RH033 Unit16 The Linux Filesystem In-Depth

Ojectives

Upon completion of this unit, you should be able to:

• Describe how filesystem information is organized
• Describe the function of dentries and inodes
• Describe how cp, mv and rm work at the inode level
• Create symbolic links and hard links
• Access removable media
• Create archives using tar and gzip

Partitions and Filesystems

1) Disk drives are divided into partitions

2) Partitions are formatted with filesystems, allowing users to store data

• Default filesystems: ext3, the Third Extended Linux Filesystem
• Other common filesystems:
• ext2 and msdos (typically used for floppies)
• iso9660 (typically used for CDs)
• GFS and GFS2 (typically used for SANs)

Inodes

1) An inode table contains a list of all files in an ext2 or ext3 filesystem

2) An inode (index node) is an entry in the table, containing information of a file (the metadata), including:

• file type, permissions, UID, GID
• the link count (count of path names pointing to this file)
• the file’s size and various stamps
• pointers to the file’s data blocks on disk
• other data about the file

Directories

1) The computer’s reference for a file is the inode number

2) The human way to reference a file is by file name

3) A directory is a mapping between the human name for the file and the computer’s inode number

cp and inodes

The cp command:

• Allocate a free inode number, placing a new entry in the inode table
• Creates a dentry in the directory, associating a name with the inode number
• Copies data into the new file

mv and inodes

1) If the destination of the mv command is on the same file system as the source, the mv command:

• Creates a new directory entry with the new file name
• Deletes the old directory entry with the old file name

2) Has no imact on the inode table (except for a time stamp) or the location of data on the disk: no data is moved!

3) If the destination is a different filesystem, mv acts as a copy and remove

rm and inodes

1) The rm command:

• Decrements the link count, thus freeing the inode number to be reused
• Places data blocks on the free list
• Removes the directory entry

2) Date is not actually removed, but will be overwritten when the data blocks are used by another file

Hard Links

1) A hard link adds an additional pathname to reference a single file

• One physical file on the filesystem
• Each directory references the same inode number
• Increments the link count
• The rm command decrements the link count
• File exists as long as at least one link remains
• When the link account is zero, the file is removed
• Cannot span drives or partitions

2) Syntax: ln filename [linkname]

Symbolic (or Soft) Links

1) A symbolic link points to another file

• ls �Cl displays the link name and the referenced file
• File type: l for symbolic link
• The content of a symbolic link is the name of the file that is references

2) Syntax: ln �Cs filename linkname

The Seven Fundamental Filetypes

ls �Cl symbol	File Type
-	regular file
d	directory
l	symbolic link
b	block special file
c	character special file
p	named pipe
s	socket

Checking Free Space

1) df �C Reports disk space usage

• Reports total kilobytes, kilobytes used, kilobytes free per file system
• -h and �CH display sizes in easier to read unit

2) du �C Reports disk space usage

• Reports kilobytes used per directory
• Includes subtotals for each subdirectory (-s option only reports single directory summary)
• Also takes �Ch and �CH options

3) Applications �C> System Tools �C> Disk Usage Analyzer or baobab �C Reports disk space usage graphically

Removable Media

1) Mounting means making a foreign filesystem look like part of the main tree

2) Before accessing, media must be mounted

3) Before removing, media must be unmouted

4) By default, non-root users may only mount certain devices (cd, dvd, floppy, usb, etc)

5) Mountpoints are usually under /media

Mounting CDs and DVDs

1) Automatically mounted in Gnome/KDE

2) Otherwise, must be manually mouted

• CD/DVD Reader �C mount /media/cdrom
• CD/DVD Writer �C mount /media/cdrecorder

3) eject command unmounts and ejects the disk

Mounting USB Media

1) Detected by the kernel as SCSI devices

• /dev/sdaX or /dev/sdbX or similar

2) Automatically mounted in Gnome/KDE

• Icon created in computer window
• Mounted under /media/Device ID �C Device ID is built into device by vendor

Mounting Floppy Disks

1) Must be manually mounted and unmounted

• mount /media/floppy
• umount /media/floppy

2) DOS floppies can be accessed with mtools

• Mounts and unmounts device transparently
• uses DOS naming conventions
• mdir a:
• mcopy /home/file.txt a:

Archiving Files and Compressing Archives

1) Archiving places many files into on target file

• Easier to backup, store, and transfer
• tar �C standard Linux archiving command

2) Archives are commonly compressed

• Algorithm applied that compressed file
• Umcompressing retores the original file
• tar natively supports compression using gzip and gunzip, or bzip2 and bunzip2

Creating, Listing, and Extracting File Archives

1) Action arguments (one is required)

• -c create an archive
• -t list an archive
• -x extracts files from an archive

2) Typically requried:

• -f archviename name of file archive

3) Optional arguments:

• -z use gzip compression
• -j use bzip2 compression
• -v be verbose

Creating File Archives: Other Tools

1) zip and unzip

• Supports pkzip-compatible archives

2) file-roller

• Graphical, multi-format archiving tool

End of Unit16

1) Questions and Answers

2) Summary

• Linux filesystem structure
• Using removable media
• Using unformatted floppies
• Archiving and compression