Linux - Directory /proc/sys/kernel/


This directory contains configuration files that directly affect the operation of the Kernel

Most important kernel files

  • acct — Controls the suspension of process accounting based on the percentage of free space available on the file system containing the log. By default, the file looks like this:
4	2	30

The second value sets the threshold percentage of free space when logging will be suspended, while the first value dictates the percentage of free space required for logging to resume. The third value sets the interval, in seconds, that the kernel polls the file system to see if logging should be suspended or resumed.

  • cap-bound — Controls the capability bounding settings, which provides a list of capabilities for any process on the system. If a capability is not listed here, then no process, no matter how privileged, can do it. The idea is to make the system more secure by ensuring that certain things cannot happen, at least beyond a certain point in the boot process.
  • ctrl-alt-del — Controls whether [Ctrl]-[Alt]-[Delete] will gracefully restart the computer using init (value 0) or force an immediate reboot without syncing the dirty buffers to disk (value 1).
  • domainname — Configures the system domain name, such as
  • hostname — Configures the system hostname, such as
  • hotplug — Configures the utility to be used when a configuration change is detected by the system. This is primarily used with USB and Cardbus PCI. The default value of /sbin/hotplug should not be changed unless you are testing a new program to fulfill this role.
  • modprobe — Sets the location of the program to be used to load kernel modules when necessary. The default value of /sbin/modprobe signifies that kmod will call it to actually load the module when a kernel thread calls kmod.
  • msgmax — Sets the maximum size of any message sent from one process to another and is set to 8192 bytes by default. You should be careful about raising this value, as queued messages between processes are stored in non-swappable kernel memory. Any increase in msgmax would increase RAM requirements for the system.
  • msgmnb — Sets the maximum number of bytes in a single message queue. The default is 16384.
  • msgmni — Sets the maximum number of message queue identifiers. The default is 16.
  • osrelease — Lists the Linux kernel release number. This file can only be altered by changing the kernel source and recompiling.
  • ostype — Displays the type of operating system. By default, this file is set to Linux, and this value can only be changed by changing the kernel source and recompiling.
  • overflowgid and overflowuid — Defines the fixed group ID and user ID, respectively, for use with system calls on architectures that only support 16-bit group and user IDs.
  • panic — Defines the number of seconds the kernel will postpone rebooting when the system experiences a kernel panic. By default, the value is set to 0, which disables automatic rebooting after a panic.
  • printk — This file controls a variety of settings related to printing or logging error messages. Each error message reported by the kernel has a loglevel associated with it that defines the importance of the message. The loglevel values break down in this order:
    • 0 — Kernel emergency. The system is unusable.
    • 1 — Kernel alert. Action must be taken immediately.
    • 2 — Condition of the kernel is considered critical.
    • 3 — General kernel error condition.
    • 4 — General kernel warning condition.
    • 5 — Kernel notice of a normal but significant condition.
    • 6 — Kernel informational message.
    • 7 — Kernel debug-level messages.

Four values are found in the printk file:

6	4	1	7

Each of these values defines a different rule for dealing with error messages. The first value, called the console loglevel, defines the lowest priority of messages that will be printed to the console. (Note that, the lower the priority, the higher the loglevel number.) The second value sets the default loglevel for messages without an explicit loglevel attached to them. The third value sets the lowest possible loglevel configuration for the console loglevel. The last value sets the default value for the console loglevel.

  • rtsig-max — Configures the maximum number of POSIX realtime signals that the system may have queued at any one time. The default value is 1024.
  • rtsig-nr — The current number of POSIX realtime signals queued by the kernel.
  • sem — Configures semaphore settings within the kernel. A semaphore is a System V IPC object that is used to control utilization of a particular process.
  • shmall — Sets the total amount of shared memory that can be used at one time on the system, in bytes. By default, this value is 2097152.
  • shmmax — Sets the largest shared memory segment size allowed by the kernel, in bytes. By default, this value is 33554432. However, the kernel supports much larger values than this.
  • shmmni — Sets the maximum number of shared memory segments for the whole system, in bytes. By default, this value is 4096
  • sysrq — Activates the System Request Key, if this value is set to anything other than the default of 0.
  • threads-max — Sets the maximum number of threads to be used by the kernel, with a default value of 2048.
  • version — Displays the date and time the kernel was last compiled. The first field in this file, such as #3, relates to the number of times a kernel was built from the source base.

The random directory stores a number of values related to generating random numbers for the kernel.

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