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5.6. The /proc Filesystem
Unix systems have come a long way with respect to providing uniform interfaces to different parts of the system; as you will learn in the next chapter, hardware is represented in Linux in the form of a special type of file. There is, however, a special filesystem called the /proc filesystem that goes even one step further: it unifies files and processes.
From the user's or the system administrator's point of view, the /proc filesystem looks just like any other filesystem; you can navigate around it with the cd command, list directory contents with the ls command, and view file contents with the cat command. However, none of these files and directories occupies any space on your hard disk. The kernel traps accesses to the /proc filesystem and generates directory and file contents on the fly. In other words, whenever you list a directory or view file contents in the /proc filesystem, the kernel dynamically generates the contents you want to see.
To make this less abstract, let's see some examples. The following example displays the list of files in the top-level directory of the /proc filesystem:
owl # ls /proc 1 1618 17613 27191 27317 2859 8929 kcore_elf rtc 11120 1621 1795 27192 27320 2860 9 kmsg scsi 11121 1649 1796 27204 27324 28746 bus ksyms self 11153 1657 1798 27205 27326 28747 cmdline loadavg slabinfo 15039 1664 1799 27221 27374 28754 config.gz locks stat 1512 1681 1800 27229 27377 29877 cpuinfo lvm swaps 1530 1689 2 27287 27379 29878 devices mdstat sys 1534 1703 20007 27289 27380 29944 dma meminfo tty 1560 1708 21391 27292 27381 3 fb misc uptime 1570 1709 21394 27297 27397 4 filesystems modules version 1578 1710 2302 27308 27515 5 fs mounts 1585 1711 2309 27310 27518 5841 ide mtrr 1586 1712 2356 27312 27521 5842 interrupts net 1587 1713 27182 27314 2786 5860 ioports partitions 1588 1731 27183 27315 28536 6100 kcore pci
The numbers will be different on your system, but the general organization will be the same. All those numbers are directories that represent each of the processes running on your system. For example, let's look at the information about the process with the ID 1534:
tigger # ls /proc/1534 cmdline environ fd mem stat status cwd exe maps root statm
You see a number of files that each contain information about this process. For example, the cmdline file shows the command line with which this process was started. status gives information about the internal state of the process and cwd links to the current working directory of this process.
Probably you'll find the hardware information even more interesting than the process information. All the information that the kernel has gathered about your hardware is collected in the /proc filesystem, even though it can be difficult to find the information you are looking for.
Let's start by checking your machine's memory. This is represented by the file /proc/meminfo:
owl # cat /proc/meminfo
total: used: free: shared: buffers: cached:
Mem: 267919360 255311872 12607488 0 40587264 77791232
Swap: 123371520 5861376 117510144
MemTotal: 261640 kB
MemFree: 12312 kB
MemShared: 0 kB
Buffers: 39636 kB
Cached: 75968 kB
BigTotal: 0 kB
BigFree: 0 kB
SwapTotal: 120480 kB
SwapFree: 114756 kBIf you then try the command free, you can see that you get exactly the same information, only the numbers are reformatted a bit. free does nothing more than read /proc/meminfo and rearrange the output a bit.
Most tools on your system that report information about your hardware do it this way. The /proc filesystem is a portable and easy way to get at this information. The information is especially useful if you want to add new hardware to your system. For example, most hardware boards need a few I/O addresses to communicate with the CPU and the operating system. If you configured two boards to use the same I/O addresses, disaster is about to happen. You can avoid this by checking which I/O addresses the kernel has already detected as being in use:
tigger # more /proc/ioports 0000-001f : dma1 0020-003f : pic1 0040-005f : timer 0060-006f : keyboard 0080-009f : dma page reg 00a0-00bf : pic2 00c0-00df : dma2 00f0-00ff : npu 01f0-01f7 : ide0 0220-022f : soundblaster 02e8-02ef : serial(auto) 0388-038b : OPL3/OPL2 03c0-03df : vga+ 03f0-03f5 : floppy 03f6-03f6 : ide0 03f7-03f7 : floppy DIR 03f8-03ff : serial(auto) 0530-0533 : WSS config 0534-0537 : MSS audio codec e000-e0be : aic7xxx e400-e41f : eth0
Now you can look for I/O addresses that are free. Of course, the kernel can show I/O addresses only for boards that it has detected and recognized, but in a correctly configured system, this should be the case for all boards.
You can use the /proc filesystem for the other information you might need when configuring new hardware as well: /proc/interrupts lists the occupied interrupt lines (IRQs) and /proc/dma lists the DMA channels in use.
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| 5.5. Shutting Down the System |  | 5.7. Managing User Accounts |
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