Unix / Linux – File System Basics.

In this topic we will learn about Unix / Linux – File System Basics.A file system is a logical collection of files on a partition or disk. A partition is a container for information and can span an entire hard drive if desired.

Your hard drive can have various partitions which usually contain only one file system, such as one file system housing the /file system or another containing the /home file system.

One file system per partition allows for the logical maintenance and management of differing file systems.

Everything in Unix is considered to be a file, including physical devices such as DVD-ROMs, USB devices, and floppy drives.

Directory Structure

Unix uses a hierarchical file system structure, much like an upside-down tree, with root (/) at the base of the file system and all other directories spreading from there.

A Unix / Linux – File System Basics is a collection of files and directories that has the following properties โˆ’

  • It has a root directory (/) that contains other files and directories.
  • Each file or directory is uniquely identified by its name, the directory in which it resides, and a unique identifier, typically called an inode.
  • By convention, the root directory has an inode number of 2 and the lost+found directory has an inode number of 3. Inode numbers 0 and 1 are not used. File inode numbers can be seen by specifying the -i option to ls command.
  • It is self-contained. There are no dependencies between one filesystem and another.

The directories have specific purposes and generally hold the same types of information for easily locating files. Following are the directories that exist on the major versions of Unix โˆ’

Sr.No.Directory & Description
1/This is the root directory which should contain only the directories needed at the top level of the file structure
2/binThis is where the executable files are located. These files are available to all users
3/devThese are device drivers
4/etcSupervisor directory commands, configuration files, disk configuration files, valid user lists, groups, ethernet, hosts, where to send critical messages
5/libContains shared library files and sometimes other kernel-related files
6/bootContains files for booting the system
7/homeContains the home directory for users and other accounts
8/mntUsed to mount other temporary file systems, such as cdrom and floppy for the CD-ROM drive and floppy diskette drive, respectively
9/procContains all processes marked as a file by process number or other information that is dynamic to the system
10/tmpHolds temporary files used between system boots
11/usrUsed for miscellaneous purposes, and can be used by many users. Includes administrative commands, shared files, library files, and others
12/varTypically contains variable-length files such as log and print files and any other type of file that may contain a variable amount of data
13/sbinContains binary (executable) files, usually for system administration. For example, fdisk and ifconfig utlities
14/kernelContains kernel files

Navigating the File System

Now that you understand the basics of the file system, as you can begin navigating to the files you need. These commands are used to navigate the system โˆ’

Sr.No.Command & Description
1cat filenameDisplays a filename
2cd dirnameMoves you to the identified directory
3cp file1 file2Copies one file/directory to the specified location
4file filenameIdentifies the file type (binary, text, etc)
5find filename dirFinds a file/directory
6head filenameShows the beginning of a file
7less filenameBrowses through a file from the end or the beginning
8ls dirnameShows the contents of the directory specified
9mkdir dirnameCreates the specified directory
10more filenameBrowses through a file from the beginning to the end
11mv file1 file2Moves the location of, or renames a file/directory
12pwdShows the current directory the user is in
13rm filenameRemoves a file
14rmdir dirnameRemoves a directory
15tail filenameShows the end of a file
16touch filenameCreates a blank file or modifies an existing file or its attributes
17whereis filenameShows the location of a file
18which filenameShows the location of a file if it is in your PATH

The df Command

The first way to manage your partition space is with the df (disk free) command. The command df -k (disk free) displays the disk space usage in kilobytes, as shown below โˆ’

$df -k
Filesystem      1K-blocks      Used   Available Use% Mounted on
/dev/vzfs        10485760   7836644     2649116  75% /
/devices                0         0           0   0% /devices

Some of the directories, such as /devices, shows 0 in the kbytes, used, and avail columns as well as 0% for capacity. These are special (or virtual) file systems, and although they reside on the disk under /, by themselves they do not consume disk space.

The df -k output is generally the same on all Unix systems. Here’s what it usually includes โˆ’

Sr.No.Column & Description
1FilesystemThe physical file system name
2kbytesTotal kilobytes of space available on the storage medium
3usedTotal kilobytes of space used (by files)
4availTotal kilobytes available for use
5capacityPercentage of total space used by files
6Mounted onWhat the file system is mounted on

You can use the -h (human readable) option to display the output in a format that shows the size in easier-to-understand notation.

The du Command

The du (disk usage) command enables you to specify directories to show disk space usage on a particular directory.

This command is helpful if you want to determine how much space a particular directory is taking. The command displays number of blocks consumed by each directory. A single block may take either 512 Bytes or 1 Kilo Byte depending on your system.

$du /etc
10     /etc/cron.d
126    /etc/default
6      /etc/dfs

The -h option makes the output easier to comprehend โˆ’

$du -h /etc
5k    /etc/cron.d
63k   /etc/default
3k    /etc/dfs

Mounting the File System

A file system must be mounted in order to be usable by the system. To see what is currently mounted (available for use) on your system, use the following command โˆ’

$ mount
/dev/vzfs on / type reiserfs (rw,usrquota,grpquota)
proc on /proc type proc (rw,nodiratime)
devpts on /dev/pts type devpts (rw)

The /mnt directory, by the Unix convention, is where temporary mounts (such as CDROM drives, remote network drives, and floppy drives) are located. If you need to mount a file system, you can use the mount command with the following syntax โˆ’

mount -t file_system_type device_to_mount directory_to_mount_to

For example, if you want to mount a CD-ROM to the directory /mnt/cdrom, you can type โˆ’

$ mount -t iso9660 /dev/cdrom /mnt/cdrom

This assumes that your CD-ROM device is called /dev/cdrom and that you want to mount it to /mnt/cdrom. Refer to the mount man page for more specific information or type mount -h at the command line for help information.

After mounting, you can use the cd command to navigate the newly available file system through the mount point you just made.

Unmounting the File System

To unmount (remove) the file system from your system, use the umount command by identifying the mount point or device.

For example, to unmount cdrom, use the following command โˆ’

$ umount /dev/cdrom

The mount command enables you to access your file systems, but on most modern Unix systems, the automount function makes this process invisible to the user and requires no intervention.

User and Group Quotas

The user and group quotas provide the mechanisms by which the amount of space used by a single user or all users within a specific group can be limited to a value defined by the administrator.

Quotas operate around two limits that allow the user to take some action if the amount of space or number of disk blocks start to exceed the administrator defined limits โˆ’

  • Soft Limit โˆ’ If the user exceeds the limit defined, there is a grace period that allows the user to free up some space.
  • Hard Limit โˆ’ If the hard limit is reached, regardless of the grace period, no further files or blocks can be allocated.

There are a number of commands to administer quotas โˆ’

There are a number of commands to administer quotas โˆ’

Sr.No.Command & Description
1quotaDisplays disk usage and limits for a user of group
2edquotaThis is a quota editor. Users or Groups quota can be edited using this command
3quotacheckScans a filesystem for disk usage, creates, checks and repairs quota files
4setquotaThis is a command line quota editor
5quotaonThis announces to the system that disk quotas should be enabled on one or more filesystems
6quotaoffThis announces to the system that disk quotas should be disabled for one or more filesystems
7repquotaThis prints a summary of the disc usage and quotas for the specified file systems

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