Understanding UNIX / Linux File System

Posted on in Categories Debian Linux, File system, FreeBSD, Linux, OpenBSD, Sys admin, Tips, UNIX last updated November 7, 2005

A conceptual understanding of file system, especially data structure and related terms will help you become a successful system administrator. I have seen many new Linux system administrator without any clue about file system. The conceptual knowledge can be applied to restore file system in an emergency situation.

What is a File?

File are collection of data items stored on disk. Or, it’s device which can store the information, data, music (mp3 files), picture, movie, sound, book etc. In fact what ever you store in computer it must be inform of file. Files are always associated with devices like hard disk ,floppy disk etc. File is the last object in your file system tree. See Linux/UNIX – rules for naming file and directory names.

What is a directory?

Directory is group of files. Directory is divided into two types:

  • Root directory – Strictly speaking, there is only one root directory in your system, which is denoted by / (forward slash). It is root of your entire file system and can not be renamed or deleted.
  • Sub directory – Directory under root (/) directory is subdirectory which can be created, renamed by the user.

Directories are used to organize your data files, programs more efficiently.

Linux supports numerous file system types

  • Ext2: This is like UNIX file system. It has the concepts of blocks, inodes and directories.
  • Ext3: It is ext2 filesystem enhanced with journalling capabilities. Journalling allows fast file system recovery. Supports POSIX ACL (Access Control Lists).
  • Isofs (iso9660): Used by CDROM file system.
  • Sysfs: It is a ram-based filesystem initially based on ramfs. It is use to exporting kernel objects so that end user can use it easily.
  • Procfs: The proc file system acts as an interface to internal data structures in the kernel. It can be used to obtain information about the system and to change certain kernel parameters at runtime using sysctl command. For example you can find out cpuinfo with following command:

# cat /proc/cpuinfo

  • Or you can enable or disable routing/forwarding of IP packets between interfaces with following command:

# cat /proc/sys/net/ipv4/ip_forward
# echo "1" > /proc/sys/net/ipv4/ip_forward
# echo "0" > /proc/sys/net/ipv4/ip_forward

  • NFS: Network file system allows many users or systems to share the same files by using a client/server methodology. NFS allows sharing all of the above file system.
  • Linux also supports Microsoft NTFS, vfat, and many other file systems. See Linux kernel source tree Documentation/filesystem directory for list of all supported filesystem.

You can find out what type of file systems currently mounted with mount command:
$ mount
OR
$ cat /proc/mounts

What is a UNIX/Linux File system?

A UNIX file system is a collection of files and directories stored. Each file system is stored in a separate whole disk partition. The following are a few of the file system:

  • / – Special file system that incorporates the files under several directories including /dev, /sbin, /tmp etc
  • /usr – Stores application programs
  • /var – Stores log files, mails and other data
  • /tmp – Stores temporary files

See The importance of Linux partitions for more information.

But what is in a File system?

Again file system divided into two categories:

  • User data – stores actual data contained in files
  • Metadata – stores file system structural information such as superblock, inodes, directories

Next time I will write more about Metadata objects – superblock, inodes, directories with actual linux commands so that you can understand and master the concepts 🙂

Continue reading rest of the Understanding Linux file system series:

  • Part II – Understanding Linux superblock
  • Part III – An example of Surviving a Linux Filesystem Failures
  • Part IV – Understanding filesystem Inodes
  • Part V – Understanding filesystem directories
  • Part VI – Understanding UNIX/Linux symbolic (soft) and hard links
  • Part VII – Why isn’t it possible to create hard links across file system boundaries?

Posted by: Vivek Gite

The author is the creator of nixCraft and a seasoned sysadmin and a trainer for the Linux operating system/Unix shell scripting. He has worked with global clients and in various industries, including IT, education, defense and space research, and the nonprofit sector. Follow him on Twitter, Facebook, Google+.

51 comment

  1. Hello Sir,

    I would like to post my comments regarding file and directories definitation.

    File:

    It contains the metadata which includes inodes, date and stamp, rights and ownership filesize and type. It also has the link to the datablock which stores data on it.

    Directories:

    Directories are also files which contains filename to inode associations in its datablock so people says there are only files in Unix.

    Thanks!
    Ashish

  2. You didn’t mention Reiserfs which I think is a little short sighted. Many users prefer it to ext3, XFS, and JFS journaling filesystems. Once reiserfs v4 is merged we’ll see some cool features being made available to all other filesystems as well.

  3. Quote “It contains the metadata which includes inodes, date and stamp, rights and ownership filesize and type. It also has the link to the datablock which stores data on it.”

    I think this definition is wrong, a file by definition is a stream of data -the meta data is exactly that, its data about data.

    The inode may be part of the operating systems fileing system, but it is NOT part of the file. For example if I copy a file from my linux system onto my windows system i lose the date/time/inode/mode/group – but I still have my “file”

    Jon

    1. When you copy a file from linux to windows or within the same OS, you are creating a new file. You are not losing the date/time or anything that you have mentioned. The new file is just getting updated with its values. Copying copies only the data, not the metadata. File “data” is not everything. Its just another attribute of a file. Data + Metadata = File.

  4. Hey Jon,

    This brings another querry in my mind.
    Where file is storing the information about date, stamp, permissions, ownership and filesize?

    BTW, I really do not have much information about windows file system. Is it same like Unix file system? Is it having inodes and other stuff?

    Thanks!
    Ashish Pathak

  5. “A conceptual understanding of file system, especially data structure and related terms will help you become a successful system administrator”
    ———————-
    But his job is easily taken away
    by future s/w that manages all objects
    files and folders , w/o a cmd line .
    Automating objects , reduces the
    kernel size , runs much faster .
    ————————

    Pocket boxes are $5 cpu , & 64MB .
    And Intel lost its grip ( 20 countries
    make ARM !). No more WinTel .

    Ppl will hack h/w and s/w into
    a pocket box that will run fast
    enuf to do everything .
    OpSys that has NO cmd line , true G.U.I.
    uses images ONLY , has no text keyboard.
    But since OpSys is non-text , one
    can create apps in hours ! Most of
    the proceedures will be in kernel ,
    all you need is to chain these proc’s
    in the proper order to create your app .

    Yes , i know Linux is your paycheck ,
    sorry …..

  6. Hey friends can any one answer this following questioon plz.. it’s urgent.. I need ur help.

    ” Now a days memories are large and cheap.Therefore,when a file is opened it is simpler and faster to fetch a new copy of the i-node into the i-node table, rather than searching the entire table to see if it is already there” True or False?? – Justify

  7. @Thomas Scott,

    Thomas, a question : How does your hypothetical keyboard-less OS stores your resume to begin with? or any data? Just curious.

    And what if someone wants to create something new? How do these “most procedures” cope with that then? Sheesh!

    If you don’t understand computers, or are plain simple drunk, why bother commenting at all? Why not just sleep off the effect of the wonderful liquor you have imbibed?

    1. I honestly do not think that this is the place that someone such as abhi, should be making disparaging remarks about someone who has a query about an issue he is facing. He approached this forum of ‘professionals’ to seek advice, and this unscrupulous individual abhi decides to attack his intelligence. For all you know “MR. ABHI”, Mr. Scott could be fairly new to this industry in a new environment about something he understands enough to get the job, but is having difficulty in overcoming an issue. Not everyone is blessed with the omniscience of computer workings. I am pretty sure that if you took your CCNA exam, and failed it the first time, you went and asked for help. Even then, IF you enrolled in school, and learned it from someone else, THEY had the patience to answer your questions when you didn’t know. Give the guy a break. As for you MR. Scott, if these people don’t help you, don’t get discouraged. Keep looking, you’ll find the answer from someone who will be patient enough to give it to you.

      1. ^ Here we have an excellent example of “white knighting.”

        Comprehension, do you know it? You didn’t even read T.Scott’s intelligible gibberish, did you? Yeah, thought not.

  8. hi, all

    i’ve to design ext3 linux file system, so please any body can suggest me, that from where i can get the proper knowledge of ‘ext3 linux file system’ architecture/ structure

  9. hi,
    I wants to find dump(crash dump) file system name and FS type and how many blocks it has? for all unix plat forms linix ,aix,hp-ux and solaris.
    please any body suggest the commands for getting above values.
    Thanks,
    Veera.

  10. Hi All,

    I have a problem.One of my collegue has mistakenly deleted / file system on one of production server[Linux based] which is not coming up.

    We have restored /etc of the same server on some other server in the network. But we are not able to logon to main server.

    Please suggest an alternative so as how to restore my server within 1 hour time.

    Thanks,
    Rahul

  11. hey hi friends,,,, i just want to know that is this uses in real life means any one can use this file system (normal operator) otherwise networking specialists required to remember this only…….. reply

  12. I am searching for the file system which is compatible to Linux kernel 3.4.1. During booting kernel thought NFS I am getting error VFS: Cannot open root device “nfs” or unknown-block(2,0). Is this because i am using filesystem for rootfs which are for kenernal 2.6.32?

  13. I want partition information in the following type…. plzz tell me… if anybdy knows

    Partition/File System Information (Threshold 80% & 85%)
    ————————————————-
    1. /dev/sda1 primary bootable ext4 100M /boot
    2. /dev/sda2 primary swap 200M
    3. /dev/sda3 extended ext3 8000M
    4. /dev/sda4 logical ext4 LVM 3000M /

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