1.dmesg介绍
dmesg 是一个在 Linux 和类 Unix 操作系统中用于显示内核环缓冲区（kernel ring buffer）内容的命令。内核环缓冲区存储了系统启动时和运行过程中产生的日志信息，主要用于诊断硬件问题、驱动问题和其他内核相关的事件。
主要功能：
查看系统启动日志：在系统启动时，内核和驱动程序会输出许多信息，如硬件检测、驱动加载、错误信息等。通过 dmesg 可以查看这些启动日志。
查看硬件信息：dmesg 命令能显示与硬件设备（如磁盘、网络接口、USB设备等）相关的信息。
诊断和排查问题：如果硬件出现故障或设备驱动有问题，dmesg 输出的日志通常能够提供有用的诊断信息。
2.dmesg用法
[root@patrolagent ~]# dmesg --help
Usage:
dmesg [options]
Options:
-C, --clear clear the kernel ring buffer
-c, --read-clear read and clear all messages
-D, --console-off disable printing messages to console
-d, --show-delta show time delta between printed messages
-e, --reltime show local time and time delta in readable format
-E, --console-on enable printing messages to console
-F, --file <file> use the file instead of the kernel log buffer
-f, --facility <list> restrict output to defined facilities
-H, --human human readable output
-k, --kernel display kernel messages
-L, --color colorize messages
-l, --level <list> restrict output to defined levels
-n, --console-level <level> set level of messages printed to console
-P, --nopager do not pipe output into a pager
-r, --raw print the raw message buffer
-S, --syslog force to use syslog(2) rather than /dev/kmsg
-s, --buffer-size <size> buffer size to query the kernel ring buffer
-T, --ctime show human readable timestamp (could be
inaccurate if you have used SUSPEND/RESUME)
-t, --notime don't print messages timestamp
-u, --userspace display userspace messages
-w, --follow wait for new messages
-x, --decode decode facility and level to readable string
-h, --help display this help and exit
-V, --version output version information and exit
Supported log facilities:
kern - kernel messages
user - random user-level messages
mail - mail system
daemon - system daemons
auth - security/authorization messages
syslog - messages generated internally by syslogd
lpr - line printer subsystem
news - network news subsystem
Supported log levels (priorities):
emerg - system is unusable
alert - action must be taken immediately
crit - critical conditions
err - error conditions
warn - warning conditions
notice - normal but significant condition
info - informational
debug - debug-level messages
For more details see dmesg(q).
[root@patrolagent ~]#3.实例
3.1.查看内核缓冲区日志
命令：
dmesg
该命令会显示当前内核缓冲区中的所有日志信息
[root@patrolagent ~]# dmesg | tail -n 10
[ 3.559989] XFS (dm-3): Ending clean mount
[ 3.561241] XFS (dm-2): Ending clean mount
[ 3.565719] systemd-journald[565]: Received request to flush runtime journal from PID 1
[ 3.583667] type=1305 audit(1731659062.040:4): audit_pid=748 old=0 auid=4294967295 ses=4294967295 subj=system_u:system_r:auditd_t:s0 res=1
[ 3.790385] NET: Registered protocol family 40
[ 3.924605] IPv6: ADDRCONF(NETDEV_UP): ens33: link is not ready
[ 3.927665] e1000: ens33 NIC Link is Up 1000 Mbps Full Duplex, Flow Control: None
[ 3.928119] IPv6: ADDRCONF(NETDEV_UP): ens33: link is not ready
[ 3.928180] IPv6: ADDRCONF(NETDEV_CHANGE): ens33: link becomes ready
[ 5.381025] floppy0: no floppy controllers found
[root@patrolagent ~]#3.2.查看特定类型的信息
命令：
dmesg |grep -i eth0
[root@patrolagent ~]# dmesg |grep -i eth0
[ 1.514492] e1000 0000:02:01.0 eth0: (PCI:66MHz:32-bit) 00:0c:29:7c:8d:7b
[ 1.514505] e1000 0000:02:01.0 eth0: Intel(R) PRO/1000 Network Connection
[root@patrolagent ~]#3.3.查看日志并实时更新
命令：
dmesg -w |head -n 5
[root@patrolagent ~]# dmesg -w | head -n 5
[ 0.000000] Initializing cgroup subsys cpuset
[ 0.000000] Initializing cgroup subsys cpu
[ 0.000000] Initializing cgroup subsys cpuacct
[ 0.000000] Linux version 3.10.0-693.el7.x86_64 (mockbuild@x86-038.build.eng.bos.redhat.com) (gcc version 4.8.5 20150623 (Red Hat 4.8.5-16) (GCC) ) #1 SMP Thu Jul 6 19:56:57 EDT 2017
[ 0.000000] Command line: BOOT_IMAGE=/vmlinuz-3.10.0-693.el7.x86_64 root=/dev/mapper/rhel_patrolagent-root ro rd.lvm.lv=rhel_patrolagent/root rd.lvm.lv=rhel_patrolagent/swap rhgb quiet LANG=en_US.UTF-8
[root@patrolagent ~]#3.4.显示特定时间段的日志
命令：
dmesg -T
使用-T选项可以将时间戳转换为可读的日期和时间格式。
[root@patrolagent ~]# dmesg -T |tail -n 5
[Fri Nov 15 16:24:20 2024] IPv6: ADDRCONF(NETDEV_UP): ens33: link is not ready
[Fri Nov 15 16:24:20 2024] e1000: ens33 NIC Link is Up 1000 Mbps Full Duplex, Flow Control: None
[Fri Nov 15 16:24:20 2024] IPv6: ADDRCONF(NETDEV_UP): ens33: link is not ready
[Fri Nov 15 16:24:20 2024] IPv6: ADDRCONF(NETDEV_CHANGE): ens33: link becomes ready
[Fri Nov 15 16:24:22 2024] floppy0: no floppy controllers found
[root@patrolagent ~]#3.5.日志清除
命令：
dmesg -C
备注：通常需要root权限
[root@patrolagent ~]# dmesg -C
You have new mail in /var/spool/mail/root
[root@patrolagent ~]# dmesg
[root@patrolagent ~]#
