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The /proc filesystem is one of Linux's great features, and this article gives you a thorough grounding in some of its most useful aspects. With it, you can administer many details of the operating system without ever having to shut down and reboot the machine, which is a boon for those who need to keep their systems as available as possible.

This article includes hints and tips for performing various administrative tasks and changing your system without rebooting. Linux provides various ways to change underlying operating system values and settings while keeping the system up and running. These come in two basic forms, those that are general to all Linux systems and are provided in the Linux kernel (you can find more information about the Linux kernel and download kernel source at the Linux Kernel Archives; see Resources for a link), and those that are distribution specific and provided by the vendor. This article deals with both types.

Note: This article is written for a 2.4-level kernel. Some of the options and features may be different for other kernel versions.

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The fact that we have some way of changing the running kernel parameters while the system is up and running gives the system administrator great power and flexibility in changing kernel settings. This sort of implementation was an inspired idea on the part of the Linux kernel developers. But can too much power be a bad thing? Sometimes. If you are going to change anything in the /proc filesystem, you must make sure that you know what you are changing and what effect this will have on the system. These are really useful techniques, but a wrong move can give you some rather undesired consequences. If you are new to this sort of thing or are not sure what effect one of your changes will have, practice on a machine that is not important to you or your business.

How to make changes
First, think about how not to make changes to the kernel. There are two good reasons why you should not just jump into the /proc filesystem, open a file in your text editor, make a bunch of changes, and save the file back out again. These are:

• Data integrity: All of these files represent the running system, and since the kernel may change any of these files at any time, if you open an editor and change some data while the system is changing it underneath you, whatever you save back is unlikely to be what the kernel is expecting.
• Virtual files: All of these files do not actually exist. How would the saved data be synchronized, etc.?

echo "Your-New-Kernel-Value" > /proc/your/file

Similarly, if you wish to view information from /proc, you should either use a command that is designed for the purpose or use the command line cat command.

What to change
You do not need to be a kernel hacker to get good use out of /proc, and a basic understanding of the structure of this filesystem will aid you greatly. You may find that you don't need to know about anything in /proc, until the day a user asks you for a certain bit of functionality that makes you glad you bothered to learn where to look to make changes. The /proc filesystem helps the system administrator in this respect via its structure and file permissions.

Each file in /proc has a very particular set of file permissions assigned to it and will be owned by a particular user ID. This is very carefully done so that the correct functionality is presented to the administrator and to the users. The following list summarizes what particular permissions may do on individual files:

• Read-only: File is not changeable by any user; used for presenting system information
• Root-write: If a file is writeable in /proc, it is usually writeable only by the root user
• Root-read: Some files may not be viewable to normal system users, only to root
• Other: You may find combinations other than the common three, above, for various reasons

The last thing to know about learning what to change in /proc is what you should actually be writing to these files. You will notice as you look at various files in /proc that some of them are human readable and some are data files. The data files can still be read by using specific utilities such as top, lspci, and free. You will also notice that the human-readable files take two different formats: some are binary switches and others contain more information. The binary switch files only contain a zero (off) or a one (on) for that particular kernel feature.

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First published by IBM developerWorks at http://www.ibm.com/developerWorks.