24

Chapter 2. The

proc

File System

When viewing different virtual files in the

/proc/

file system, you will notice some of the information

is easily understandable while some is not human readable. This is in part why utilities exist to pull

data from virtual files and display it in a useful way. Some examples of such applications are

lspci

,

apm

,

free

, and

top

.

Note

Some of the virtual files in the /proc/ directory are only readable by the root user.

2.1.2. Changing Virtual Files

As a general rule, most virtual files within the

/proc/

directory are read only. However, some can be

used to adjust settings in the kernel. This is especially true for files in the

/proc/sys/

subdirectory.

To change the value of a virtual file, use the

echo

command and a

symbol to redirect the new value

to the file. For instance, to change your hostname on the fly, you can type:

echo bob.subgenius.com > /proc/sys/kernel/hostname

Other

files

act

as

binary

or

boolean

switches.

For

instance,

if

you

type

cat

/proc/sys/net/ipv4/ip_forward

, you will see either a

0

or a

1

. A

0

indicates the kernel is not

forwarding network packets. By using the

echo

command to change the value of the

ip_forward

file to

1

, you can immediately turn packet forwarding on.

Tip

Another command used to alter settings in the /proc/sys/ subdirectory is /sbin/sysctl. For more

information on this command, see Section 2.4

For a listing of some of the kernel configuration files available in the

/proc/sys/

, see Section 2.3.9.

2.2. Top level Files in the

proc

Filesystem

Below is a list of some of the more useful virtual files in the top level of the

/proc/

directory.

Note

In most cases, the content of files listed in this section will not be the same on your machine. This is

because much of the information pertains to the hardware on which you are running Red Hat Linux.

2.2.1.

/proc/apm

This file provides information about the state of the Advanced Power Management (APM) system and

is used by the

apm

command. If the system with no battery is connected to an AC power source, this

virtual file looks similar to this:

1.16 1.2 0x07 0x01 0xff 0x80  1%  1 ?