It's not just a file system full of odd looking files that only the kernel understands. Instead, it's really something of a peep hole into your system. And there a quite a number of useful things that you can learn from the files that it contains.
So, what do you see when you cd over to /proc? Well, run ls and the first thing you're likely to notice is the very large group of directories with just numbers for names. These numbers correspond to the process IDs (PID) of processes that are running on your system -- everything from the init process that started the boot time ball rolling to the shell you're using right now. And you're likely to see quite a lot of them -- probably several hundred or more.
$ cd /proc $ ls 1 15878 38 433 5266 579 67 7521 devices 10 1589 39 434 5267 5792 6788 7523 diskstats 10052 16 393 435 5268 58 6793 7525 dma 1021 1623 3956 436 5269 580 6794 7529 driver 10522 16571 3957 437 5270 581 6795 7531 execdomains 10552 16585 3958 438 5271 5810 6796 7533 fb 11 1695 3959 439 5272 582 6797 7535 filesystems 11984 17 3960 44 5273 583 6798 7537 fs ...
If you were to count the numeric (process) directories, your total should be the same as the response you'd get if you ran the command ps -ef --no-headers | wc -l (ps output without the header line).
The bulk of these directories will likely be owned by root but, depending on how your system is being used, you'll also see application service accounts (such as oracle in the example below) and usernames among the process owners listed.
# ls -l | more total 0 dr-xr-xr-x 5 root root 0 Oct 3 2013 1 dr-xr-xr-x 5 root root 0 Oct 3 2013 10 dr-xr-xr-x 5 root root 0 Oct 3 2013 1021 dr-xr-xr-x 5 root root 0 Oct 3 2013 11 dr-xr-xr-x 5 oracle oinstall 0 Feb 4 07:11 1167 dr-xr-xr-x 5 root root 0 Jan 26 11:00 11920 dr-xr-xr-x 5 root root 0 Mar 7 2014 11923 dr-xr-xr-x 5 gdm gdm 0 Jan 26 11:01 11950
Notice that none of these are files in the same sense as files we see in our file systems. They don't take up space on the disk and they don't have content even if the cat command displays their data for you. Unlike the directories that we see in "real" file systems, these show up as using 0 bytes of data. Many will have dates and times that correspond to the last time the system was booted (i.e., when the related processes started) while other files in /proc may appear to be updated almost constantly. Only the /proc/kcore file will have a signficant size and it might appear to be huge, though even it isn't really using disk space) as it relates to the RAM on your system.
# ls -l kcore -r-------- 1 root root 39460016128 Feb 8 09:10 kcore
You'll also see a collection of other files in /proc with names like cpuinfo, key-users and schedstat -- names that provide clues to what these files contain. In fact, you can think of the files in /proc as falling into two categories -- those that are represent processes running on your system and those represent some aspect of the system itself.
So, what are some useful things these interesting pseudo files can tell you?
For one thing, they can tell you how long the system has been up. Check out the /proc/uptime file. This file reports the system uptime, even though it might not be immediately obvious. The number 74216960.58 in the output below probably likely doesn't look like an uptime report to you. But type "cat uptime" a couple times in a row and you'll notice that the numbers are constantly changing. It's obviously keeping up.
$ cat /proc/uptime;sleep 10;cat /proc/uptime 74216960.58 73912315.63 74216970.58 73912325.61
As you'll note, this file actually contains two numbers. The first is the uptime of the system (as you'd expect from the name) while the second is the amount of time the system has spent idle. The numbers are constantly changing because we're always getting further from the time the system was last booted. After sleeping for ten seconds, the number on the left just happens to be 10 units larger, so it's clear that these numbers are reporting time in seconds.
No problem. A little command line math can turn those seconds into days. If we then compare the result of our calculation with the uptime command output, we'll the some connection between the numbers.
$ expr 74216970 / 60 / 60 / 24 858 $ uptime 14:30:17 up 858 days, 23:50, 1 user, load average: 0.08, 0.04, 0.00
Of course, almost no one would want to go through all the trouble of calculating uptime with an expr command when the uptime command can tell us what we want to know directly, especially if we have to think through the sixty seconds per minute, sixty minutes per hour, and 24 hours per day conversions.
And think your system is busy? Do a little more math with these numbers and you might see something like this. Notice how I added two zeroes to the end of the idle time figure to get an answer that would represent the percentage of the time this system has been idle. Yes, that's 99%. This system is clearly not straining -- at least not most of the time.
$ expr 7391232500 / 74216970 99
This uptime exercise is useful because it reinforces the idea that these "files" are plucking information from the system to update the virtual file content many times a second. Note, though, that the dates and times associated with this file keep up with the current time.
$ ls -l /proc | tail -11 -r--r--r-- 1 root root 0 Feb 9 14:42 stat -r--r--r-- 1 root root 0 Feb 9 14:42 swaps dr-xr-xr-x 11 root root 0 Oct 3 2013 sys --w------- 1 root root 0 Feb 9 14:42 sysrq-trigger dr-xr-xr-x 2 root root 0 Feb 9 14:42 sysvipc dr-xr-xr-x 4 root root 0 Feb 9 14:42 tty -r--r--r-- 1 root root 0 Feb 9 14:42 uptime -r--r--r-- 1 root root 0 Feb 9 14:42 version -r-------- 1 root root 0 Feb 9 14:42 vmcore -r--r--r-- 1 root root 0 Feb 9 14:42 vmstat -r--r--r-- 1 root root 0 Feb 9 14:42 zoneinfo
Another file with information that will likely seem familiar is the version file. This file supplies information on your operating system version, much like the output of the uname -a command and undoubtedly tapping the same system resources.
$ cat /proc/version Linux version 2.6.18-128.el5 (firstname.lastname@example.org) (gcc version 4.1.2 20080704 (Red Hat 4.1.2-44)) #1 SMP Wed Dec 17 11:41:38 EST 2008
$ uname -a Linux sea-aveksa-1.telecomsys.com 2.6.18-128.el5 #1 SMP Wed Dec 17 11:41:38 EST 2008 x86_64 x86_64 x86_64 GNU/Linux
Another file -- the cpuinfo file -- supplies fairly extensive information on your system CPUs. While I don't want to insert all 500+ lines into this post, you can see some of the details below. The second command is simply counting up the number of CPUs.
$ head -11 /proc/cpuinfo processor : 0 vendor_id : GenuineIntel cpu family : 6 model : 44 model name : Intel(R) Xeon(R) CPU X5650 @ 2.67GHz stepping : 2 cpu MHz : 2660.126 cache size : 12288 KB physical id : 1 siblings : 12 core id : 0
$ more cpuinfo | grep processor | wc -l 24
The vmstat file provides virtual memory statistics. Want to see what's happening with page swapping? The numbers below represent swapping activity (pages swapped in and out) since the system was booted.
$ grep pswp /proc/vmstat pswpin 229269 pswpout 316559
If these names look familiar, you may be remembering them from sar output like that shown below.
# sar -W 10 2 Linux 3.14.35-28.38.amzn1.x86_64 (ip-172-30-0-28) 02/10/2016 _x86_64_(1 CPU) 12:17:03 PM pswpin/s pswpout/s 12:17:13 PM 0.00 0.00 12:17:23 PM 0.00 0.00 Average: 0.00 0.00
We can also look at memory statistics. These details can come in very handy if you want to get a very detailed understanding of the memory on your system and how it is being used.
$ more /proc/meminfo MemTotal: 37037804 kB MemFree: 18605268 kB Buffers: 323740 kB Cached: 14919556 kB SwapCached: 12068 kB Active: 13878148 kB Inactive: 3846048 kB HighTotal: 0 kB HighFree: 0 kB LowTotal: 37037804 kB LowFree: 18605268 kB SwapTotal: 16778232 kB SwapFree: 16309048 kB Dirty: 9896 kB Writeback: 0 kB AnonPages: 2468880 kB Mapped: 7089292 kB Slab: 442900 kB PageTables: 189648 kB NFS_Unstable: 0 kB Bounce: 0 kB CommitLimit: 35297132 kB Committed_AS: 12768916 kB VmallocTotal: 34359738367 kB VmallocUsed: 271696 kB VmallocChunk: 34359466659 kB HugePages_Total: 0 HugePages_Free: 0 HugePages_Rsvd: 0 Hugepagesize: 2048 kB
Want to check on what file system types are supported by your kernel? Take a look at /proc/filesystems.
$ head -11 /proc/filesystems nodev sysfs nodev rootfs nodev bdev nodev proc nodev cpuset nodev binfmt_misc nodev debugfs nodev securityfs nodev sockfs nodev usbfs nodev pipefs
To view all the mounts used by your system, look at the /proc/mounts file.
$ cat /proc/mounts rootfs / rootfs rw 0 0 /dev/root / ext3 rw,data=ordered,usrquota 0 0 /dev /dev tmpfs rw 0 0 /proc /proc proc rw 0 0 /sys /sys sysfs rw 0 0 /proc/bus/usb /proc/bus/usb usbfs rw 0 0 devpts /dev/pts devpts rw 0 0 /dev/sda1 /boot ext3 rw,data=ordered 0 0 tmpfs /dev/shm tmpfs rw 0 0 none /proc/sys/fs/binfmt_misc binfmt_misc rw 0 0 sunrpc /var/lib/nfs/rpc_pipefs rpc_pipefs rw 0 0 /etc/auto.misc /misc autofs rw,fd=6,pgrp=5541,timeout=300,minproto=5,maxproto=5,indirect 0 0 -hosts /net autofs rw,fd=12,pgrp=5541,timeout=300,minproto=5,maxproto=5,indirect 0 0 oracleasmfs /dev/oracleasm oracleasmfs rw 0 0 //windows-server/outgoing /mnt/ActAccts cifs rw,mand,unc=\\windows-server \outgoing,username=xferSvc,uid=0,gid=0,file_mode=02767,dir_mode=0777,rsize=16384,wsize=57344 0 0
The /proc/net directory contains a wealth of network information including data for your network interfaces.
$ ls /proc/net anycast6 ip_conntrack netfilter rt6_stats tcp6 arp ip_conntrack_expect netlink rt_acct tr_rif bonding ip_mr_cache netstat rt_cache udp dev ip_mr_vif packet snmp udp6 dev_mcast ip_tables_matches protocols snmp6 unix dev_snmp6 ip_tables_names psched sockstat wireless if_inet6 ip_tables_targets raw sockstat6 igmp ipv6_route raw6 softnet_stat igmp6 mcfilter route stat ip6_flowlabel mcfilter6 rpc tcp
Examples of some /proc/net data include your arp cache and routing table.
$ cat arp IP address HW type Flags HW address Mask Device 172.30.0.1 0x1 0x2 0a:ee:74:5c:40:bd * eth0 172.30.0.2 0x1 0x2 0a:ee:74:5c:40:bd * eth0 $ cat route Iface Destination Gateway Flags RefCnt Use Metric Mask MTU Window IRTT eth0 00000000 01001EAC 0003 0 0 0 000000000 0 0 eth0 FEA9FEA9 00000000 0005 0 0 0 FFFFFFFF0 0 0 eth0 00001EAC 00000000 0001 0 0 0 00FFFFFF0 0 0
For some files in /proc, you'll need to use your superpowers. Here we're looking into some aspects of our host-based firewall.
$ sudo cat /proc/net/ip_tables_names filter nat
You can view arp (address resolution protocol) data that your system has collected using the /proc/net/arp file. This is much the same information that you'd see using the arp command.
$ cat /proc/net/arp IP address HW type Flags HW address Mask Device 10.20.30.128 0x1 0x2 00:50:56:B1:2E:01 * bond0 10.20.30.110 0x1 0x2 A4:BA:88:12:2C:5D * bond0 10.20.30.154 0x1 0x2 00:50:56:B3:0E:33 * bond0 10.20.30.1 0x1 0x2 00:00:0C:07:AC:2A * bond0 10.20.30.33 0x1 0x2 00:50:52:B6:32:33 * bond0
Or maybe you want to look into page faults.
$ cat vmstat | grep "fault" pgfault 2426152809 pgmajfault 79826
You can examine your swap partitions and swap files through the /proc/swaps file.
$ more /proc/swaps Filename Type Size Used Priority /dev/mapper/VolGroup00-LogVol01 partition 16777208 514200 /swapfile file 1024 0 -2
Details about your system's devices are available in the /proc/sys/dev directory. Below, we look at the cdrom and raid devices.
# ls -l /proc/sys/dev/cdrom total 0 -rw-r--r-- 1 root root 0 Feb 8 17:59 autoclose -rw-r--r-- 1 root root 0 Feb 8 17:59 autoeject -rw-r--r-- 1 root root 0 Feb 8 17:59 check_media -rw-r--r-- 1 root root 0 Feb 8 17:59 debug -r--r--r-- 1 root root 0 Feb 8 17:59 info -rw-r--r-- 1 root root 0 Feb 8 17:59 lock
# ls -l /proc/sys/dev/raid total 0 -rw-r--r-- 1 root root 0 Feb 8 17:59 speed_limit_max -rw-r--r-- 1 root root 0 Feb 8 17:59 speed_limit_min
Examining the contents of one of these files, we see the maximum speed (RAID rebuild speed) that is set for the device.
# cat /proc/sys/dev/raid/speed_limit_max 200000
A lot of the information available through /proc can also be viewed using commands like arp, netstat, and sar. Still, it's useful to be able to pull data from the kernel in one convenient location and /proc provides a tremendous wealth of stats for anyone who's want to dive deeply into their system.
This tour of /proc and some of the extensive information that it provides was just a taste of the detail available to you. The key to making good use of all this data is deciding what kind of information you want to see and devising scripts or aliases to fetch it from the tremendously detailed files always waiting for you in /proc.
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