: 24k freed
[    0.011699] ACPI: Core revision 20121018
[    0.016015] ftrace: allocating 29358 entries in 115 pages
[    0.028544] ..TIMER: vector=0x30 apic1=0 pin1=2 apic2=-1 pin2=-1
[    0.068468] smpboot: CPU0: Intel(R) Pentium(R) 4 CPU 3.00GHz (fam: 0f, model: 04, stepping: 09)
[    0.072000] Performance Events: Netburst events, Netburst P4/Xeon PMU driver.
[    0.072000] ... version:                0

and so on.

So my question is, how are the two related?  When should we use dmesg and when boot.log?

Answer 1

dmesg output is diagnostic messages of the Linux kernel, i.e. some information about hardware and driver initialization.

boot.log seems to be a specific feature of your distribution (I don't have it), which shows you a log of successful/failed starts of your daemons and other initscripts which are located at /etc/rc.d/ or /etc/init.d/. These initscripts are started after the Linux kernel is booted.

Answer 2

dmesg displays messages in the kernel’s ring buffer. This is where the kernel logs all messages, not just messages from boot.

/var/log/boot.log is create by Plymouth. Plymouth can be used to display graphics instead of the traditional boot messages. Plymouth records those traditional boot messages in /var/log/boot.log.

Use /var/log/boot.log to get only boot messages. Use dmesg to get all of the kernel’s messages.

Answer 3

dmesg is a kernels ring buffer, it can be used by the kernel to log messages before device drivers have been loaded (to access disk, etc). boot.log is the permanent store of boot messages.