08.file system

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Linux File Systems and File

Minsuk LeeHansung University, Seoul, Korea

[email protected]

http://oss.or.kr/koss_forum_1


Linux File System

• In Linux, everything is a file.– Its attribute is different

• Regular File, Directory, Link, Device

• Every file has owner and group.• Every file has permission for– owner, group, and others

• Prebuilt directory architecture :– /etc, /dev, /usr, /sbin, /…

DON’T CHANGE IT !YOU MANAGE YOUR OWN HOME DIRECTORY ~/



Attribute of a File

• $ ls -l

Attr perm #link owner group file size date time filename----------------------------------------------------------------------------------------rw-r--r-- 1 myuser myuser 164 2011-08-28 14:25 myfile - rwx r-x

r— 0 7 5 4Attribute user group other

Attribute:- : regular file b : block devicec : character de-viced : directoryl : symbolic link p : pipes : socket

File name :256 charactersCase sensitiveNo extensionMost special characters ac-cepted

$ chmod, $chown work here !

permission:r : readablew : writablex : executable (traversable)



• Inode has information of a file– Inode also exists in disk– All the attributes, times, owner– Data block locations

Inode (metadata for a File)

inode



File link• Link (hard link), Symbolic Link

– myfile and myfile-link shares same disk space• One good usage of hard link is to protect not to be erased acciden-

tally

– myfile-sym-link is a link file with the target file name in it

Inode 블럭

Data 블럭디렉토리



Permission Masking

• Default permission setting– ‘$ umask mask ‘ : masking 666 (rw-rw-rw-) with mask – default mask value = 022 (rw-r--r--)– ‘x’ field cannot be masked



• Some commands needs to change other’s file

Need some mechanism to run command with file owners permis-sion

• $ chmod u+s <executable-file>– Sets UID field in inode attributes– Makes rwx to rws

Set UID option !

THINKSECURITY

!!



The Root Directory ‘/’ (1)



The Root directory (2)• / : the root directory (top most directory in the system)

– All absolute directory path starts with ‘/’

–There is NO ‘C:\’, ‘D:\’,– every storage is mounted somewhere in directory hier-

archy• /cdrom, /mnt/floppy, …



/dev (1)• Device special files : the abstraction to access hardware

devices– block devices : hdd, cdrom, floppy disk, tape, USB memory– character devices : serial port, …– other devices



/dev (2)

• IDE hdd– brw-rw---- : b block device, user/group :

rw– Owner : root, group : disk– Device Major, minor number : 8,0– /dev/sda : the Whole IDE disk• /dev/sda1, /dev/sda2, /dev/sda5 : partitions



Partition table



Other directories (1)• /bin : Most frequently used commands • /etc : Configuration File directory– System itself, servers, applications, …

• /home : user’s home directories• /lib : shared libraries, kernel modules• /lost+found : area for file system check utility• /mnt : default mount point (not mandatory)– For removable storages (cdrom, usb, floppy, ...)

• /root : home directory for superuser• /sbin : commands for system management– fsck, ifconfig, lsmod, mkfs, reboot…



Other directories (2)

• /tmp : directory for temporary data • /var : directory for frequently changed data

– Logs, mail spool, print spool, lock files• /sys. /proc : system monitoring (virtual)• /usr : commands/library/data for users

• /boot : files for Boot– Booting Process

• PowerON ROM-BIOS (read MBR) BootLoader Kernel• For embedded devices there is no ROM-BIOS



Locating files• $ which <filename>

– Search execution file in PATH

• $ whereis <filename>– Search Execution file, source, man page file

• $ locate <filename-pattern>– Search system for the pattern

• Works like ‘$ find ./ -name *pattern*’

– Needs ‘$ updatedb’ to refresh database



Let’s practice !

• Make link and symbolic link for a file– See what happen if erase files• Erasing the link file, erasing the original file

• Apply umask and generate file with $vi


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08.file system