7 个用于排除 Linux 系统故障和收集信息的 dmesg 命令

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“ dmesg ”命令显示来自内核环形缓冲区的消息。系统通过多个运行级别，从中我们可以获得大量信息，如系统架构、CPU、附加设备、RAM 等。当计算机启动时，内核（操作系统的核心）被加载到内存中。在此期间，将显示大量消息，我们可以看到内核检测到的硬件设备。

另请参阅：10 个用于收集系统和硬件信息的 Linux 命令

在设备发生故障时，这些消息对于诊断目的非常重要。当我们在系统上连接或断开硬件设备时，借助 dmesg 命令，我们可以实时了解检测到的或断开的信息。dmesg 命令在大多数基于Linux 和 Unix 的操作系统上都可用。

让我们通过下面讨论的实际示例来介绍最著名的工具“dmesg”命令。dmesg 的确切语法如下。

# dmseg [options...]

1. 列出内核中所有已加载的驱动程序

我们可以使用文本处理工具，即“ more ”、“ tail ”、“ less ”或“ grep ”与 dmesg 命令。由于 dmesg 日志的输出无法放在一页上，因此使用带有管道 more 或 less 的 dmesg 命令将在一页中显示日志。

[root@example.com ~]# dmesg | more
[root@example.com ~]# dmesg | less

示例输出

[    0.000000] Initializing cgroup subsys cpuset
[    0.000000] Initializing cgroup subsys cpu
[    0.000000] Initializing cgroup subsys cpuacct
[    0.000000] Linux version 3.11.0-13-generic (buildd@aatxe) (gcc version 4.8.1 (Ubuntu/Linaro 4.8.1-10ubuntu8) ) #20-Ubuntu SMP Wed Oct 23 17:26:33 UTC 2013 
(Ubuntu 3.11.0-13.20-generic 3.11.6)
[    0.000000] KERNEL supported cpus:
[    0.000000]   Intel GenuineIntel
[    0.000000]   AMD AuthenticAMD
[    0.000000]   NSC Geode by NSC
[    0.000000]   Cyrix CyrixInstead
[    0.000000]   Centaur CentaurHauls
[    0.000000]   Transmeta GenuineTMx86
[    0.000000]   Transmeta TransmetaCPU
[    0.000000]   UMC UMC UMC UMC
[    0.000000] e820: BIOS-provided physical RAM map:
[    0.000000] BIOS-e820: [mem 0x0000000000000000-0x000000000009fbff] usable
[    0.000000] BIOS-e820: [mem 0x00000000000f0000-0x00000000000fffff] reserved
[    0.000000] BIOS-e820: [mem 0x0000000000100000-0x000000007dc08bff] usable
[    0.000000] BIOS-e820: [mem 0x000000007dc08c00-0x000000007dc5cbff] ACPI NVS
[    0.000000] BIOS-e820: [mem 0x000000007dc5cc00-0x000000007dc5ebff] ACPI data
[    0.000000] BIOS-e820: [mem 0x000000007dc5ec00-0x000000007fffffff] reserved
[    0.000000] BIOS-e820: [mem 0x00000000e0000000-0x00000000efffffff] reserved
[    0.000000] BIOS-e820: [mem 0x00000000fec00000-0x00000000fed003ff] reserved
[    0.000000] BIOS-e820: [mem 0x00000000fed20000-0x00000000fed9ffff] reserved
[    0.000000] BIOS-e820: [mem 0x00000000fee00000-0x00000000feefffff] reserved
[    0.000000] BIOS-e820: [mem 0x00000000ffb00000-0x00000000ffffffff] reserved
[    0.000000] NX (Execute Disable) protection: active
.....

另请阅读：使用 head、tail 和 cat 命令有效管理 Linux 文件

2.列出所有检测到的设备

要发现内核检测到了哪些硬盘，您可以搜索关键字“ sda ”以及“ grep ”，如下所示。

[root@example.com ~]# dmesg | grep sda

[    1.280971] sd 2:0:0:0: [sda] 488281250 512-byte logical blocks: (250 GB/232 GiB)
[    1.281014] sd 2:0:0:0: [sda] Write Protect is off
[    1.281016] sd 2:0:0:0: [sda] Mode Sense: 00 3a 00 00
[    1.281039] sd 2:0:0:0: [sda] Write cache: enabled, read cache: enabled, doesn't support DPO or FUA
[    1.359585]  sda: sda1 sda2 < sda5 sda6 sda7 sda8 >
[    1.360052] sd 2:0:0:0: [sda] Attached SCSI disk
[    2.347887] EXT4-fs (sda1): mounted filesystem with ordered data mode. Opts: (null)
[   22.928440] Adding 3905532k swap on /dev/sda6.  Priority:-1 extents:1 across:3905532k FS
[   23.950543] EXT4-fs (sda1): re-mounted. Opts: errors=remount-ro
[   24.134016] EXT4-fs (sda5): mounted filesystem with ordered data mode. Opts: (null)
[   24.330762] EXT4-fs (sda7): mounted filesystem with ordered data mode. Opts: (null)
[   24.561015] EXT4-fs (sda8): mounted filesystem with ordered data mode. Opts: (null)

注意：'sda' 表示第一个 SATA 硬盘，'sdb' 表示第二个 SATA 硬盘，依此类推。如果是 IDE 硬盘，请使用 'hda' 或 'hdb' 搜索。

3. 仅打印前 20 行输出

'head' 与 dmesg 一起将显示起始行，即 'dmesg | head -20' 将仅从起始点打印 20 行。

[root@example.com ~]# dmesg | head  -20

[    0.000000] Initializing cgroup subsys cpuset
[    0.000000] Initializing cgroup subsys cpu
[    0.000000] Initializing cgroup subsys cpuacct
[    0.000000] Linux version 3.11.0-13-generic (buildd@aatxe) (gcc version 4.8.1 (Ubuntu/Linaro 4.8.1-10ubuntu8) ) #20-Ubuntu SMP Wed Oct 23 17:26:33 UTC 2013 (Ubuntu 3.11.0-13.20-generic 3.11.6)
[    0.000000] KERNEL supported cpus:
[    0.000000]   Intel GenuineIntel
[    0.000000]   AMD AuthenticAMD
[    0.000000]   NSC Geode by NSC
[    0.000000]   Cyrix CyrixInstead
[    0.000000]   Centaur CentaurHauls
[    0.000000]   Transmeta GenuineTMx86
[    0.000000]   Transmeta TransmetaCPU
[    0.000000]   UMC UMC UMC UMC
[    0.000000] e820: BIOS-provided physical RAM map:
[    0.000000] BIOS-e820: [mem 0x0000000000000000-0x000000000009fbff] usable
[    0.000000] BIOS-e820: [mem 0x00000000000f0000-0x00000000000fffff] reserved
[    0.000000] BIOS-e820: [mem 0x0000000000100000-0x000000007dc08bff] usable
[    0.000000] BIOS-e820: [mem 0x000000007dc08c00-0x000000007dc5cbff] ACPI NVS
[    0.000000] BIOS-e820: [mem 0x000000007dc5cc00-0x000000007dc5ebff] ACPI data
[    0.000000] BIOS-e820: [mem 0x000000007dc5ec00-0x000000007fffffff] reserved

4. 仅打印最后 20 行输出

'tail' 与 dmesg 命令一起将仅打印最后 20 行，这在我们插入可移动设备时很有用。

[root@example.com ~]# dmesg | tail -20

parport0: PC-style at 0x378, irq 7 [PCSPP,TRISTATE]
ppdev: user-space parallel port driver
EXT4-fs (sda1): mounted filesystem with ordered data mode
Adding 2097144k swap on /dev/sda2.  Priority:-1 extents:1 across:2097144k
readahead-disable-service: delaying service auditd
ip_tables: (C) 2000-2006 Netfilter Core Team
nf_conntrack version 0.5.0 (16384 buckets, 65536 max)
NET: Registered protocol family 10
lo: Disabled Privacy Extensions
e1000: eth0 NIC Link is Up 1000 Mbps Full Duplex, Flow Control: None
Slow work thread pool: Starting up
Slow work thread pool: Ready
FS-Cache: Loaded
CacheFiles: Loaded
CacheFiles: Security denies permission to nominate security context: error -95
eth0: no IPv6 routers present
type=1305 audit(1398268784.593:18630): audit_enabled=0 old=1 auid=4294967295 ses=4294967295 res=1
readahead-collector: starting delayed service auditd
readahead-collector: sorting
readahead-collector: finished

5. 搜索检测到的设备或特定字符串

由于 dmesg 输出的长度，搜索特定字符串很困难。因此，过滤包含字符串（如“ usb ”、“ dma ”、“ tty ”和“ memory ”等）的行。“-i”选项指示grep 命令忽略大小写（大写或小写字母）。

[root@example.com log]# dmesg | grep -i usb
[root@example.com log]# dmesg | grep -i dma
[root@example.com log]# dmesg | grep -i tty
[root@example.com log]# dmesg | grep -i memory

示例输出

[    0.000000] Scanning 1 areas for low memory corruption
[    0.000000] initial memory mapped: [mem 0x00000000-0x01ffffff]
[    0.000000] Base memory trampoline at [c009b000] 9b000 size 16384
[    0.000000] init_memory_mapping: [mem 0x00000000-0x000fffff]
[    0.000000] init_memory_mapping: [mem 0x37800000-0x379fffff]
[    0.000000] init_memory_mapping: [mem 0x34000000-0x377fffff]
[    0.000000] init_memory_mapping: [mem 0x00100000-0x33ffffff]
[    0.000000] init_memory_mapping: [mem 0x37a00000-0x37bfdfff]
[    0.000000] Early memory node ranges
[    0.000000] PM: Registered nosave memory: [mem 0x0009f000-0x000effff]
[    0.000000] PM: Registered nosave memory: [mem 0x000f0000-0x000fffff]
[    0.000000] please try 'cgroup_disable=memory' option if you don't want memory cgroups
[    0.000000] Memory: 2003288K/2059928K available (6352K kernel code, 607K rwdata, 2640K rodata, 880K init, 908K bss, 56640K reserved, 1146920K highmem)
[    0.000000] virtual kernel memory layout:
[    0.004291] Initializing cgroup subsys memory
[    0.004609] Freeing SMP alternatives memory: 28K (c1a3e000 - c1a45000)
[    0.899622] Freeing initrd memory: 23616K (f51d0000 - f68e0000)
[    0.899813] Scanning for low memory corruption every 60 seconds
[    0.946323] agpgart-intel 0000:00:00.0: detected 32768K stolen memory
[    1.360318] Freeing unused kernel memory: 880K (c1962000 - c1a3e000)
[    1.429066] [drm] Memory usable by graphics device = 2048M

6.清除 dmesg 缓冲区日志

是的，如果需要，我们可以使用以下命令清除 dmesg 日志。它将清除 dmesg 环形缓冲区消息日志，直到您执行以下命令。您仍然可以查看存储在“ /var/log/dmesg ”文件中的日志。如果您连接任何设备，都会生成 dmesg 输出。

[root@example.com log]# dmesg -c

7.实时监控dmesg

一些发行版也允许命令“tail -f /var/log/dmesg”进行实时 dmesg 监控。

[root@example.com log]# watch "dmesg | tail -20"

结论：dmesg 命令很有用，因为 dmesg 会实时记录所有已完成或发生的系统更改。与往常一样，您可以man dmesg来获取更多信息。