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Chapter 5. The proc File System

The Linux kernel has two primary functions: to control access to physical devices on the computer and to schedule when and how processes interact with these devices. The /proc/ directory contains a hierarchy of special files which represent the current state of the kernel — allowing applications and users to peer into the kernel's view of the system.

Within the /proc/ directory, one can find a wealth of information detailing the system hardware and any processes currently running. In addition, some of the files within the /proc/ directory tree can be manipulated by users and applications to communicate configuration changes to the kernel.

5.1. A Virtual File System

Under Linux, all data are stored as files. Most users are familiar with the two primary types of files: text and binary. But the /proc/ directory contains another type of file called a virtual file. It is for this reason that /proc/ is often referred to as a virtual file system.

These virtual files have unique qualities. Most of them are listed as zero bytes in size and yet when one is viewed, it can contain a large amount of information. In addition, most of the time and date settings on virtual files reflect the current time and date, indicative of the fact they are constantly updated.

Virtual files such as /proc/interrupts, /proc/meminfo, /proc/mounts, and /proc/partitions provide an up-to-the-moment glimpse of the system's hardware. Others, like /proc/filesystems and the /proc/sys/ directory provide system configuration information and interfaces.

For organizational purposes, files containing information on a similar topic are grouped into virtual directories and sub-directories. For instance, /proc/ide/ contains information for all physical IDE devices. Likewise, process directories contain information about each running process on the system.

5.1.1. Viewing Virtual Files

By using the cat, more, or less commands on files within the /proc/ directory, users can immediately access an enormous amount of information about the system. For example, to display the type of CPU a computer has, type cat /proc/cpuinfo to receive output 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

When viewing different virtual files in the /proc/ file system, 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. Examples of these utilities include lspci, apm, free, and top.

NoteNote
 

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

5.1.2. 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 example, to change the hostname on the fly, type:

echo www.example.com > /proc/sys/kernel/hostname 

Other files act as binary or boolean switches. Typing cat /proc/sys/net/ipv4/ip_forward returns either a 0 or a 1. 0 indicates that the kernel is not forwarding network packets. Using the echo command to change the value of the ip_forward file to 1 immediately turns packet forwarding on.

TipTip
 

Another command used to alter settings in the /proc/sys/ subdirectory is /sbin/sysctl. For more information on this command, see Section 5.4 Using the sysctl Command

For a listing of some of the kernel configuration files available in the /proc/sys/, see Section 5.3.9 /proc/sys/.

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