3.3.9. /proc/sys/

3.3.9. /proc/sys/

The /proc/sys/ directory is different from others in /proc/ because it not only provides information about the system but also allows the system administrator to immediately enable and disable kernel features.


Use caution when changing settings on a production system using the various files in the /proc/sys/ directory. Changing the wrong setting may render the kernel unstable, requiring a system reboot.

For this reason, be sure the options are valid for that file before attempting to change any value in /proc/sys/.

A good way to determine if a particular file can be configured, or if it is only designed to provide information, is to list it with the -l option at the shell prompt. If the file is writable, it may be used to configure the kernel. For example, a partial listing of /proc/sys/fs looks like the following:

-r--r--r--    1 root     root            0 May 10 16:14 dentry-state
-rw-r--r--    1 root     root            0 May 10 16:14 dir-notify-enable 
-r--r--r--    1 root     root            0 May 10 16:14 dquot-nr 
-rw-r--r--    1 root     root            0 May 10 16:14 file-max 
-r--r--r--    1 root     root            0 May 10 16:14 file-nr

In this listing, the files dir-notify-enable and file-max can be written to and, therefore, can be used to configure the kernel. The other files only provide feedback on current settings.

Changing a value within a /proc/sys/ file is done by echoing the new value into the file. For example, to enable the System Request Key on a running kernel, type the command:

echo 1 > /proc/sys/kernel/sysrq

This changes the value for sysrq from 0 (off) to 1 (on).

A few /proc/sys/ configuration files contain more than one value. To correctly send new values to them, place a space character between each value passed with the echo command, such as is done in this example:

echo 4 2 45 > /proc/sys/kernel/acct


Any configuration changes made using the echo command disappear when the system is restarted. To make configuration changes take effect after the system is rebooted, refer to Section 3.4, “Using the sysctl Command”.

The /proc/sys/ directory contains several subdirectories controlling different aspects of a running kernel.

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