Exploring /proc By Trevor Warren <trevorwarren@yahoo.com> Posted: ( 2000-12-12 10:03:00 EST by ) The Proc psuedo file system is a real time, memory resident file system that tracks the processes running on your machine and the state of your system. Read on to learn how to get the most out of the /proc file system.
The most striking factor about the /proc file system is the fact that the file system doesn''''t exist on any particular media. The /proc File System is a pseudo file system residing in the virtual memory and maintains highly dynamic data on the state of your operating system. Most of the information in the /proc file system is updated to match the current state of the operating system. The contents of the /proc file system can be read by anyone who has the requisite permissions. However, certain parts of the /proc file system can be read only by the owner of the process and of course root. The contents of the /proc filesystem are used by many utilities which grab the data from the particular /proc directory and display it. Under Linux, we have utilities like lscpi, scanpci and pnpdump which help us detect the various PCI, ISA hardware chipsets and help us make the best choices as far as the io, dma and irq values are concerned. Similarly by looking at the values of the various parameters in the /proc file system, one can determine a lot about the current state of the kernel and the processes under way. For example, we have the dmesg command. bash# dmesg Dmesg helps us determine the devices that have been detected and initialized by the Linux kernel. We have utilities like "ps" and "top" which give an accurate snapshot about the status of the processes running on the machine and a list of awake and sleeping processes currently running on the machine. Have you ever wondered where exactly the information dished out to you by the "ps" and the "top" process comes from? The information for these processes come from the /proc file system which is updated on the fly as changes take place in the processes. Lets take a look at a snapshot of the directory listing of the root (/) directory of a Linux machine. drwxr-xr-x 14 root root 291 Oct 25 18:47 opt dr-xr-xr-x 86 root root 0 Nov 30 2000 proc <-- drwx--x--x 16 root root 841 Nov 20 00:10 root drwxr-xr-x 5 root root 4627 Oct 15 11:42 sbin Since the /proc file system is a virtual file system and resides in memory, a new /proc file system is created every time your Linux machine reboots. Take a look at the snapshot of the root directory shown above. The size of the proc directory is 0 and the last time of modification is the current date. Using the /proc/sys file system to parse kernel parameters. Another very important part of the /proc file system is the /proc/sys directory. Making changes in this directory enables you to make real time changes to certain kernel parameters. One fine example that can illustrate this is as follows. /proc/sys/net/ipv4/ip_forward
The above file ip_forward has a default value of "0" which you can see when you cat the contents of this file. This means that IP forwarding by this machine isn''''t allowed by the kernel. But this configuration can be changed in real time by just changing the value stored in this file from "0" to "1". Thus we have enabled IP forwarding on our Linux machine.
Contents of the /proc File System The output of the directory listing of the /proc directory is as follows. The actual listing was really lengthy. What''''s given below is a much shorter version.
1 114 1210 1211 1212 1227 133 137 148 160 161 163 167 168 169 170 171 172 173 174 186 190 193 194 195 203 206 207 208 209 210 211 220 221 222 223 224 225 226 227 229 230 234 246 253 279 296 3 4 5 500 501 6 667 668 669 683 684 685 7 711 712 713 737 763 764 765 766 773 774 775 782 79 88 92 asound bus cmdline config.gz cpuinfo devices dma fb filesystems fs ide interrupts ioports kcore kcore_elf kmsg ksyms loadavg locks lvm mdstat meminfo memstat misc modules mounts net partitions pci rtc scsi self slabinfo stat swaps sys tty uptime version Every number and word that you see in the above snapshot are the contents of the /proc directory. Lets learn a little more about the contents of the directories with the numerical naming scheme. The numerical named directories 1 114 1210 1211 1212 1227 133 137 148 160 161 163 167 168 169 170 171 172 173 174 186 190 193 194 195 203 206 207 208 209 210 211 220 221 222 223 224 225 226 227 229 230 234 246 253 279 296 3 4 5 500 501 6 667 668 669 683 684 685 7 711 712 713 737 763 764 765 766 773 774 775 782 79 88 92 The various directories that you see out here are the processes that were running on our machine at the instant we took a snapshot of the /proc file system. Let''''s look at the contents of one of these directories. freeos:~ # cd /proc freeos:/proc # ls -la 114 total 0 dr-xr-xr-x 3 named named 0 Nov 30 12:20 . dr-xr-xr-x 89 root root 0 Nov 30 2000 .. -r--r--r-- 1 root root 0 Nov 30 12:20 cmdline lrwx------ 1 root root 0 Nov 30 12:20 cwd -> /var/named -r-------- 1 root root 0 Nov 30 12:20 environ lrwx------ 1 root root 0 Nov 30 12:20 exe -> /usr/sbin/named dr-x------ 2 root root 0 Nov 30 12:20 fd pr--r--r-- 1 root root 0 Nov 30 12:20 maps -rw------- 1 root root 0 Nov 30 12:20 mem lrwx------ 1 root root 0 Nov 30 12:20 root -> / -r--r--r-- 1 root root 0 Nov 30 12:20 stat -r--r--r-- 1 root root 0 Nov 30 12:20 statm -r--r--r-- 1 root root 0 Nov 30 12:20 status Before executing this command you need to log in as root because the various processes running on the system may be owned by various other users. As usual you have access only to the processes that you have started. After having logged in as root just execute the above command for any of the directories and compare the output to what is listed above. Did you notice any similarity in the output you got and the one listed above? Yes, the contents of all the directories are the same irrespective of the directory you choose to view as these directories contain the various parameters and the status of the process whose PID is the name of the current directory that you are in. The values of the various parameters and status information would of course vary from process to process. Look at the first line of the output as viewed above. -r--r--r-- 1 root root 0 Nov30 12:20 cmdline "cmdline", this file contains the whole command line used to invoke the process. The contents of this file are the command line arguments with all the parameters that one has passed to start the particular process. All the information contained in the file is without any formatting and any space between the command line arguments and various arguments. lrwx------ 1 root root 0 Nov 30 12:20 cwd -> /var/named "cwd", as is visible from above this is a symbolic link to the current working directory of that particular process. -r-------- 1 root root 0 Nov 30 12:20 environ "environ" contains all the environment variables defined for this particular process in the form VARIABLE=value. As with cmdline, all the information contained in the file is without any formatting or space between the command line a
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