Tutorials Point

  Unix for Beginners
  Unix Shell Programming
  Advanced Unix
  Unix Useful References
  Unix Useful Resources
  Selected Reading

Copyright © 2014 by tutorialspoint

  Home     References     Discussion Forums     About TP  

crash - Unix, Linux Command

previous next AddThis Social Bookmark Button



crash - Analyze Linux crash data or a live system


crash [ -h [ opt ] ] [ -v ] [ -s ] [ -i file ] [ -d num ] [ -S ] [ mapfile ] [ namelist ] [ dumpfile ]


Crash is a tool for interactively analyzing the state of the Linux system while it is running, or after a kernel crash has occurred and a core dump has been created by the Red Hat netdump, diskdump, kdump, or xendump facilities. It is loosely based on the SVR4 UNIX crash command, but has been significantly enhanced by completely merging it with the gdb debugger. The marriage of the two effectively combines the kernel-specific nature of the traditional UNIX crash utility with the source code level debugging capabilities of gdb.

The current set of commands consist of common kernel core analysis tools such as kernel stack back traces of all processes, source code disassembly, formatted kernel structure and variable displays, virtual memory data, dumps of linked-lists, etc., along with several commands that delve deeper into specific kernel subsystems. Appropriate gdb commands may also be entered, which in turn are passed on to the gdb module for execution.

The crash utility is designed to be independent of Linux version dependencies. When new kernel source code impacts the correct functionality of crash and its command set, the utility will be updated to recognize new kernel code changes, while maintaining backwards compatibility with earlier releases.


-h opt Crash displays a help message. If the optional opt argument is a crash command name, the help page for that command is displayed. If it is the string "input", a page describing the various crash command line input options is displayed. If it is the string "output", a page describing command line output options is displayed.
-v Crash displays the versions of the original gdb and crash libraries that make up the crash executable.
-s Crash does not display any version, GPL, or crash initialization data during startup. It proceeds directly to the "crash>" prompt.
-i file Crash reads and executes the crash command(s) contained in file before accepting any user input.
-d num Crash sets its internal debug level. The higher the number, the more debugging data will be printed while crash runs.
-S Crash uses "/boot/System.map" as the mapfile.
  This is a pathname to an uncompressed kernel image (a vmlinux file) that has been compiled with the "-g" option, or that has an accessible, associated, debuginfo file. If the dumpfile argument is entered, then this argument must also be used. If the namelist argument is not entered and no dumpfile argument is entered, crash will search in several typical directories for a kernel namelist that matches the live system.
  If the live system kernel, or the kernel from which the dumpfile was derived, was not compiled with the -g switch, then the additional mapfile argument is required. It may be either the associated System.map file, or the non-debug kernel namelist. However, if the mapfile argument is used, then the namelist argument must be a kernel namelist of a similar kernel version that was built with the -g switch.
  This is a pathname to a kernel memory core dump file. If the dumpfile argument is not entered, the session will be invoked on the live system using /dev/mem, which usually requires root privileges.


Each crash command generally falls into one of the following categories:
Symbolic display
  Displays of kernel text/data, which take full advantage of the power of gdb to format and display data structures symbolically.
System state
  The majority of crash commands come consist of a set of "kernel-aware" commands, which delve into various kernel subsystems on a system-wide or per-task basis.
Utility functions
  A set of useful helper commands serving various purposes, some simple, others quite powerful.
Session control
  Commands that control the crash session itself.
The following alphabetical list consists of a very simple overview of each crash command. However, since individual commands often have several options resulting in significantly different output, it is suggested that the full description of each command be viewed by entering the command crash -h command, or during a crash session by simply entering help command.
* "pointer to" is shorthand for either the struct or union commands. It displays the contents of a kernel structure or union.
alias creates a single-word alias for a command.
ascii displays an ascii chart or translates a numeric value into its ascii components.
bt displays a task’s kernel-stack backtrace. If it is given the -a option, it displays the stack traces of the active tasks on all CPUs. It is often used with the foreach command to display the backtraces of all tasks with one command.
dev displays data concerning the character and block device assignments, I/O port usage, I/O memory usage, and PCI device data.
dis disassembles memory, either entire kernel functions, from a location for a specified number of instructions, or from the start of a function up to a specified memory location.
eval evalues an expression or numeric type and displays the result in hexadecimal, decimal, octal and binary.
exit causes crash to exit.
extend dynamically loads or unloads crash extension shared object libraries.
files displays information about open files in a context.
  repeats a specified command for the specified (or all) tasks in the system.
fuser displays the tasks using the specified file or socket.
gdb passes its argument to the underlying gdb program. It is useful for executing GDB commands that have the same name as crash commands.
help alone displays the command menu; if followed by a command name, a full description of a command, its options, and examples are displayed. Its output is far more complete and useful than this man page.
irq displays data concerning interrupt request numbers and bottom-half interrupt handling.
kmem displays information about the use of kernel memory.
list displays the contents of a linked list.
log displays the kernel log_buf contents in chronological order.
mach displays data specific to the machine type.
mod displays information about the currently installed kernel modules, or adds or deletes symbolic or debugging information about specified kernel modules.
mount displays information about the currently-mounted filesystems.
net display various network related data.
p passes its arguments to the gdb "print" command for evaluation and display.
ps displays process status for specified, or all, processes in the system.
pte translates the hexadecimal contents of a PTE into its physical page address and page bit settings.
ptob translates a page frame number to its byte value.
ptov translates a hexadecimal physical address into a kernel virtual address.
q is an alias for the "exit" command.
rd displays the contents of memory, with the output formatted in several different manners.
repeat repeats a command indefinitely, optionally delaying a given number of seconds between each command execution.
runq displays the tasks on the run queue.
set either sets a new context, or gets the current context for display.
sig displays signal-handling data of one or more tasks.
struct displays either a structure definition or the contents of a kernel structure at a specified address.
swap displays information about each configured swap device.
sym translates a symbol to its virtual address, or a static kernel virtual address to its symbol -- or to a symbol-plus-offset value, if appropriate.
sys displays system-specific data.
task displays the contents of a task_struct.
timer displays the timer queue entries, both old- and new-style, in chronological order.
union is similar to the struct command, except that it works on kernel unions.
vm displays basic virtual memory information of a context.
vtop translates a user or kernel virtual address to its physical address.
waitq walks the wait queue list displaying the tasks which are blocked on the specified wait queue.
whatis displays the definition of structures, unions, typedefs or text/data symbols.
wr modifies the contents of memory. When writing to memory on a live system, this command should obviously be used with great care.


  Initialization commands. The file can be located in the user’s HOME directory and/or the current directory. Commands found in the .crashrc file in the HOME directory are executed before those in the current directory’s .crashrc file.


EDITOR Command input is read using readline(3). If EDITOR is set to emacs or vi then suitable keybindings are used. If EDITOR is not set, then vi is used. This can be overridden by set vi or set emacs commands located in a .crashrc file, or by entering -e emacs on the crash command line.
  If CRASHPAGER is set, its value is used as the name of the program to which command output will be sent. If not, then command output is sent to /usr/bin/less -E -X by default.


If crash does not work, look for a newer version: kernel evolution frequently makes crash updates necessary.

The command set scroll off will cause output to be sent directly to the terminal rather than through a paging program. This is useful, for example, if you are running crash in a window of emacs.


Dave Anderson <anderson@redhat.com> wrote crash
Jay Fenlason <fenlason@redhat.com> wrote this man page.


The help command within crash provides more complete and accurate documentation than this man page.

http://people.redhat.com/anderson - the home page of the crash utility.

netdump(8), gdb(1)

previous next Printer Friendly