1 File System of Unix 2 References “UNIX&Linux Shell 設計程式篇 ” 第一章 – 請參考 pages 18 - 32 “Practical UNIX Programming - A Guide to Concurrency, Communication,

1

File System of

Unix

2

References

• “UNIX&Linux Shell 設計程式篇” 第一章 – 請參考 pages 18 - 32

• “Practical UNIX Programming - A Guide to Concurrency, Communication, and Multithreading” ： Chapter 3 Files

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Online

• Introduction of File System

• Original File

• Some important Shell Commands

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Introduction of File System

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ls -la

-rw-r----- 1 wrlai 980 Mar 23 12:00 .cshrc

-rw------ 1 wrlai 2302 Sep 10 16:32 .login

-rw----r-- 1 wrlai 2354 Feb 22 10:35 .mailrc

--x--x--- 1 wrlai 3302 Jan 14 16:32 makefile

-rw-rw--- 1 wrlai 3302 Jan 14 16:32 myfile

-rw-r--r-- 1 wrlai 3880 Feb 12 10:00 abc

drwxr-xr-x 1 wrlai 880 Feb 12 10:00 test

drwxr-xr-x 1 wrlai 880 Feb 12 10:00 project

An Example of Files

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File System

• Files– Ordinary files （ - ）– Directory files （ d ） – Special Files ： socket （ s ）、 names pipe （ p ）、

symbolic link （ l ）、 character device(terminal)（ c ）、 block device(disk) （ b ）

• Hierarchical File System – Tree-like structure– Each node presents a file

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The Tree Structure

wrlai

.cshrc

.login

.mailrc projectCprogram

proj1 proj2

a.out file1.cct1.cc t2.cc h1*

myfileabc

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• The directory you are working in, is called working directory or current directory.</home1/staff/wrlai>cd Cprogram

</home1/staff/wrlai/Cprogram>

• When you are first log in, the working directory is your home directory.– Use 〜 to present the home directory

</usr/bin>cd 〜</home1/staff/wrlai>

Current/ Home Directory

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cd

• cd 用於改變 shell 的 working directory– cd 是 shell 的內建命令（不會執行 fork 和 exec ）– Child shell 用 cd 改變目錄不會影響 Parent shell 的目錄

• Example ：</usr/home/wrlai>cd / （改變目錄到 / ）</>pwd （顯示目前所在的目錄位置）/

</>

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Original Files

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Files

• Every file has a filename.

• No two files in the same directory can have the same name.

• A full (absolute) pathname is its filename and the complete path. Ex: /usr/prof/alex/project/assign1.c

• A relative file name is its filename and path from current directory.Ex: alex/project/assign1.c in </usr/prof>

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Unix File Representation

• A Unix file has a description which is stored in a structure called an inode.

• Each file is segmented and located in a tree-structured representation.

• Features of inode ：– fairly efficient for small files – flexible if the size of the file changes

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inodefile information: size (in bytes) owner UID and GID relevant times (3) link and block counts permissions

direct pointers to beginning file blocks

single indirect pointers

double indirect pointers

triple indirect pointers

the time of creation the time of last access the time of last modification

...

...

......

...

...

...

...

...

...

...

...file block

point block

...

...

......

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inode – Small Filefile information: size (in bytes) owner UID and GID relevant times (3) link and block counts permissions

direct pointers 1, 2, 3





1

2

3

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inode – Larger Filefile information: size (in bytes) owner UID and GID relevant times (3) link and block counts permissions

direct pointers 1, 2, …, 12





1

12

...

...13

...

...

...

file block

point block

...

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ls -la

-rw-rw---- 1 wrlai cs 3302 Jan 14 16:32 myfile

File Information

The name of file

The time and date the file was

created or last modified

The size of the file in bytes

The name of group

The name of the owner

The number of hard links

Access Permissions ( 9 bits )

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• Type of file (one bit) :d : directory

- : ordinary file

s : socket

p : names pipe (FIFO)

l : symbolic link

c : character device (terminal)

b : block device (disk)

Access Permissions （ 1/3 ）

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• Three types of users can access a file （ Owner/Group/Other ）

• A user can attempt to access a file in three ways （ Read/Write/Execute ） – Ordinary file

• Read : you can read from file• Write : you can write to the file• Execute : you can execute the file

– Directory• Read : you can read the directory• Write : you can create, move, copy or remove entries• Execute : you can search (i.e., enter) the directory


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- r w - r w - r - -

File mode (- indicates an ordinary file)

Owner’s privileges(rw- means readable and writeable but not executable)

Group’s privileges

(rw- means readable and writable but not executable)

Other’s privileges(r-- means readable but not writable or executable)


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• You can use chmod to change the access permissions of a file

• Example : % ls -l myfile

-rw-rw-rw- 1 wrlai cs 3302 May 20 12:20 myfile

% chmod g+x myfile

-rw-rwxrw- 1 wrlai cs 3302 May 20 12:20 myfile

% chmod o-w myfile

-rw-rwxr-- 1 wrlai cs 3302 May 20 12:20 myfile

Change Mode （ 1/4 ）

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WHO u User g Group o Other a All (ugo)

Operator - Remove permission + Add permission = Set permission

Permissions r Read w Write x Execute l Set locking privilege s Set user or group ID mode t Set save text (sticky bit) mode u User’s current permissions g Group’s current permission o Other’s current permissions

Change Mode （ 2/4 ）

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Change Mode （ 3/4 ） -r--r--r-- 1 wrlai cs 3302 May 20 12:20 myfile

% chmod 666 myfile


read write execute Value read write execute - - - 0 0 + 0 + 0 - - Yes 1 0 + 0 + 1 - Yes - 2 0 + 2 + 0 - Yes Yes 3 0 + 2 + 1 Yes - - 4 4 + 0 + 0 Yes - Yes 5 4 + 0 + 1 Yes Yes - 6 4 + 2 + 0 Yes Yes Yes 7 4 + 2 + 1

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Change Mode （ 4/4 ）% chmod 666 myfile


% chmod 676 myfile

-rw-rwxrw- 1 wrlai cs 3302 May 20 12:20 myfile

% chmod 674 myfile

-rw-rwxr-- 1 wrlai cs 3302 May 20 12:20 myfile

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16 bits Attribute （ 1/3 ）• Every file has 16 attribute bits.

• File Type （ 12-15 bits ）– 12 ： S_IFIC （ Named pipe ）– 13 ： S_IFCHR/S_IFBLK （ Character or

Block ）– 14 ： S_IFDIR （ Directory ）– 15 ： S_IFREG （ Original file ）– Only one bit can be set in bits 12-15.

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16 bits Attribute （ 2/3 ）• Access permissions （ 0-8 bits ） rwxrwxrwx• Sticky bit 黏著位元（ bit 9 ）： S_ISVTX

– 當此 bit=1 ， file 會一直留在記憶體中而不會被 swap out 。

– 對於 Original files ，使用者執行位元為 t （而非 x ），表示此 bit=1 。例如： ls 、 sh 、 vi 。

– 對於 Directory ，使用者執行位元為 t ，表示只有超級使用者可刪除此目錄。

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16 bits Attribute （ 3/3 ）• Set Group ID （ bit 10 ）： S_ISGID

– 當此 bit=1 ，表示對此 file ，每個人均有和 file 擁有者相同的執行權（ bit 6 = ‘s’ ）。

– 執行此 file 的使用者， EUID 改成和 file 擁有者相同的 UID 。

• Set User ID （ bit 11 ）： S_ISUID– 當此 bit=1 ，表示對此 file ， Group 內的人均有和 file 擁有者相同的執行權。

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UID 、 GID 、 EUID 、 EGID

• In /etc/passwd, every user has his– UID ： real User ID

– GID ： Group ID （可以有很多個）• Login shell 時， EUID=UID 、 EGID=GID 。

– EUID ： Effective User ID

– EGID ： Effective Group ID

• 這四個 ID 用於決定某一使用者對於某一檔案的使用權。

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Example of UID & EUID

• 某一使用者的 UID = ncliu ，但可以利用 passwd 修改 /etc/passwd 的內容，如同擁有和 root 相同的權限（ EUID = root ）。

被執行的指令 UID EUID SUID

/bin/passwd ncliu root on

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Some important Shell Commands

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• The rm command allows you to remove files.

• You need to have write permission.

• Is it possible to restore a file that has been

removed ? NO !

rm - remove （ 1/3 ）

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% ls -l myfile.*

-rw-rw-rw- 1 wrlai cs 1898 Jan 5 ... myfile.1

-r--r--r-- 1 wrlai cs 1898 jan 7 ... myfile.2

% rm myfile.*

rm : override protection 444 for myfile.2 ? y

% rm -i testfile.* i （ interactive ） : ask user before remove

rm : remove testfile.1 ? y

rm : remove testfile.2 ? y

rm - remove （ 2/3 ）

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rm - remove （ 3/3 ）% rm -f myfile.*

f （ force ） : regardless the file permissions

% ls -l

drw-rw-rw- 1 wrlai cs 1898 Jan 5 ... dummy

% rm -r dummy r （ recursive ） : remove all files and subdirectories

within dummy

How about these commands list below :% rm -r * % rm -ir dummy % rm -fr dummy

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Wildchar

• ? 代表單一 character.

• * 代表任何的 string. 萬用字元• `v*[0-9]` 是 regular expression, v 開頭 , 最後

是數字的檔案 .% ls

sample1.c sample111.c

% ls sample?.c sample1.c

% ls sample*.csample1.c sample111.c

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alias

• You can add the following command in your .bachrc

alias rm=“rm –i”

• Then you open this account next time, you type

rm abc

It will ask user before remove

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• To move a file to a different directory% mv [ -if ] file ... directory

• To rename a file or directory% mv [ -if ] oldname newname

• If the target to which you move a file already exists, mv will replace the file.

• If you want to be caution about losing data, use -i% mv -i original destination

mv - move

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• Make a copy of a file% cp [-ip ] file1 file2– i （ interactive ）： ask the permission before

replacing a file that already exists– p （ preserve ）： the destination file have the

same modification times and permissions as the source file

• Copy files to a different directory%cp [-ip ] file ... directory

cp - copy （ 1/2 ）

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• Copy a directory to another directory% cp -r [-ip ] directory1 directory2

% cp -r essays backups– r （ recurive ）： will copy essays and all its

files and subdirectories to backups (directory)

cp - copy （ 2/2 ）

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• To make a directory% mkdir [-mp] directory ...– m : Specify the mode, in octal, for the new

directory– p : Create both the directory, and any parent

directories that don’t already exist.

mkdir - make directory

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• To remove a directory% rmdir [-ps] directory ...– p : Remove the directory and any empty parent

directories– s : Suppress warning messages

• Is it possible to remove your working directory ?

rmdir - remove directory

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• To change the owner and group associated with a file.% chown

% chgrp [-Rh] groupID files– R : Recursive operation– h : Change the ownership or group of the symbolic

link file itself, not the file to which it refers

• Note: In some system, these commands are not permit to anyone but the superuser.

chown, chgrp

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• To locate misplaced files% find . -name sample1.c -print

>./C-language/sample1.c– to search through the current

subtree（ . ） for a file named “sample1.c”

% find /usr -name `v*[0-9]` -print– to search the file with its name start with v and

end with a digit in the /usr subtree

find （ 1/2 ）

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% find /etc -size +1000 -print– to search files with size larger than 1000

blocks (512 bytes) in the /etc subtree– ‘+’ : larger than ； ‘ =‘ : equal to (exactly) ； ‘ -’ : less than

% find / -name core -exec rm { } \ ;– to search the entire file system for the files

named core and remove them)

• Note : Be careful to use find to locate files starting from the root （ / ） .

find （ 2/2 ）

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Simple Test

• Try to change one of your executable programs– Only you can read/write/execution. The other

can’t do anything.– All of people can read it, but only you can write

and execute it.

• Use the command “man” to find the detail about “ls”.

Documents

1 File System of Unix 2 References “UNIX&Linux Shell 設計程式篇 ” 第一章 – 請參考 pages 18 - 32 “Practical UNIX Programming - A Guide to Concurrency, Communication,