Red Hat LINUX 7.2 - OFFICIAL LINUX CUSTOMIZATION GUIDE Guida di Installazione Scaricare il pdf (Pagina 24)

24 Chapter 2. The /proc File System

As you view different virtual ﬁles in /proc, you will notice that some of the information makes sense.

Others are not human readable. This is why utilities exist pull data from virtual ﬁles in /proc and

display it in a useful way. Some examples of such applications are apm, free, and top.

Note

Some of the virtual ﬁles in /proc 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.su/jointfilesconvert/370363/bgenius.com > /proc/sys/kernel/hostname

Other ﬁles act as binary switches. For instance, if you type cat

/proc/sys/net/ipv4/ip_forward, you will get 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.

For a listing of some of the kernel conﬁguration ﬁles available in the /proc/sys/, see Section 2.3.8.

2.2. Top-Level Files in /proc

Below is a list of some of the more useful virtual ﬁles in the top-level of the /proc directory.

2.2.1. /proc/apm

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

This information is used by the apm command.

The output of this ﬁle on a system without a battery and constantly connected to an AC power source

looks similar to this:

1.16 1.2 0x03 0x01 0xff 0x80 -1% -1 ?

Executing apm -v command on these systems results in something similar to this:

APM BIOS 1.2 (kernel driver 1.16)

AC on-line, no system battery

For these systems, apm may be able to do little more than put the machine in standby mode. The apm

command is much more useful on laptops. This is also reﬂected in their /proc/apm ﬁles. This is the

output from a sample ﬁle on a laptop running Linux while plugged into a power outlet:

1.16 1.2 0x03 0x01 0x03 0x09 100% -1 ?

When the same machine is unplugged from its power source and running on its own batteries for a

few minutes, you will see the contents of the apm ﬁle change:

1 2 ... 19 20 21 22 23 24 25 26 27 28 29 ... 281 282

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Red Hat LINUX 7.2 - OFFICIAL LINUX CUSTOMIZATION GUIDE Guida di Installazione Pagina 24