Advanced Operating Systems

1
Advanced Operating Systems
Tonga Institute of Higher Education
Lecture 5 Filesystems
2
Linux Filesystems

• The Linux file system plays a big role in the way
  you interact with Linux. It's important to know
  where things go, how to access them and where to
  find things that you need
• The typical Linux distribution will have a root
  directory at / and a bunch of folders coming
  from that root directory. Usually like,
• cdrom home opt tmp
• dev lib proc usr
• bin etc lostfound
• root var boot floppy
• mnt sbin

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What's Inside those Folders?

• The /bin directory (which you can see by typing
  ls /bin contains many of the most common
  commands you'll use.
• This is where cp and rm and mv all live,
  among hundreds of other of commands that have
  some purpose in Linux
• The /etc directory is generally for
  configuration files.
• This is where the kernel and the operating system
  will expect to find things like the list of users
  on the system, the list of passwords, services,
  modules and so on

4
Looking inside /etc

• Files that are important
• /etc/passwd this contains a list of users and
  their usernames. It used to contain passwords,
  but it was insecure
• /etc/shadow this is where passwords are saved
  in encrypted format, only the root user can see
  this file
• /etc/lilo.conf will show the different
  operating systems available to boot off of
• /etc/group the list of groups on the system
• /etc/hosts contains a list of hostnames and IP
  addresses. Used a lot before DNS became popular
• /etc/fstab information about your partitions
• You'll also find config files for other programs,
  as well as folders about services that we'll talk
  about

5
Filesystems

• The /usr directory is usually reserved for
  programs and commands that all users can use on
  the system.
• Most of your programs will get saved in
  /usr/bin and /usr/sbin
• It's good that this partition has a lot of space,
  because you'll always be adding new programs to
  your system for different users to use.
• The /boot directory is where the Linux kernel
  is saved along with other things necessary to
  boot up your computer after you turn it on.
• If you want to make your computer not work, start
  deleting things from /boot

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Filesystems

• The /root folder is a special place just for
  the root user. No one else will be able to open
  or see anything inside. It's usually a good place
  to save things that you don't want anyone to see
  or use
• The /sbin folder is a special folder of
  commands and programs that only the root user can
  use. These include things like shutdown to turn
  off the computer
• The /tmp folder is for temporary files. Things
  that are not important. Everyone has access to
  read, write and delete files in this folder. It
  will be used by different programs to save things
  that it will delete later

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Filesystems

• The /var is a directory for certain files that
  may change their size (ie. variable size)
• For example, there are a few excellent databases
  for Linux. One is called MySQL. Normally, MySQL
  keeps its data in a subdirectory of /var called
  /var/mysql/. It is also the normal place where
  email servers store their incoming mail. Again,
  email varies in size as well. (/var/mail/ folder)
• The /lib is for library files, that's where the
  name /lib comes from. Programs may use libraries
  to carry out their functions. Different programs
  use the same libraries, so Linux will store them
  here so that every program knows where to find
  them. You will probably not have to worry about
  this directory much unless you start getting
  messages like 'can't find shared library...'.
  That will sometimes happens when you've
  downloaded some program and had to compile it
  yourself from source

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Filesystems

• The /home directory contains users files and
  folders. When you log in, this is the first
  folder you'll find yourself in. Every user on the
  system will have their own home directory that
  they can use to save whatever they need.
• The /proc directory contains information about
  processes that are running and things like how
  much memory they're using. It will also contain
  important things about the system that the kernel
  will need to use. Things like memory, devices and
  filesystems. You can see the files are not even
  writeable for the root user. This is because no
  one should change these things manually

9
Filesystems

• The /dev directory is a folder that contains
  files that represent actually devices that are
  connected to your computer. Things like hard
  drives, CD-Roms, DVD-Drives, floppy drives and so
  on.
• Everything in Linux is just a file and it's no
  different in the /dev directory. When a program
  needs to access a device, like a floppy drive, it
  will access the file for the floppy drive
• If you look inside the /dev drive you'll see
  hundreds of files. You don't have that many
  things connected to your computer, so what are
  they all? Mostly, just in case you want to add
  more things to your computer in the future.

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The real filesystem

• We talked about where data is stored in the
  filesystem, now we need to know exactly about
  filesystems
• MS-Windows, from Windows 95 on up uses file
  systems called 'vfat' (old) or 'ntfs.' (new).
  Linux uses a different system called 'ext3'.
  There are also other types of file systems out
  there. There are actually hundreds of types. They
  are really just different ways to save data on
  your hard disk. You may have chosen to have both
  Windows and Linux installed in your computer.
• Sometimes, you might have to access files in the
  Windows partition of your hard drive.
• You would use a command called 'mount' to do
  that. You would also have to indicate as an
  option in that command that the file system you
  want to access or "mount" is a Windows 'vfat'
  file system

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Mount

• The mount command is the way for Linux to connect
  to a different device or file system. For
  example, to connect to your Microsoft partition,
  if that partition is named /dev/hdb, use...
• root_at_usp root mount -t vfat /dev/hdb
• On the same idea, you may want to store some
  files on a floppy or access data on a floppy that
  someone has given to you.
• You would also have to use the command 'mount'
  before you copied data to or got data from a
  floppy disk. If that person is a Windows user,
  then you would have to indicate once again that
  the file system is 'vfat'. If you want to copy
  data from your Linux partition to a floppy, you
  would have to format and then "mount" the floppy
  as a Linux 'ext2' file system.

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Making floppies

• Most floppies sold on the market are
  pre-formatted for MS-Windows. You can copy data
  from Linux to a Windows formatted floppy with no
  problem. If you want to copy Linux files to a
  floppy to be used in another Linux machine or to
  be used by you later in Linux, then it might be
  better to format the floppy for Linux's ext2 file
  system (one that is a little older than ext3).
• To create a linux floppy "by hand", you would
  place a floppy in the drive and type
• mke2fs /dev/fd0
• The command is an abbreviated way of saying "make
  an ext2 file system". The other part is the
  device, your floppy drive, which is known to
  Linux as "fd0" (floppy drive 0). As we mentioned
  in the first part of the course, everything is a
  file in Linux, including your floppy drive.

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Mounting file systems

• Though some windows managers for Linux have ways
  for clicking on an icon to access a floppy drive,
  usually in Linux the floppy drive or other device
  must be "mounted". That means basically, making
  it temporarily a part of your Linux file system.
  In other words, telling Linux that it is a file
  to be written to or copied from.
• To access a floppy disk from the command line of
  our shell, we would use the command mount and
  type the following (remember you need to be
  working as 'root' to do this)
• mount -t ext2 /dev/fd0 /floppy

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Mounting

• Mounting this floppy assumes a couple of things
• you have a floppy disk in the drive
• in this example, the floppy type
  (indicated by the option -t) is a Linux formatted
  floppy. If you want to mount a Windows floppy,
  change the option to -t vfat
• in your root directory (you can get to
  it by typing cd /), you have a directory called
  floppy. If you don't, you should create it (mkdir
  floppy). Some Linux distributions create this
  automatically during the installation process.
  Some don't.

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Mounting

• When we typed
• mount -t ext2 /dev/fd0 /floppy
• We told Linux that our floppy disk is now part of
  our Linux file system (/dev/fd0) and that any
  files we would like to store on that disk will be
  copied to /floppy, as if it were just another
  directory on our Linux system.
• If you did it right, typing the command 'df' (the
  command to see how much free disk space) should
  include something like this
• /dev/fd0 1390 649 669 49 /floppy

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Mounting

• Now, to copy to and from the floppy disk, you
  would type
• cp my_file /floppy
• If you wanted to create individual subdirectories
  on the floppy, you would first change to the
  /floppy directory
• cd /floppy
• Then you would use the mkdir command to create
  the directories you want. You can also use the
  command
• cp -r my_directory/ /floppy
• to copy the directory automatically to the
  floppy.

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Mounting other devices

• You can also use the mount command to copy to and
  from other devices.
• If you would like to get some files from a
  CD-ROM, the standard command to do this is
• mount -t iso9660 /dev/hdb /cdrom
• The type, iso9660 is the standard file system for
  a CD. The device (/dev/hdb) is the type of CD-ROM
  and the mount point (/cdrom) should exist. If it
  doesn't, you should create it in the root
  directory with 'mkdir', just as you may have done
  with the /floppy directory.
• Remember that the concept of CD-ROM is read only.
  You won't be able to write to this type of CD-ROM
  drive. A message will tell you that when you
  mount this type of device.

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Mounting another partition of the hard disk.

• Many people may have preferred to install Linux
  along with another operating system. You may have
  Linux and Windows installed in the same computer.
  If you would like to access files on the Windows
  partition you would type the following command
• mount -t vfat /dev/hda1 /mnt
• Windows is always in the primary partition, so
  that's why we've used the device /hda1 (hard disk
  partition 1). The choice for /mnt is the standard
  mount point in this case. You may use the /mnt
  directory to mount the other devices (floppies,
  CDs) as well.

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Unmounting file system

• Mounting file systems that aren't part of the
  standard Linux system is considered a temporary
  condition in Linux. Now that we know how to mount
  these outside file systems in Linux, the
  important thing now is to learn how to unmount it
  when we're finished using it.
• In the old days, if you didn't unmount, you could
  mess up your system. Today you can usually get
  away with it, but notice that you can't eject a
  CD until you have unmounted the disc
• To unmount a device, use the command below
• umount /floppy
• Notice, it's umount and NOT unmount. You also
  type the place of the thing you have mounted, not
  the device.

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Permissions for Files

• Linux has inherited from UNIX the concept of
  ownerships and permissions for files.
• This is basically because it was conceived as a
  networked system where different people would be
  using a variety of programs, files, etc.
• Obviously, there's a need to keep things
  organized and secure. We don't want an ordinary
  user using a program that could potentially trash
  the whole system.
• We also don't want someone's wife reading their
  love letters to another woman

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File Permissions

• If you run the command 'ls -l' in your home
  directory, you will get a list of files that may
  include something like this
• -rw-r--r-- 1 bob users 1892 Jul 10 1830
  notes.txt
• This basically says, interpreting this from RIGHT
  to LEFT that the file, notes.txt was created at
  630 PM on July 10 and is 1892 bytes large. It
  belongs to the group users (i.e, the people who
  use this computer). It belongs to bob in
  particular and it is one (1) file. Then come the
  file permission symbols.

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File Permissions

• -rw-r--r-- 1 bob users 1892 Jul 10 1830
  notes.txt
• So what do the symbols 'rw-rr--' mean?
• The slash marks - separate the permissions into
  three types
• The first part refers to the owner's (bob's)
  permissions.
• The slash mark - before the rw means that this
  is a normal file that contains any type of data.
  A directory, for example, would have a 'd'
  instead of a slash mark.
• The rw that follows means that bob can read and
  write to (modify) his own file. That's pretty
  logical. If you own it, you can do what you want
  with it.

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File Permissions

• -rw-r--r-- 1 bob users 1892 Jul 10 1830
  notes.txt
• The second part of the these symbols after the
  second slash, are the permissions for the group.
  Linux can establish different types of groups for
  file access. In a one home computer environment
  anyone who uses the computer can read this file
  but cannot write to (modify) it. This is a
  completely normal situation. You, as a user, may
  want to take away the rights of others to read
  your file.
• After the two slash marks (two here because
  there is no write permissions for the group) come
  the overall user permissions. Anyone who might
  have access to the computer from inside or
  outside (in the case of a network) can read this
  file. Once again, we can take away the
  possibility of people reading this file if we so
  choose.

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Changing Permissions

• 'chmod' is the program that is used to change
  file permissions on a Linux system.
• Others cannot modify your personal user files but
  you may not want other people to even read these
  files. You can use this command to take away the
  possibility of others prying into your private
  stuff.
• The syntax (parts separated by brackets) for
  using this command is the following
• chmod a/o/g/u or - r/w/x

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Chmod

• chmod a/o/g/u or - r/w/x
• The symbols in the first brackets refer to the
  four concepts of users that Linux has.
• aall, all users
• oothers, (other people)
• ggroup, the members of your group
• uuser, (this means you)
• The symbol adds permissions and the symbol -
  takes them away.
• Your actual rights to files - rread rights,
  wwrite rights and xexecutable rights

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Chmod Examples

• Take away read rights for everybody except you
• chmod og-r notes.txt
• Take away executable rights for everybody?
• chmod a-x file.exe
• Give read and executable rights to everyone?
• chmod arx notes.txt
• Take away read right for group?
• chmod g-r notes.txt
• Take away read and write rights from yourselves?
• Chmod u-rw notes.txt

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Chmod with numbers

• You don't need to use the letter abbreviations
  with chmod. You can also use a bunch of numbers
  that will be very difficult to remember at first
• We have three sections to permissions those of
  the owner of the file followed by those of the
  group and then the permissions of others. So if
  you use numbers with 'chmod', there will be three
  numbers following the command. One number
  corresponds to each group. Here's an example of a
  chmod command with numbers
• chmod 644 grocery_list.txt
• The 6 is from the owner, the 4 is for the group
  and the next 4 is for the other people.
• So what do those numbers mean?

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Chmod with numbers

• The three types of permissions correspond to
  three numbers. Read permission is given a value
  of 4. Write permission is given a value of 2.
  Execute permission is given a value of 1. Here's
  what it would look like
• User (or the owner) Group Others
• read-write-execute read-write-execute
  read-write-execute
• 4-2-1 4-2-1 4-2-1
• It's really just simple arithmetic.
• In our example of chmod 644 grocery_list.txt,
  we've added the 4 for read permission and to the
  2 for write permission for the owner to get 6.
  We've just given the group and others read
  permission, so there's nothing to add in these
  two groups. Just give it a 4 in each case. 644.
  And done.

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Chmod examples with numbers

• RWE for owner, RW for group, RW for others?
• chmod 755 file.txt
• RE for owner, RW for group, nothing for others?
• chmod 560 file.txt
• WE for owner, E for group, W for others?
• chmod 312 file.txt
• RWE for owner, nothing for everyone else?
• chmod 700 file.txt
• RW for owner, R for group, R for others?
• chmod 644 file.txt

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Chown Changing owners

• The command 'chown' is the chmod's cousin. It is
  used for changing the ownership rights of a file.
• It does not change the read, write and execution
  permissions however, it only changes who owns the
  file and what group it is in. The command is uses
  like this
• chown ownergroup the_file
• Pretend there is a file in /home/user/ called
  the_file and if you type 'ls -l' the_file you'll
  get something like this
• -rw-r--r-- 1 root root 2428 Nov
  17 1318 the_file
• You can see, root is the owner of the file.
  Therefore, root is the only one who has write
  permissions for the file. If you want to use the
  file, you won't be able to read at all, so let's
  change it to your username
• chown useruser the_file

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Chown

• This example assumes your Linux version creates
  groups for each user. There are others that will
  create a generic group called users for everybody
  who uses the computer.
• On a network, groups are created according to the
  needs of the organization. On your single home
  computer, just type ls -l and see what system
  corresponds to you.
• As you can see, 'chown' is absolutely necessary
  if you're working as more than one user with the
  computer.

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Learning more about filesystems

• Filesystems are created on disk partitions to
  enable applications to store and organize data in
  the form of files and directories.
• Linux, like Unix systems, uses a hierarchical
  filesystem composed of files and directories,
  which can contain either files or other
  directories.
• The files and directories in a Linux filesystem
  are made available to users by mounting them
  using the Linux "mount" command, which is
  generally done as part of a Linux system's
  startup process.
• The list of filesystems that are available for
  use is stored in the file /etc/fstab (which
  stands for filesystem table),
• While the list of filesystems that are currently
  mounted is maintained by Linux in the file
  /etc/mtab (which stands for mount table).

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More filesystems

• As each filesystem is mounted during the boot
  process, a bit in the filesystem header
  (popularly known as the "clean bit") is cleared,
  which indicates that the filesystem is in use and
  that the data structures used to manage
  allocation and file and directory organization in
  that filesystem may actively be changing.
• When a Linux system crashes or is simply turned
  off without going through the standard shutdown
  procedure, the clean bit in filesystem headers is
  not set. The next time the system boots, the
  mount process detects filesystems that are are
  not marked as being clean, and manually verifies
  their consistency using the Linux/Unix 'fsck'
  (filesystem check) utility.

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Journaling

• In order to reduce filesystem inconsistencies and
  minimize system restart time, journaling
  filesystems keep track of the changes that they
  will be making to the filesystem before they
  actually make them to the filesystem.
• These records are stored in a separate part of
  the filesystem, typically known as the "journal"
  or "log".
• Once these journal records (also commonly known
  as "log" records) are safely written, a
  journaling filesystem applies those changes to
  the filesystem and then purges those entries from
  the log.
• Journal records are organized into sets of
  related filesystem changes, much like changes
  made to a database are organized into
  transactions.

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Journaling filesystems

• There are a number of journaling filesystems
  available for Linux.
• The best known of these are XFS, a journaling
  filesystem originally developed by Silicon
  Graphics
• The ReiserFS, a journaling filesystem developed
  especially for Linux,
• JFS, a journaling filesystem originally developed
  by IBM
• And now, the ext3 filesystem

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Ext3 Filesystem

• RedHat 8.0 and above now use the ext3 filesystem
  (you can see it the /etc/fstab file)
• The ext3 filesystem is a journaling version of
  the Linux ext2 filesystem.
• The ext3 filesystem has one significant advantage
  that no other journaling filesystem has - it is
  totally compatible with the ext2 filesystem. It
  can therefore make use of all of the existing
  applications that have already been developed to
  manipulate and fine-tune the ext2 filesystem.
• Reiserfs is another, well developed, journaling
  file system. Slackware lets you choose this type

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Conclusion

• The layout of the filesystem in Linux is one of
  the most important things you can know about.
• Knowing where to find things and where to put
  things is a big key in keeping your system up to
  date and working well
• Knowing about the filesystem that underlies your
  system will give you a better understanding about
  what is happening to your Linux computer.