The UNIX File System

The UNIX

File System

CS465

File Systems • What is a file system?

A means of organizing information on the computer. A file system is a logical view, not necessarily a physical view.

• What does the file system provide:

– Ways to create, move, and remove files

– Ways to order files

– Security

• Examples of file systems:

– DOS, Macintosh, CD-ROM, UNIX, NFS (networked file system)

UNIX File System

• Heirarchical storage

– Basic storage unit is a FILE

• Unix File: a collection of data

– Just a sequence of bytes

– No record or block structure is required

Unix File Types

• Ordinary file (stores info)

• Directory (holds other files and directories)

– Directory file contains information about files

• Special file (represents physical devices like printers, terminals, etc)

• Pipe (temporary file for command linkage)

File and Directory Names

• Names are case sensitive

• Can use any character, however:

– Avoid using: / > , ! ( ) # & < > | @ $ ^ * ?

• Legal but hard to use with shells

• Length: Up to 14 characters guaranteed

• Names must be unique inside a directory

• Files beginning with “.” are “invisible”

The Unix File SystemThe Unix File System

/ is the “root” directory.In our example there are three directories

in root.

The “home” directory contains directories of all

users on a system.

Each user can create and administer his/her own directories,

subdirectories and files.

progs notes .login

jones smith username

home etc tmp

/

A rooted tree file structure (inverted tree)

Some Common DirectoriesSome Common Directories

/ root ancestor of all files in the file system

/bin binary executable files

/dev special device files

/etc administrative files

/home user home directories

/tmp temporary files

/usr special user files or home directories

Pathnames• Absolute Path Name

– Every file and directory in the file system can be identified by a “full path name” (route from root to file)/home/sue/email/f1

• Relative path name– Location is relative to working directory

. Working directory.. Parent directory

– If working directory is /home/sue:email/f1

fred

/

home

sue

docs email

f1 f2

Home and Working Directories• Home directory

– The directory you are in when you first login in– This is your space; you control security– Place to put your personalized startup files– Your working directory after typing cd with no arguments

• Working (current) directory– Can access files in your working directory by simply typing the

filename (relative pathname)– To access files in other directories, must use the absolute

pathname pwd prints the working directory

cd changes the working directory

Directory ShorthandsDirectory abbreviations:

. the directory itself

.. the parent directory Every directory contains . and .. files

In most shells “~” means your home directory)~user means user’s home directory

Example: ~small000/.login

is file .login in /home/small000, which is my home directory.

Finding YourselfFinding Yourself

• The command pwd tells you where you are in the file hierarchy.

• It gives you the absolute path to your location.

$ pwd

/home/small000

Changing DirectoriesChanging Directories

cd change directory (home)

cd .. go to parent directory

cd / go to the root directory

cd ~ go to my home directory

cd ~user go to user’s home directory

cd /etc go to the etc directory from root

cd ../sub go to the sub directory in myparent directory

Creating and Removing Directories

• A new directory can be created with the mkdir command– Note command cannot be shortened to md

• A directory can be removed using the rmdir command– The directory must be empty (no files or

subdirectories)– Note command cannot be shortened to rd

Links

• Can have many links to the same file

• One directory contains the actual file, and the others contain a name only, which links to the actual file

• ln -command for creating links

$ ln filename linkname

Linksln creates a new link, not a new file. The new link and the original filename are equivalent pointers to the file.

Example:

$ ln names lnames

jones

lnames namesletter3

007

Golden Eye

Tomorrow Never Dies

FileContents

Creating Directories

mkdir – Makes a new directory (if you have permission to do

so) – With a simple pathname, mkdir makes a new

directory in your working directory.

$ pwd/home/jbond$ mkdir newdir$ ls –ltotal 3-rw-r--r-- 1 jbond cs 154 Feb 4 15:00 letter3-rw-r--r-- 1 jbond cs 64 Feb 4 15:00 namesdrwxr-xr-x 2 jbond cs 512 Feb 4 15:26 newdir/

Deleting Directories

rmdir deletes a directory (if you have permission).

$ rmdir newdir

$ ls -l

total 6-rw-r--r-- 1 jbond cs 154 Feb 4 15:00 letter3

-rw-r--r-- 1 jbond cs 64 Feb 4 15:00 names

$ rmdir /usrrmdir: directory "/usr": Search or write permission needed

Moving Stuff

mv can move a directory into a directory

$ lsnames newdir/ secret/$ mv newdir secret$ lsnames secret/$ ls secretletter newdir/

Access Permissions

• Every user has:– A username– A numeric uid (user identification) – A default group association– Optional assocations with other groups

• Command:id view your uid and default group

and which groups you belong to

Access Permissions• Every file has:

– A single owner

– An association with a single group

– A set of access permissions associated with it

• For a File, permissions control what can be done to the file contents

• For a Directory, permissions control whether a file in that directory can be listed, searched, renamed or removed

Changing Your Group Association

• When you first log in, you're group is set to the default group specified in your /etc/passwd file.  

• To change groups, use the newgrp command$ newgrp admin

• Once you've changed to the new group, if you create a new file, it will be owned by the new group.

Access Permissions• First character shows the file type:

– directory (d)

– plain file (-)

– link (l) -rwxr-xr-x-rwxr-xr-x Rest specify three types of users:

- user owner- group- others

rwxrwxr-xr-xr-xr-x

who are allowed to: (r) read (w) write (x) execute

Permission Settings

Permission For a File For a Directory

r (read) Contents can be viewed or printed.

Contents can be listed, but not searched. Normally r and x are used together.

w (write) Contents can be changed or deleted.

File entries can be added or removed.

x (execute) File can be run as a program.

Directory can be searched and you can cd to it.

Permission SettingsPermission Settings• Permission settings use octal numbers.

– r = 100 = 4

– w = 010 = 2

– x = 001 = 1

– None = 000 = 0

• These numbers are additive.

– rwx = 7 (4 + 2 + 1) = 111

– rw = 6 (4 + 2) = 110

– rx = 5 (4 + 1) = 101

• [rwx][r-x][r--] = [111][101][100] = [7][5][4] = 754

owner group others

-rwxrwxrwx-rwxr-xr-x-rw-r--r---r--------

Numerical Access Permissions

111111111 777777

101101111 755755

100100110 644644

000000100 400400

Permission Settings: ExamplesUse ls –l to view permission settings-r-- --- --- (400) protect it from accidental editing-rw- --- --- (600) only you can edit/read the file -rw- r-- r-- (644) only you can edit, others can read-rw- rw- rw- (666) public read file!

dr-x r-x r-x (555) anyone can list but can’t create/delete/rename files

dr-x --- --- (500) only you can listdrwx --- --- (700) you can do anything, but not othersdrwx r-x r-x (755) you can do anything, others only listdrwx rwx rwx (777) anyone can do anything!

Changing Permissions

chmod command is used to modify permissions.can only be used by the owner of a file/dir

(or the administrator root).

Format:

chmod [ugoa] [+-=] [rwx] [file/dir]

– Optionally, one of the characters: u (user/owner), g (group), o (other), or a (all).

– Optionally, one of the characters: + (add permission), - (remove permission), or = (set permission).

– Any combination of the characters r (read), w (write), or x (execute).

chmod Examples

• Character Method$ chmod a=r-x file$ chmod u=rw- file$ chmod ugo+r file$ chmod go-w file

• Numerical Method$ chmod 744 file$ chmod 600 file

Default Permissions

• Every time a directory or file is created it must immediately have some permissions

• The umask instruction sets the default permissions

$ umask ddd• ddd is a three digit octal number

umask• The system’s initial permission value, used when a file

is created, is 666 (rw-rw-rw-).

• The system’s initial permission value used when a directory is created, is 777 (rwxrwxrwx).

• To determine the umask value you want to set, subtract the value of the permissions you want from the initial permissions. The remainder is the value to use with the umask command.

• To change the default mode for files to 644 (rw-r--r--), subtract 644 from 666. You get 022, which is the value you would use as an argument to the umask command.

File and Directory Permissions for umask Values

unmask Octal Value File Permissions Directory Permissions

0 rw- rwx

1 rw- rw-

2 r-- r-x

3 r- r--

4 -w- -wx

5 -w -w-

6 --x --x

7 --- --- (none)

umask Scope

• The new umask value affects only those files and directories that are created from this point forward.

• Place a umask command in your .profile file to permanently set your default permissions.

The SuperUser

• Every Unix system has at least one userid which is special

• This is referred to as root although the name may be different

• root may access any and all files or directories, no matter what their protection bits are set to