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 –l 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 –s filename linkname
The Seven Fundamental Filetypes
| ls –l 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 – Reports disk space usage
- Reports total kilobytes, kilobytes used, kilobytes free per file system
- -h and –H display sizes in easier to read unit
2) du – Reports disk space usage
- Reports kilobytes used per directory
- Includes subtotals for each subdirectory (-s option only reports single directory summary)
- Also takes –h and –H options
3) Applications –> System Tools –> Disk Usage Analyzer or baobab – 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 – mount /media/cdrom
- CD/DVD Writer – 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 – 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 – 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
