configure kernel parameters at runtime


Show all available variables and their values

$ sysctl -a

Set a changeable kernel state variable

$ sysctl -w [section.tunable]=[value]

Get currently open file handlers

$ sysctl fs.file-nr

Get limit for simultaneous open files

$ sysctl fs.file-max

Apply changes from /etc/sysctl.conf

$ sysctl -p


sysctl [ options ][ variable [ = value ]][...]
sysctl -p [ file or regexp ][...]


sysctl is used to modify kernel parameters at runtime. The parameters available are those listed under /proc/sys/. Procfs is required for sysctl support in Linux. You can use sysctl to both read and write sysctl data.


variable The name of a key to read from. An example is kernel.ostype. The '/' separator is also accepted in place of a '.'.

variable=value To set a key, use the form variable=value where variable is the key and value is the value to set it to. If the value contains quotes or characters which are parsed by the shell, you may need to enclose the value in double quotes.

-n , --values Use this option to disable printing of the key name when printing values.

-e , --ignore Use this option to ignore errors about unknown keys.

-N , --names Use this option to only print the names. It may be useful with shells that have programmable completion.

-q , --quiet Use this option to not display the values set to stdout.

-w , --write Use this option when all arguments prescribe a key to be set.

-p [ FILE ], --load [= FILE ] Load in sysctl settings from the file specified or /etc/sysctl.conf if none given. Specifying -as filename means reading data from standard input. Using this option will mean arguments to sysctl are files, which are read in the order they are specified. The file argument may be specified as regular expression.

-a , --all Display all values currently available.

--deprecated Include deprecated parameters to --all values listing.

-b , --binary Print value without new line.

--system Load settings from all system configuration files. Files are read from directories in the following list in given order from top to bottom. Once a file of a given filename is loaded, any file of the same name in subsequent directories is ignored.

-r , --pattern pattern Only apply settings that match pattern. The pattern uses extended regular expression syntax.

-A Alias of -a

-d Alias of -h

-f Alias of -p

-X Alias of -a

-o Does nothing, exists for BSD compatibility.

-x Does nothing, exists for BSD compatibility.

-h , --help Display help text and exit.

-V , --version Display version information and exit.


/sbin/sysctl -a
/sbin/sysctl -n kernel.hostname
/sbin/sysctl -w kernel.domainname=""
/sbin/sysctl -p/etc/sysctl.conf
/sbin/sysctl -a --pattern forward
/sbin/sysctl -a --pattern forward$
/sbin/sysctl -a --pattern 'net.ipv4.conf.(eth|wlan)0.arp'
/sbin/sysctl --system --pattern '^net.ipv6'


The base_reachable_time and retrans_time are deprecated. The sysctl command does not allow changing values of these parameters. Users who insist to use deprecated kernel interfaces should push values to /proc file system by other means. For example:

echo 256 > /proc/sys/net/ipv6/neigh/eth0/base_reachable_time




Please send bug reports to


sysctl.conf(5) regex(7)

AUTHOR George Staikos

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