1. BILKENT UNIVERSITY DEPARTMENT OF COMPUTER TECHNOLOGY AND INFORMATION SYSTEMS CTIS156 INFORMATION TECHNOLOGIES II FILES AND FILE SYSTEM STRUCTURE

2. Files And File System Structure ● Objectives – To explain the Linux file concept – To discuss various types of files supported by Linux – To describe the attributes of a file – To explain the notion of pathnames – To explain the user view of a linux file system – To discuss representation of a file inside the Linux file system – To describe how a Linux file is stored on the disk 2 CTIS156 INFORMATION TECHNOLOGIES II

3. Files And File System Structure ● Introduction: – Most computer system users work with the file system structure: ● Creating, ● reading, ● writing/modifying and ● executing files – Therefore the user needs to understand: ● What a file in Linux ● How files can be organized and managed ● How they are represented inside the OS ● How they are stored on the disk 3 CTIS156 INFORMATION TECHNOLOGIES II

4. Files And File System Structure ● The Linux File Concept – In Linux, a file is a sequence of bytes – Everything (including all devices) is treated as a file in Linux ● A simple file (text/executable) ● A directory ● A keyboard ● A printer ● A disk drive ● A network interface card, etc.. 4 CTIS156 INFORMATION TECHNOLOGIES II

5. Files And File System Structure ● Types of Files – Linux supports five file types: ● Simple/ordinary files ● Directory ● Symbolic(soft) link ● Special file ● Named pipe 5 CTIS156 INFORMATION TECHNOLOGIES II

6. Files And File System Structure ● Types of Files (cont) – Simple/ordinary files: ● Used to store information and data on a secondary storage device (a disk) ● Can contain a – C, C++, Java source code – an executable program (a compiler, a text editor, etc.) – Pictures, audio, graphics, etc ● They are not treated differently by Linux ● However they are treated differently by application programs (e.g. a C compiler treats.c file as a source file) 6 CTIS156 INFORMATION TECHNOLOGIES II

7. Files And File System Structure ● Types of Files (cont) – Simple/ordinary files (cont): ● File names can be up to 255 characters ● Any characters can be used in a filename ● Nonprintable characters are not recommended (e.g. A space character) ● File extensions mean nothing to Linux ● Certain file extensions are given and may be required by the applications (e.g..c file extension is required by C compilers) 7 CTIS156 INFORMATION TECHNOLOGIES II

8. Files And File System Structure ● Types of Files (cont) – Simple/ordinary files (cont): ● Commonly used extensions for some applications: 8 CTIS156 INFORMATION TECHNOLOGIES II

9. Files And File System Structure ● Types of Files (cont) – Directory: ● Contains the names of files and/or other directories ● A directory file contains an array of directory entries. – Structure of a directory entry: 9 CTIS156 INFORMATION TECHNOLOGIES II

10. Files And File System Structure ● Types of Files (cont) – Directory: (cont) ● Inode number (unique number) is a four byte long, index value of an array on the disk ● An element of this array, known as an index node (or inode) contains file attributes: – File size – File owner – Time created – Time last modified – Access permissions, etc. ● Linux allocates and inode, whenever a new file is created 10 CTIS156 INFORMATION TECHNOLOGIES II

11. Files And File System Structure ● Types of Files (cont) – Link File: ● Contains a symbolic link to a file in another directory ● Can have a different name than the original file ● Allows multiple access points (or names) to the original file 11 CTIS156 INFORMATION TECHNOLOGIES II

12. Files And File System Structure ● Types of Files (cont) – Special File (Device): ● Every hardware device (screen, keyboard, harddisk, Cdrom, etc) is associated a special file in Linux ● All hardware devices are accessed through those special files ● Special files reside in /dev directory ● Two types of special files: – Character special files: For character-oriented devices (keyboard) – Block special files: For block-oriented devices (a harddisk) 12 CTIS156 INFORMATION TECHNOLOGIES II

13. Files And File System Structure ● Types of Files (cont) – Special File (Device)(cont): ● Some special files are: – fd0 (for floppy drive 0) – hda (for harddisk a) – lp0 ( for line printer 0) – tty (for teletype terminal) 13 CTIS156 INFORMATION TECHNOLOGIES II

14. Files And File System Structure ● Types of Files (cont) – Named Pipe (FIFO): ● A file used by the two processes to communicate with each other (IPC) ● File System Structure: 14 CTIS156 INFORMATION TECHNOLOGIES II

15. Files And File System Structure 15 CTIS156 INFORMATION TECHNOLOGIES II

16. Files And File System Structure ● File System Structure (cont): – Home and Present Working Directories: ● When you logon, you are placed in a special directory, called your home directory (To see enter “echo $HOME”) ● Home directory is denoted ~ (tilda) character ● The directory that you are in at a particular time is called your present working directory (or current directory) (To see enter “pwd”) ● Present working directory is denoted by. (dot) ● The parent of the present directory is denoted.. (dotdot) 16 CTIS156 INFORMATION TECHNOLOGIES II

17. Files And File System Structure ● File System Structure (cont): – Pathnames: Absolute And Relative ● A file or directory in a hierarchical file system is specified by a pathname ● Pathnames can be specified in three ways: – Starting with the root directory – Starting with the present working directory – Starting with the users home directory ● Pathnames that start with the root directory is called absolute pathname ● Pathnames that start with the home or present directories are called relative pathnames 17 CTIS156 INFORMATION TECHNOLOGIES II

18. Files And File System Structure ● File System Structure (cont): – Pathnames: Absolute And Relative (cont) ● Examples: /home/faculty/sarwar/courses/ee446/exams/mid1 (absolute pathname for file mid1) ~/courses/ee446/exams/mid1 (relative pathname: user sarwar's home directory is /home/faculty/sarwar) If the user is in ee446 directory (present directory), the relative pathname can be./exams/mid1 or just exams/mid1 18 CTIS156 INFORMATION TECHNOLOGIES II

19. Files And File System Structure ● File System Structure (cont): – Some Standard Directories and Files: ● Every Linux system contains a set of standard files and directories according to the File System Standard (FSSTND) proposed in 1994. ● The purposes of each directory: Root Directory (/): At the top of the file system hierarchy. It is the master cabinet that contains all directories and files /bin : Contains binary (executable) images of most essential Linux commands for system administrators and users 19 CTIS156 INFORMATION TECHNOLOGIES II

20. Files And File System Structure ● File System Structure (cont): /boot : Contains all the files needed to boot the Linux system, including the binary image of the LINUX kernel (e.g. vmlinuz-2.6.8.1-10mdk) /dev : Contains special files corresponding to the devices (terminals, disk drives, cdrom, printer, etc) /etc : Contains system configuration files (text files) All these files can only be edited by system administrators. /etc/passwd contains user information /home : Contains users' home directories 20 CTIS156 INFORMATION TECHNOLOGIES II

21. Files And File System Structure ● File System Structure (cont): /lib : Contains shared libraries for C, C++, and Fortran programming languages /lost+found : Contains all files that are not connected to a directory. These files are found by a Linux tool fsck. /mnt : It is used by the system administrators to mount file systems to devices (disk, cdrom, floopy, etc.) temporarily 21 CTIS156 INFORMATION TECHNOLOGIES II

22. Files And File System Structure ● File System Structure (cont): /proc : Contains process and system information. Used by the kernel /root : It is the home directory of the root user /sbin : /usr/sbin : /usr/local/sbin : Contains system administration command files that can only be run by the root user /tmp : Contains temporary files and directories. The contents are deleted periodically set by the administrator 22 CTIS156 INFORMATION TECHNOLOGIES II

23. Files And File System Structure ● File System Structure (cont): /usr : One of the largest sections in Linux file system. Contains all shared add-on software packages, libraries and documents. A new software package should be installed here /var : Contains the variable data. / var/spool/mail contains incoming mail of users, /var/log contains all system logs. 23 CTIS156 INFORMATION TECHNOLOGIES II

24. Files And File System Structure ● Navigating The File Structure – Commands : ● echo $HOME (to see the home dir) ● pwd (to see the present working dir) ● cd(to change dir) ● ls (to display the contents of a dir) ls -a (to display all files) ls -l (to display in long format) ls -i (to display the inode of a file or dir) ls -F(to display the file types) ● mkdir (to create directories) ● rmdir (to remove an empty directory) ● mount(to see which device is mounted to which directory in the file system) 24 CTIS156 INFORMATION TECHNOLOGIES II

25. Files And File System Structure ● Navigating The File Structure – Determining File Attributes : ● You can see the file attributes by “ ls -l ” command 25 CTIS156 INFORMATION TECHNOLOGIES II

26. Files And File System Structure ● Navigating The File Structure – Determining File Attributes : ● Using shell metacharacters with ls command: ls -l course* ls -l *.c ls -l ???.c ls -l ~/courses/ee446/lab[!5]*.c ls -i [a-zA-Z]??[1-5].html ls ~/ [!0-9]*.[c,C] ● Determining file types: file *(to see file type) 26 CTIS156 INFORMATION TECHNOLOGIES II