The Linux kernel's primary functions are to control access to physical
devices on the computer and to schedule when and how processes interact with
these devices. The /proc directory contains hierarchy
of virtual folders and files which represent the current state of the
kernel. Viewing files within /proc allows the user to
peer into the kernel's view of the system. In addition, the user can use
these virtual files to communicate configuration changes to the kernel.
In Linux, everything is stored as a file. Most users are familiar with
the two primary types of files, text and binary. However, the
/proc directory contains files that are not part of
any file system associated with any physical storage device connected to
your system. Instead, the virtual files in /proc
are part of a virtual file system.
The virtual files in /proc have unique
qualities. Most of them are 0 bytes in size. Yet when the
file is viewed, it can contain quite a bit of information. In
addition, most of their time and date settings reflect the current time
and date, meaning that they are constantly changing.
Both applications and system administrators can use
/proc as a method of accessing information about
the state of the kernel, the attributes of the machine, the state of
individual processes, and so on. Most of the files in the directory,
such as interrupts, meminfo,
mounts, and partitions,
provide an up-to-the-moment glimpse of a system's physical
environment. Others, like file systems and the
/proc/sys/ directory provide software configuration
information.
To make things easier, files that contain information covering a similar
topic are grouped into virtual directories and sub-directories, such as
/proc/ide/ for all physical IDE devices.
By using cat, more, or
less commands on the files within
/proc, you can immediately access an enormous
amount of information about the system. For example, if you want to
see what sort of CPU your computer has, type cat
cpuinfo and you will see something similar to the following:
processor : 0
vendor_id : AuthenticAMD
cpu family : 5
model : 9
model name : AMD-K6(tm) 3D+ Processor
stepping : 1
cpu MHz : 400.919
cache size : 256 KB
fdiv_bug : no
hlt_bug : no
f00f_bug : no
coma_bug : no
fpu : yes
fpu_exception : yes
cpuid level : 1
wp : yes
flags : fpu vme de pse tsc msr mce cx8 pge mmx syscall 3dnow k6_mtrr
bogomips : 799.53 |
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.
|
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.subgenious.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 the Section called /proc/sys/.
