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Manuál Linux
[Linux manuál]

proc: zpracovat informační pseudosouborový systém

Originální popis anglicky: proc - process information pseudo-filesystem


The proc filesystem is a pseudo-filesystem which is used as an interface to kernel data structures. It is commonly mounted at /proc. Most of it is read-only, but some files allow kernel variables to be changed.
The following outline gives a quick tour through the /proc hierarchy.
There is a numerical subdirectory for each running process; the subdirectory is named by the process ID. Each such subdirectory contains the following pseudo-files and directories.
This holds the complete command line for the process, unless the whole process has been swapped out or the process is a zombie. In either of these latter cases, there is nothing in this file: i.e. a read on this file will return 0 characters. The command line arguments appear in this file as a set of null-separated strings, with a further null byte after the last string.
This is a link to the current working directory of the process. To find out the cwd of process 20, for instance, you can do this:
cd /proc/20/cwd; /bin/pwd
Note that the pwd command is often a shell builtin, and might not work properly. In bash, you may use pwd -P.
This file contains the environment for the process. The entries are separated by null characters, and there may be a null character at the end. Thus, to print out the environment of process 1, you would do:
(cat /proc/1/environ; echo) | tr "\000" "\n"
(For a reason why one should want to do this, see lilo(8).)
Under Linux 2.2 and later, this file is a symbolic link containing the actual path name of the executed command. This symbolic link can be dereferenced normally - attempting to open it will open the executable. You can even type /proc/[number]/exe to run another copy of the same process as [number].
Under Linux 2.0 and earlier /proc/[number]/exe is a pointer to the binary which was executed, and appears as a symbolic link. A readlink(2) call on this file under Linux 2.0 returns a string in the format:
For example, [0301]:1502 would be inode 1502 on device major 03 (IDE, MFM, etc. drives) minor 01 (first partition on the first drive).
find(1) with the -inum option can be used to locate the file.
This is a subdirectory containing one entry for each file which the process has open, named by its file descriptor, and which is a symbolic link to the actual file. Thus, 0 is standard input, 1 standard output, 2 standard error, etc.
Programs that will take a filename, but will not take the standard input, and which write to a file, but will not send their output to standard output, can be effectively foiled this way, assuming that -i is the flag designating an input file and -o is the flag designating an output file:
foobar -i /proc/self/fd/0 -o /proc/self/fd/1 ...
and you have a working filter.
/proc/self/fd/N is approximately the same as /dev/fd/N in some UNIX and UNIX-like systems. Most Linux MAKEDEV scripts symbolically link /dev/fd to /proc/self/fd, in fact.
A file containing the currently mapped memory regions and their access permissions.
The format is:

address perms offset dev inode pathname 08048000-08056000 r-xp 00000000 03:0c 64593 /usr/sbin/gpm 08056000-08058000 rw-p 0000d000 03:0c 64593 /usr/sbin/gpm 08058000-0805b000 rwxp 00000000 00:00 0 40000000-40013000 r-xp 00000000 03:0c 4165 /lib/ld-2.2.4.so 40013000-40015000 rw-p 00012000 03:0c 4165 /lib/ld-2.2.4.so 4001f000-40135000 r-xp 00000000 03:0c 45494 /lib/libc-2.2.4.so 40135000-4013e000 rw-p 00115000 03:0c 45494 /lib/libc-2.2.4.so 4013e000-40142000 rw-p 00000000 00:00 0 bffff000-c0000000 rwxp 00000000 00:00 0

where address is the address space in the process that it occupies, perms is a set of permissions:

r = read w = write x = execute s = shared p = private (copy on write)

offset is the offset into the file/whatever, dev is the device (major:minor), and inode is the inode on that device. 0 indicates that no inode is associated with the memory region, as the case would be with bss.
Under Linux 2.0 there is no field giving pathname.
This file can be used to access the pages of a process's memory through open(2), read(2), and fseek(3).
Unix and Linux support the idea of a per-process root of the filesystem, set by the chroot(2) system call. This file is a symbolic link that points to the process's root directory, and behaves as exe, fd/*, etc. do.
Status information about the process. This is used by ps(1). It is defined in /usr/src/linux/fs/proc/array.c.
The fields, in order, with their proper scanf(3) format specifiers, are:
pid %d
The process id.
comm %s
The filename of the executable, in parentheses. This is visible whether or not the executable is swapped out.
state %c
One character from the string "RSDZTW" where R is running, S is sleeping in an interruptible wait, D is waiting in uninterruptible disk sleep, Z is zombie, T is traced or stopped (on a signal), and W is paging.
ppid %d
The PID of the parent.
pgrp %d
The process group ID of the process.
session %d
The session ID of the process.
tty_nr %d
The tty the process uses.
tpgid %d
The process group ID of the process which currently owns the tty that the process is connected to.
flags %lu
The kernel flags word of the process. For bit meanings, see the PF_* defines in <linux/sched.h>. Details depend on the kernel version.
minflt %lu
The number of minor faults the process has made which have not required loading a memory page from disk.
cminflt %lu
The number of minor faults that the process's waited-for children have made.
majflt %lu
The number of major faults the process has made which have required loading a memory page from disk.
cmajflt %lu
The number of major faults that the process's waited-for children have made.
utime %lu
The number of jiffies that this process has been scheduled in user mode.
stime %lu
The number of jiffies that this process has been scheduled in kernel mode.
cutime %ld
The number of jiffies that this process's waited-for children have been scheduled in user mode. (See also times(2).)
cstime %ld
The number of jiffies that this process's waited-for children have been scheduled in kernel mode.
priority %ld
The standard nice value, plus fifteen. The value is never negative in the kernel.
nice %ld
The nice value ranges from 19 (nicest) to -19 (not nice to others).
0 %ld
This value is hard coded to 0 as a placeholder for a removed field.
itrealvalue %ld
The time in jiffies before the next SIGALRM is sent to the process due to an interval timer.
starttime %lu
The time in jiffies the process started after system boot.
vsize %lu
Virtual memory size in bytes.
rss %ld
Resident Set Size: number of pages the process has in real memory, minus 3 for administrative purposes. This is just the pages which count towards text, data, or stack space. This does not include pages which have not been demand-loaded in, or which are swapped out.
rlim %lu
Current limit in bytes on the rss of the process (usually 4294967295 on i386).
startcode %lu
The address above which program text can run.
endcode %lu
The address below which program text can run.
startstack %lu
The address of the start of the stack.
kstkesp %lu
The current value of esp (stack pointer), as found in the kernel stack page for the process.
kstkeip %lu
The current EIP (instruction pointer).
signal %lu
The bitmap of pending signals (usually 0).
blocked %lu
The bitmap of blocked signals (usually 0, 2 for shells).
sigignore %lu
The bitmap of ignored signals.
sigcatch %lu
The bitmap of catched signals.
wchan %lu
This is the "channel" in which the process is waiting. It is the address of a system call, and can be looked up in a namelist if you need a textual name. (If you have an up-to-date /etc/psdatabase, then try ps -l to see the WCHAN field in action.)
nswap %lu
Number of pages swapped - not maintained.
cnswap %lu
Cumulative nswap for child processes.
exit_signal %d
Signal to be sent to parent when we die.
processor %d
CPU number last executed on.
Provides information about memory status in pages. The columns are:
size total program size
resident resident set size
share shared pages
trs text (code)
drs data/stack
lrs library
dt dirty pages
Provides much of the information in /proc/[number]/stat and /proc/[number]/statm in a format that's easier for humans to parse.
Advanced power management version and battery information when CONFIG_APM is defined at kernel compilation time.
Contains subdirectories for installed busses.
Subdirectory for pcmcia devices when CONFIG_PCMCIA is set at kernel compilation time.
Contains various bus subdirectories and pseudo-files containing information about pci busses, installed devices, and device drivers. Some of these files are not ASCII.
Information about pci devices. They may be accessed through lspci(8) and setpci(8).
Arguments passed to the Linux kernel at boot time. Often done via a boot manager such as lilo(1).
This is a collection of CPU and system architecture dependent items, for each supported architecture a different list. Two common entries are processor which gives CPU number and bogomips; a system constant that is calculated during kernel initialization. SMP machines have information for each CPU.
Text listing of major numbers and device groups. This can be used by MAKEDEV scripts for consistency with the kernel.
/proc/diskstats (since Linux 2.5.69)
This file contains disk I/O statistics for each disk device. See the kernel source file Documentation/iostats.txt for further information.
This is a list of the registered ISA DMA (direct memory access) channels in use.
Empty subdirectory.
List of the execution domains (ABI personalities).
Frame buffer information when CONFIG_FB is defined during kernel compilation.
A text listing of the filesystems which were compiled into the kernel. Incidentally, this is used by mount(1) to cycle through different filesystems when none is specified.
Empty subdirectory.
This directory exists on systems with the ide bus. There are directories for each ide channel and attached device. Files include:
cache              buffer size in KB
capacity           number of sectors
driver             driver version
geometry           physical and logical geometry
identify           in hexidecimal
media              media type
model              manufacturer's model number
settings           drive settings
smart_thresholds   in hexidecimal
smart_values       in hexidecimal
The hdparm(8) utility provides access to this information in a friendly format.
This is used to record the number of interrupts per each IRQ on (at least) the i386 architechure. Very easy to read formatting, done in ASCII.
I/O memory map in Linux 2.4.
This is a list of currently registered Input-Output port regions that are in use.
/proc/kallsyms (since Linux 2.5.71)
This holds the kernel exported symbol definitions used by the modules(X) tools to dynamically link and bind loadable modules. In Linux 2.5.47 and earlier, a similar file with slightly different syntax was named ksyms.
This file represents the physical memory of the system and is stored in the ELF core file format. With this pseudo-file, and an unstripped kernel (/usr/src/linux/vmlinux) binary, GDB can be used to examine the current state of any kernel data structures.
The total length of the file is the size of physical memory (RAM) plus 4KB.
This file can be used instead of the syslog(2) system call to read kernel messages. A process must have superuser privileges to read this file, and only one process should read this file. This file should not be read if a syslog process is running which uses the syslog(2) system call facility to log kernel messages.
Information in this file is retrieved with the dmesg(8) program.
/proc/ksyms (Linux 1.1.23-2.5.47)
See /proc/kallsyms.
The load average numbers give the number of jobs in the run queue (state R) or waiting for disk I/O (state D) averaged over 1, 5, and 15 minutes. They are the same as the load average numbers given by uptime(1) and other programs.
This file shows current file locks (flock(2) and fcntl(2)) and leases (fcntl(2)).
This file is only present if CONFIGDEBUGMALLOC was defined during compilation.
This is used by free(1) to report the amount of free and used memory (both physical and swap) on the system as well as the shared memory and buffers used by the kernel.
It is in the same format as free(1), except in bytes rather than KB.
This is a list of all the file systems currently mounted on the system. The format of this file is documented in fstab(5).
A text list of the modules that have been loaded by the system. See also lsmod(8).
Memory Type Range Registers. See /usr/src/linux/Documentation/mtrr.txt for details.
various net pseudo-files, all of which give the status of some part of the networking layer. These files contain ASCII structures and are, therefore, readable with cat. However, the standard netstat(8) suite provides much cleaner access to these files.
This holds an ASCII readable dump of the kernel ARP table used for address resolutions. It will show both dynamically learned and pre-programmed ARP entries. The format is:

IP address HW type Flags HW address Mask Device 0x1 0x2 00:50:BF:25:68:F3 * eth0 0x1 0xc 00:00:00:00:00:00 * eth0

Here 'IP address' is the IPv4 address of the machine and the 'HW type' is the hardware type of the address from RFC 826. The flags are the internal flags of the ARP structure (as defined in /usr/include/linux/if_arp.h) and the 'HW address' is the data link layer mapping for that IP address if it is known.
The dev pseudo-file contains network device status information. This gives the number of received and sent packets, the number of errors and collisions and other basic statistics. These are used by the ifconfig(8) program to report device status. The format is:

Inter-| Receive | Transmit face |bytes packets errs drop fifo frame compressed multicast|bytes packets errs drop fifo colls carrier compressed lo: 2776770 11307 0 0 0 0 0 0 2776770 11307 0 0 0 0 0 0 eth0: 1215645 2751 0 0 0 0 0 0 1782404 4324 0 0 0 427 0 0 ppp0: 1622270 5552 1 0 0 0 0 0 354130 5669 0 0 0 0 0 0 tap0: 7714 81 0 0 0 0 0 0 7714 81 0 0 0 0 0 0
Defined in /usr/src/linux/net/core/dev_mcast.c:

indx ifterface_name dmi_u dmi_g dmi_address 2 eth0 1 0 01005e000001 3 eth1 1 0 01005e000001 4 eth2 1 0 01005e000001
Internet Group Management Protocol. Defined in /usr/src/linux/net/core/igmp.c.
This file uses the same format as the arp file and contains the current reverse mapping database used to provide rarp(8) reverse address lookup services. If RARP is not configured into the kernel, this file will not be present.
Holds a dump of the RAW socket table. Much of the information is not of use apart from debugging. The 'sl' value is the kernel hash slot for the socket, the 'local address' is the local address and protocol number pair."St" is the internal status of the socket. The "tx_queue" and "rx_queue" are the outgoing and incoming data queue in terms of kernel memory usage. The "tr", "tm->when", and "rexmits" fields are not used by RAW. The uid field holds the creator euid of the socket.
This file holds the ASCII data needed for the IP, ICMP, TCP, and UDP management information bases for an snmp agent.
Holds a dump of the TCP socket table. Much of the information is not of use apart from debugging. The "sl" value is the kernel hash slot for the socket, the "local address" is the local address and port number pair. The "remote address" is the remote address and port number pair (if connected). 'St' is the internal status of the socket. The 'tx_queue' and 'rx_queue' are the outgoing and incoming data queue in terms of kernel memory usage. The "tr", "tm->when", and "rexmits" fields hold internal information of the kernel socket state and are only useful for debugging. The uid field holds the creator euid of the socket.
Holds a dump of the UDP socket table. Much of the information is not of use apart from debugging. The "sl" value is the kernel hash slot for the socket, the "local address" is the local address and port number pair. The "remote address" is the remote address and port number pair (if connected). "St" is the internal status of the socket. The "tx_queue" and "rx_queue" are the outgoing and incoming data queue in terms of kernel memory usage. The "tr", "tm->when", and "rexmits" fields are not used by UDP. The uid field holds the creator euid of the socket. The format is:

sl local_address rem_address st tx_queue rx_queue tr rexmits tm->when uid 1: 01642C89:0201 0C642C89:03FF 01 00000000:00000001 01:000071BA 00000000 0 1: 00000000:0801 00000000:0000 0A 00000000:00000000 00:00000000 6F000100 0 1: 00000000:0201 00000000:0000 0A 00000000:00000000 00:00000000 00000000 0
Lists the UNIX domain sockets present within the system and their status. The format is:
Num RefCount Protocol Flags Type St Path 0: 00000002 00000000 00000000 0001 03 1: 00000001 00000000 00010000 0001 01 /dev/printer
Here 'Num' is the kernel table slot number, 'RefCount' is the number of users of the socket, 'Protocol' is currently always 0, 'Flags' represent the internal kernel flags holding the status of the socket. Currently, type is always '1' (Unix domain datagram sockets are not yet supported in the kernel). 'St' is the internal state of the socket and Path is the bound path (if any) of the socket.
Contains major and minor numbers of each partition as well as number of blocks and partition name.
This is a listing of all PCI devices found during kernel initialization and their configuration.
A directory with the scsi midlevel pseudo-file and various SCSI lowlevel driver directories, which contain a file for each SCSI host in this system, all of which give the status of some part of the SCSI IO subsystem. These files contain ASCII structures and are, therefore, readable with cat.
You can also write to some of the files to reconfigure the subsystem or switch certain features on or off.
This is a listing of all SCSI devices known to the kernel. The listing is similar to the one seen during bootup. scsi currently supports only the add-single-device command which allows root to add a hotplugged device to the list of known devices.
An echo 'scsi add-single-device 1 0 5 0' > /proc/scsi/scsi will cause host scsi1 to scan on SCSI channel 0 for a device on ID 5 LUN 0. If there is already a device known on this address or the address is invalid, an error will be returned.
[drivername] can currently be NCR53c7xx, aha152x, aha1542, aha1740, aic7xxx, buslogic, eata_dma, eata_pio, fdomain, in2000, pas16, qlogic, scsi_debug, seagate, t128, u15-24f, ultrastore, or wd7000. These directories show up for all drivers that registered at least one SCSI HBA. Every directory contains one file per registered host. Every host-file is named after the number the host was assigned during initialization.
Reading these files will usually show driver and host configuration, statistics etc.
Writing to these files allows different things on different hosts. For example, with the latency and nolatency commands, root can switch on and off command latency measurement code in the eata_dma driver. With the lockup and unlock commands, root can control bus lockups simulated by the scsi_debug driver.
This directory refers to the process accessing the /proc filesystem, and is identical to the /proc directory named by the process ID of the same process.
Information about kernel caches. The columns are:
See slabinfo(5) for details.
kernel/system statistics. Varies with architecture. Common entries include:
cpu 3357 0 4313 1362393
The number of jiffies (1/100ths of a second) that the system spent in user mode, user mode with low priority (nice), system mode, and the idle task, respectively. The last value should be 100 times the second entry in the uptime pseudo-file.
page 5741 1808
The number of pages the system paged in and the number that were paged out (from disk).
swap 1 0
The number of swap pages that have been brought in and out.
intr 1462898
The number of interrupts received from the system boot.
disk_io: (2,0):(31,30,5764,1,2) (3,0):...
(major,minor):(noinfo, read_io_ops, blks_read, write_io_ops, blks_written)
ctxt 115315
The number of context switches that the system underwent.
btime 769041601
boot time, in seconds since the epoch (January 1, 1970).
processes 86031
Number of forks since boot.
Swap areas in use. See also swapon(8).
This directory (present since 1.3.57) contains a number of files and subdirectories corresponding to kernel variables. These variables can be read and sometimes modified using the proc file system, and the sysctl(2) system call. Presently, there are subdirectories abi, debug, dev, fs, kernel, net, proc, rxrpc, sunrpc and vm that each contain more files and subdirectories.
This directory may contain files with application binary information. On some systems, it is not present.
This directory may be empty.
This directory contains device specific information (eg dev/cdrom/info). On some systems, it may be empty.
This contains the subdirectory binfmt_misc and files dentry-state, dir-notify-enable, dquot-nr, file-max, file-nr, inode-max, inode-nr, inode-state, lease-break-time, leases-enable, overflowgid, overflowuid super-max and super-nr with function fairly clear from the name.
Documentation for files in this directory can in the kernel sources in Documentation/binfmt_misc.txt.
This file contains six numbers, nr_dentry, nr_unused, age_limit (age in seconds), want_pages (pages requested by system) and two dummy values. nr_dentry seems to be 0 all the time. nr_unused seems to be the number of unused dentries. age_limit is the age in seconds after which dcache entries can be reclaimed when memory is short and want_pages is nonzero when the kernel has called shrink_dcache_pages() and the dcache isn't pruned yet.
This file can be used to disable or enable the dnotify interface described in fcntl(2) on a system-wide basis. A value of 0 in this file disables the interface, and a value of 1 enables it.
This file shows the maximum number of cached disk quota entries. On some (2.4) systems, it is not present. If the number of free cached disk quota entries is very low and you have some awesome number of simultaneous system users, you might want to raise the limit.
This file shows the number of allocated disk quota entries and the number of free disk quota entries.
This file defines a system-wide limit on the number of open files for all processes. (See also setrlimit(2), which can be used by a process to set the per-process limit, RLIMIT_NOFILE, on the number of files it may open.) If you get lots of error messages about running out of file handles, try increasing this value:
echo 100000 > /proc/sys/fs/file-max
The kernel constant NR_OPEN imposes an upper limit on the value that may be placed in file-max.
If you increase /proc/sys/fs/file-max, be sure to increase /proc/sys/fs/inode-max to 3-4 times the new value of /proc/sys/fs/file-max, or you will run out of inodes.
This (read-only) file gives the number of files presently opened. It contains three numbers: The number of allocated file handles, the number of free file handles and the maximum number of file handles. The kernel allocates file handles dynamically, but it doesn't free them again. If the number of allocated files is close to the
maximum, you should consider increasing the maximum. When the number of free file handles is large, you've encountered a peak in your usage of file handles and you probably don't need to increase the maximum.
This file contains the maximum number of in-memory inodes. On some (2.4) systems, it may not be present. This value should be 3-4 times larger than the value in file-max, since stdin, stdout and network sockets also need an inode to handle them. When you regularly run out of inodes, you need to increase this value.
This file contains the first two values from inode-state.
This file contains seven numbers: nr_inodes, nr_free_inodes, preshrink and four dummy values. nr_inodes is the number of inodes the system has allocated. This can be slightly more than inode-max because Linux allocates them one pageful at a time. nr_free_inodes represents the number of free inodes. preshrink is nonzero when the nr_inodes > inode-max and the system needs to prune the inode list instead of allocating more.
This file specifies the grace period that the kernel grants to a process holding a file lease (fcntl(2)) after it has sent a signal to that process notifying it that another process is waiting to open the file. If the lease holder does not remove or downgrade the lease within this grace period, the kernel forcibly breaks the lease.
This file can be used to enable or disable file leases (fcntl(2)) on a system-wide basis. If this file contains the value 0, leases are disabled. A non-zero value enables leases.
/proc/sys/fs/overflowgid and /proc/sys/fs/overflowuid
These files allow you to change the value of the fixed UID and GID. The default is 65534. Some filesystems only support 16-bit UIDs and GIDs, although in Linux UIDs and GIDs are 32 bits. When one of these filesystems is mounted with writes enabled, any UID or GID that would exceed 65535 is translated to the overflow value before being written to disk.
This file controls the maximum number of superblocks, and thus the maximum number of mounted filesystems the kernel can have. You only need to increase super-max if you need to mount more filesystems than the current value in super-max allows you to.
This file contains the number of filesystems currently mounted.
This directory contains files acct, cad_pid, cap-bound, core_pattern, core_uses_pid, ctrl-alt-del, dentry-state, domainname, hotplug, hostname, htab-reclaim (PowerPC only), java-appletviewer (binfmt_java, obsolete), java-interpreter (binfmt_java, obsolete), l2cr (PowerPC only), modprobe, msgmax, msgmnb, msgmni, osrelease, ostype, overflowgid, overflowuid, panic, panic_on_oops, pid_max, powersave-nap (PowerPC only), printk, pty, random, real-root-dev, reboot-cmd (SPARC only), rtsig-max, rtsig-nr, sem, sg-big-buff, shmall, shmmax, shmmni, sysrq, tainted, threads-max, version and zero-paged (PowerPC only) with function fairly clear from the name.
This file contains three numbers: highwater, lowwater and frequency. If BSD-style process accounting is enabled these values control its behaviour. If free space on filesystem where the log lives goes below lowwater percent accounting suspends. If free space gets above highwater percent accounting resumes. Frequency determines how often the kernel checks the amount of free space (value is in seconds). Default values are 4, 2 and 30. That is, suspend accounting if <= 2% of space is free; resume it if >= 4% of space is free; consider information about amount of free space valid for 30 seconds.
This file holds the value of the kernel capability bounding set (expressed as a signed decimal number). This set is ANDed against the capabilities permitted to a process during exec.
This file (new in Linux 2.5) provides finer control over the form of a core filename than the obsolete /proc/sys/kernel/core_uses_pid file described below. The name for a core file is controlled by defining a template in /proc/sys/kernel/core_pattern. The template can contain % specifiers which are substituted by the following values when a core file is created:
  %%  A single % character
  %p  PID of dumped process
  %u  real UID of dumped process
  %g  real GID of dumped process
  %s  number of signal causing dump
  %t  time of dump (secs since 0:00h, 1 Jan 1970)
  %h  hostname (same as the 'nodename' 
      returned by  uname(2))
  %e  executable filename
A single % at the end of the template is dropped from the core filename, as is the combination of a % followed by any character other than those listed above. All other characters in the template become a literal part of the core filename. The maximum size of the resulting core filename is 64 bytes. The default value in this file is "core". For backward compatibility, if /proc/sys/kernel/core_pattern does not include "%p" and /proc/sys/kernel/core_uses_pid is non-zero, then .PID will be appended to the core filename.
This file can be used control the naming of a core dump file on Linux 2.4. If this file contains the value 0, then a core dump file is simply named core. If this file contains a non-zero value, then the core dump file includes the process ID in a name of the form core.PID.
This file controls the handling of Ctrl-Alt-Del from the keyboard. When the value in this file is 0, Ctrl-Alt-Del is trapped and sent to the init(1) program to handle a graceful restart. When the value is > 0, Linux's reaction to a Vulcan Nerve Pinch (tm) will be an immediate reboot, without even syncing its dirty buffers. Note: when a program (like dosemu) has the keyboard in 'raw' mode, the ctrl-alt-del is intercepted by the program before it ever reaches the kernel tty layer, and it's up to the program to decide what to do with it.
This file contains the path for the hotplug policy agent. The default value in this file "/sbin/hotplug".
/proc/sys/kernel/domainname and /proc/sys/kernel/hostname
can be used to set the NIS/YP domainname and the hostname of your box in exactly the same way as the commands domainname and hostname, i.e.:
# echo "darkstar" > /proc/sys/kernel/hostname
# echo "mydomain" > /proc/sys/kernel/domainname
has the same effect as
# hostname "darkstar"
# domainname "mydomain"
Note, however, that the classic darkstar.frop.org has the hostname "darkstar" and DNS (Internet Domain Name Server) domainname "frop.org", not to be confused with the NIS (Network Information Service) or YP (Yellow Pages) domainname. These two domain names are in general different. For a detailed discussion see the hostname(1) man page.
(PowerPC only) If this file is set to a non-zero value, the PowerPC htab (see kernel file Documentation/powerpc/ppc_htab.txt) is pruned each time the system hits the idle loop.
(PowerPC only) This file contains a flag that controls the L2 cache of G3 processor boards. If 0, the cache is disabled. Enabled if nonzero.
This file is described by the kernel source file Documentation/kmod.txt.
This file defines a system-wide limit specifying the maximum number of bytes in a single message written on a System V message queue.
This file defines the system-wide limit on the number of message queue identifiers. (This file is only present in Linux 2.4 onwards.)
This file defines a system-wide paramter used to initialise the msg_qbytes setting for subsequenly created message queues. The msg_qbytes setting specifies the maximum number of bytes that may be written to the message queue.
/proc/sys/kernel/ostype and /proc/sys/kernel/osrelease
These files give substrings of /proc/version.
/proc/sys/kernel/overflowgid and /proc/sys/kernel/overflowuid
These files duplicate the files /proc/sys/fs/overflowgid and /proc/sys/fs/overflowuid.
gives read/write access to the kernel variable panic_timeout. If this is zero, the kernel will loop on a panic; if nonzero it indicates that the kernel should autoreboot after this number of seconds. When you use the software watchdog device driver, the recommended setting is 60.
This file (new in Linux 2.5) controls the kernel's behaviour when an oops or BUG is encountered. If this file contains 0, then the system tries to continue operation. If it contains 1, then the system delays a few seconds (to give klogd time to record the oops output) and then panics. If the /proc/sys/kernel/panic file is also non-zero then the machine will be rebooted.
This file (new in Linux 2.5) specifies the value at which PIDs wrap around (i.e., the value in this file is one greater than the maximum PID). The default value for this file, 32768, results in the same range of PIDs as on earlier kernels. The value in this file can be set to any value up to 2^22 (PID_MAX_LIMIT, approximately 4 million).
/proc/sys/kernel/powersave-nap (PowerPC only)
This file contains a flag. If set, Linux-PPC will use the 'nap' mode of powersaving, otherwise the 'doze' mode will be used.
The four values in this file are console_loglevel, default_message_loglevel, minimum_console_level and default_console_loglevel. These values influence printk() behavior when printing or logging error messages. See syslog(2) for more info on the different loglevels. Messages with a higher priority than console_loglevel will be printed to the console. Messages without an explicit priority will be printed with priority default_message_level. minimum_console_loglevel is the minimum (highest) value to which console_loglevel can be set. default_console_loglevel is the default value for console_loglevel.
/proc/sys/kernel/pty (since Linux 2.6.4)
This directory contains two files relating to the number of Unix 98 pseudo-terminals (see pts(4)) on the system.
This file defines the maximum number of pseudo-terminals.
This read-only file indicates how many pseudo-terminals are currently in use.
This directory contains various parameters controlling the operation of the file /dev/random.
This file is documented in the kernel source file Documentation/initrd.txt.
/proc/sys/kernel/reboot-cmd (Sparc only)
This file seems to be a way to give an argument to the SPARC ROM/Flash boot loader. Maybe to tell it what to do after rebooting?
This file can be used to tune the maximum number of POSIX realtime (queued) signals that can be outstanding in the system.
This file shows the number POSIX realtime signals currently queued.
/proc/sys/kernel/sem (since Linux 2.4)
This file contains 4 numbers defining limits for System V IPC semaphores. These fields are, in order:
The maximum semaphores per semaphore set.
A system-wide limit on the number of semaphores in all semaphore sets.
The maximum number of operations that may be specified in a semop(2) call.
A system-wide limit on the maximum number of semaphore identifiers.
This file shows the size of the generic SCSI device (sg) buffer. You can't tune it just yet, but you could change it on compile time by editing include/scsi/sg.h and changing the value of SG_BIG_BUFF. However, there shouldn't be any reason to change this value.
This file contains the system-wide limit on the total number of pages of System V shared memory.
This file can be used to query and set the run time limit on the maximum (System V IPC) shared memory segment size that can be created. Shared memory segments up to 1Gb are now supported in the kernel. This value defaults to SHMMAX.
(available in Linux 2.4 and onwards) This file specifies the system-wide maximum number of System V shared memory segments that can be created.
contains a string like:
#5 Wed Feb 25 21:49:24 MET 1998.TP
The '#5' means that this is the fifth kernel built from this source base and the date behind it indicates the time the kernel was built.
/proc/sys/kernel/zero-paged (PowerPC only)
This file contains a flag. When enabled (non-zero), Linux-PPC will pre-zero pages in the idle loop, possibly speeding up get_free_pages.
This directory contains networking stuff.
This directory may be empty.
This directory supports Sun remote procedure call for network file system (NFS). On some systems, it is not present.
This directory contains files for memory management tuning, buffer and cache management.
This file contains the kernel virtual memory accounting mode. Values are:
0: heuristic overcommit (this is the default)
1: always overcommit, never check
2: always check, never overcommit
In mode 0, calls of mmap(2) with MAP_NORESERVE set are not checked, and the default check is very weak, leading to the risk of getting a process "OOM-killed". Under Linux 2.4 any nonzero value implies mode 1. In mode 2 (available since Linux 2.6), the total virtual address space on the system is limited to (SS + RAM*(r/100)), where SS is the size of the swap space, and RAM is the size of the physical memory, and r is the contents of the file /proc/sys/vm/overcommit_ratio.
See the description of /proc/sys/vm/overcommit_memory.
Subdirectory containing the pseudo-files msg, sem and shm. These files list the System V Interprocess Communication (IPC) objects (respectively: message queues, semaphores, and shared memory) that currently exist on the system, providing similar information to that available via ipcs(1). These files have headers and are formatted (one IPC object per line) for easy understanding. ipc(5) provides further background on the information shown by these files.
Subdirectory containing the psuedo-files and subdirectories for tty drivers and line disciplines.
This file contains two numbers: the uptime of the system (seconds), and the amount of time spent in idle process (seconds).
This string identifies the kernel version that is currently running. It includes the contents of /proc/sys/ostype, /proc/sys/osrelease and /proc/sys/version. For example:

Linux version 1.0.9 (quinlan@phaze) #1 Sat May 14 01:51:54 EDT 1994
/proc/vmstat (since Linux 2.6)
This file displays various virtual memory statistics.


cat(1), find(1), free(1), mount(1), ps(1), tr(1), uptime(1), chroot(2), mmap(2), readlink(2), syslog(2), slabinfo(5), hier(7), arp(8), dmesg(8), hdparm(8), ifconfig(8), init(8), lsmod(8), lspci(8), netstat(8), procinfo(8), route(8)


Note that many strings (i.e., the environment and command line) are in the internal format, with sub-fields terminated by NUL bytes, so you may find that things are more readable if you use od -c or tr "\000" "\n" to read them. Alternatively, echo `cat <file>` works well.
This manual page is incomplete, possibly inaccurate, and is the kind of thing that needs to be updated very often.


The material on /proc/sys/fs and /proc/sys/kernel is closely based on kernel source documentation files written by Rik van Riel.
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