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Chapter 2. The /proc File System 45

A good way to determine if a particular ﬁle can conﬁgured or is only designed to provide information

is to list it. If the ﬁle is writable, you may use it to conﬁgure the kernel in a certain way. For example,

a partial listing of /proc/sys/fs looks like this:

-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 will change the sysrq ﬁle’s value from 0 (off) to 1 (on).

The purpose of the System Request Key is to allow you to immediately instruct the kernel to do

a number of important activities by using a simple key combination, such as immediately shutting

down or restarting a system, syncing all mounted ﬁle systems, or dumping important information to

your console. This feature is most useful when using a development kernel or if you are experienc-

ing system freezes. For more information on the System Request Key, refer to /usr/src/linux-

2.4/Documentation/sysrq.txt.

A few /proc/sys/ conﬁguration ﬁles contain more than one value. In order 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

Note

Any conﬁguration changes you make using the echo command will disappear when the system is

restarted. To make your conﬁguration changes take effect at the time the system is booted, see

Section 2.4.

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

kernel.

2.3.8.1. /proc/sys/dev/

This directory provides parameters for particular devices on the system. Most systems have at least

two directories, cdrom and raid, but customized kernels can have others, such as parport, which

provides the ability to share one parallel port between multiple device drivers.

The cdrom directory contains a ﬁle called info, which reveals a number of important CD-ROM

parameters:

CD-ROM information, Id: cdrom.c 3.12 2000/10/18

drive name: hdc

drive speed: 32

drive # of slots: 1

1 2 ... 40 41 42 43 44 45 46 47 48 49 50 ... 281 282

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Red Hat LINUX 7.2 - OFFICIAL LINUX CUSTOMIZATION GUIDE Guida di Installazione Pagina 45