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 dmesg output piped through grep | |
| Operating system | Unix-like |
|---|---|
| Platform | Cross-platform |
| Type | Command |
dmesg (diagnostic messages [1] ) is a command on most Unix-like operating systems that prints the message buffer of the kernel. [2] [3] The output includes messages produced by the device drivers.
When initially booted, a computer system loads its kernel into memory. At this stage device drivers present in the kernel are set up to drive relevant hardware. Such drivers, as well as other elements within the kernel, may produce output ("messages") reporting both the presence of modules and the values of any parameters adopted. (It may be possible to specify boot parameters which control the level of detail in the messages.) The booting process typically happens at a speed where individual messages scroll off the top of the screen before an operator can read/digest them. The dmesg command allows the review of such messages in a controlled manner after the system has started. [4]
Even after the system has fully booted, the kernel may occasionally produce further diagnostic messages. Common examples of when this might happen are when I/O devices encounter errors, or USB devices are hot-plugged. dmesg provides a mechanism to review these messages at a later time. When first produced, they will be directed to the system console. If the console is in use, these messages may be confused with or quickly overwritten by the output of user programs. [5]
The output of dmesg can amount to many complete screens. For this reason, this output is normally reviewed using standard text-manipulation tools such as more, tail, less or grep. [6] Size of the dmesg buffer is limited and the output is often captured in a permanent system logfile via a logging daemon, such as syslog.
Traditionally, dmesg lines begin with a device name followed by a colon, followed with detailed text. Often these come in clusters, with the same device showing up on multiple lines in succession. Each cluster is usually associated with a single device enumeration, by one particular device driver (or device facility) associated with the device name.
Each driver or facility emits diagnostic information in its own chosen format. Device drivers may specify the format in the manual page by convention called identically to the device file name without the trailing number.
Scroll Lock is a lock key on most IBM-compatible computer keyboards. Depending on the operating system, it may be used for different purposes, and applications may assign functions to the key or change their behavior depending on its toggling state. The key is not frequently used, and therefore some reduced or specialized keyboards lack Scroll Lock altogether.
The Linux framebuffer (fbdev) is a linux subsystem used to show graphics on a computer monitor, typically on the system console.
In computer programming, standard streams are preconnected input and output communication channels between a computer program and its environment when it begins execution. The three input/output (I/O) connections are called standard input (stdin), standard output (stdout) and standard error (stderr). Originally I/O happened via a physically connected system console, but standard streams abstract this. When a command is executed via an interactive shell, the streams are typically connected to the text terminal on which the shell is running, but can be changed with redirection or a pipeline. More generally, a child process inherits the standard streams of its parent process.
A kernel panic is a safety measure taken by an operating system's kernel upon detecting an internal fatal error in which either it is unable to safely recover or continuing to run the system would have a higher risk of major data loss. The term is largely specific to Unix and Unix-like systems. The equivalent on Microsoft Windows operating systems is a stop error, often called a "blue screen of death".
NTLDR is the boot loader for all releases of Windows NT operating system from 1993 with the release of Windows NT 3.1 up until Windows XP and Windows Server 2003. From Windows Vista onwards it was replaced by the BOOTMGR bootloader. NTLDR is typically run from the primary storage device, but it can also run from portable storage devices such as a CD-ROM, USB flash drive, or floppy disk. NTLDR can also load a non NT-based operating system given the appropriate boot sector in a file.
In computing, a loadable kernel module (LKM) is an object file that contains code to extend the running kernel, or so-called base kernel, of an operating system. LKMs are typically used to add support for new hardware and/or filesystems, or for adding system calls. When the functionality provided by an LKM is no longer required, it can be unloaded in order to free memory and other resources.
udev is a device manager for the Linux kernel. As the successor of devfsd and hotplug, udev primarily manages device nodes in the /dev directory. At the same time, udev also handles all user space events raised when hardware devices are added into the system or removed from it, including firmware loading as required by certain devices.
The USB mass storage device class is a set of computing communications protocols, specifically a USB Device Class, defined by the USB Implementers Forum that makes a USB device accessible to a host computing device and enables file transfers between the host and the USB device. To a host, the USB device acts as an external hard drive; the protocol set interfaces with a number of storage devices.
The proc filesystem (procfs) is a special filesystem in Unix-like operating systems that presents information about processes and other system information in a hierarchical file-like structure, providing a more convenient and standardized method for dynamically accessing process data held in the kernel than traditional tracing methods or direct access to kernel memory. Typically, it is mapped to a mount point named /proc at boot time. The proc file system acts as an interface to internal data structures about running processes in the kernel. In Linux, it can also be used to obtain information about the kernel and to change certain kernel parameters at runtime (sysctl).
sysfs is a pseudo file system provided by the Linux kernel that exports information about various kernel subsystems, hardware devices, and associated device drivers from the kernel's device model to user space through virtual files. In addition to providing information about various devices and kernel subsystems, exported virtual files are also used for their configuration.
The SRM firmware is the boot firmware written by Digital Equipment Corporation (DEC) for computer systems based on the DEC Alpha microprocessor. SRM are the initials of (Alpha) System Reference Manual, the publication detailing the Alpha AXP architecture and which specified various features of the SRM firmware.
The magic SysRq key is a key combination understood by the Linux kernel, which allows the user to perform various low-level commands regardless of the system's state. It is often used to recover from freezes, or to reboot a computer without corrupting the filesystem. Its effect is similar to the computer's hardware reset button but with many more options and much more control.
In computing, ioctl is a system call for device-specific input/output operations and other operations which cannot be expressed by regular file semantics. It takes a parameter specifying a request code; the effect of a call depends completely on the request code. Request codes are often device-specific. For instance, a CD-ROM device driver which can instruct a physical device to eject a disc would provide an ioctl request code to do so. Device-independent request codes are sometimes used to give userspace access to kernel functions which are only used by core system software or still under development.
Das U-Boot is an open-source boot loader used in embedded devices to perform various low-level hardware initialization tasks and boot the device's operating system kernel. It is available for a number of computer architectures, including M68000, ARM, Blackfin, MicroBlaze, IBM S360, My66, MOS 6502, ARM64, MIPS, Nios, SuperH, PPC, RISC-V and x86.
lspci is a command on Unix-like operating systems that prints ("lists") detailed information about all PCI buses and devices in the system. It is based on a common portable library libpci which offers access to the PCI configuration space on a variety of operating systems.
The Linux booting process involves multiple stages and is in many ways similar to the BSD and other Unix-style boot processes, from which it derives. Although the Linux booting process depends very much on the computer architecture, those architectures share similar stages and software components, including system startup, bootloader execution, loading and startup of a Linux kernel image, and execution of various startup scripts and daemons. Those are grouped into 4 steps: system startup, bootloader stage, kernel stage, and init process. When a Linux system is powered up or reset, its processor will execute a specific firmware/program for system initialization, such as Power-on self-test, invoking the reset vector to start a program at a known address in flash/ROM, then load the bootloader into RAM for later execution. In personal computer (PC), not only limited to Linux-distro PC, this firmware/program is called BIOS, which is stored in the mainboard. In embedded Linux system, this firmware/program is called boot ROM. After being loaded into RAM, bootloader will execute to load the second-stage bootloader. The second-stage bootloader will load the kernel image into memory, decompress and initialize it then pass control to this kernel image. Second-stage bootloader also performs several operation on the system such as system hardware check, mounting the root device, loading the necessary kernel modules, etc. Finally, the first user-space process starts, and other high-level system initializations are performed.
In Unix-like operating systems, a device file, device node, or special file is an interface to a device driver that appears in a file system as if it were an ordinary file. There are also special files in DOS, OS/2, and Windows. These special files allow an application program to interact with a device by using its device driver via standard input/output system calls. Using standard system calls simplifies many programming tasks, and leads to consistent user-space I/O mechanisms regardless of device features and functions.
The Linux console is a system console internal to the Linux kernel. A system console is the device which receives all kernel messages and warnings and which allows logins in single user mode. The Linux console provides a way for the kernel and other processes to send text output to the user, and to receive text input from the user. The user typically enters text with a computer keyboard and reads the output text on a computer monitor. The Linux kernel supports virtual consoles – consoles that are logically separate, but which access the same physical keyboard and display. The Linux console are implemented by the VT subsystem of the Linux kernel, and do not rely on any user space software. This is in contrast to a terminal emulator, which is a user space process that emulates a terminal, and is typically used in a graphical display environment.
In Linux, and other Unix-like operating systems, the /boot/ directory holds files used in booting the operating system. The usage is standardized in the Filesystem Hierarchy Standard.
printk is a C function from the Linux kernel interface that prints messages to the kernel log. It accepts a string parameter called the format string, which specifies a method for rendering an arbitrary number of varied data type parameter(s) into a string. The string is then printed to the kernel log.
