summer 2015 silicon valley university confidential 1 introduction to unix / linux - 4 dr. jerry...

Summer 2015SILICON VALLEY UNIVERSITY

CONFIDENTIAL 1

Introduction to UNIX / Linux - 4

Dr. Jerry Shiao, Silicon Valley University

SILICON VALLEY UNIVERSITY CONFIDENTIAL 2Summer 2015

Introduction UNIX/Linux Course

Section 4 The UNIX File System

File Types and Representation. File Attributes File Organization and Management. File Storage in UNIX File System. File Command and Primitives

The UNIX File Security File Protection From Unauthorized Access

User Login. Encrypt File. User Access Privileges.

File and Directory Access Privileges Special Access Bits

SILICON VALLEY UNIVERSITY CONFIDENTIAL 3

Copyright @2005 Pearson Addison-Wesley.


The UNIX File System UNIX File Concept

File is sequence of Bytes. Everything (Network Card, Disk Drive, Keyboard, File, Directory)

can be treated as a File. Meaning is attached to the file’s contents by the application that

uses/processes the file. Types of Files

Simple/ordinary File Directory Symbolic (soft) Link Special (Device) Files

Block Special Files Character Special Files

Named Pipe (FIFO) Socket




The UNIX File System Simple / Ordinary Files

Store information and data on secondary storage device.

File Extensions

Meanings are attached to a file’s content by the application that uses/processes the file.Compiler expects “.c” extension, Web browser expects “.html” extension, Loader expects “.o” extension.



Introduction UNIX/Linux Course The UNIX File System Directory

Names of files/directories and Inode Numbers. Array of Entries:

Inode (Index Node) Number: Index into an array on disk. Disk Array Entry:

Files attributes Inode for every file in UNIX

Inode Table Table of Inodes in memory for all open files. File’s Inode copied from disk to Inode Table. Access File attributes from memory, not from disk.



Introduction UNIX/Linux Course The UNIX File System Link File

Created when a symbolic link is created to an existing file. Allows sharing file without duplicating the file. Alias of existing file.

Device File Special File is the means of accessing hardware devices.

Each hardware device associated with at least one Special File. To access hardware (device), applications accesses Device File with file operations

(i.e. open/read/write). /dev Directory. Character Device File: Character-oriented devices for streaming data (i.e.

keyboard, mice, console connection, virtual terminals). Block Device File: Block-oriented devices for data movement in blocks (i.e. disks,

CD-ROM, Flash Drives). Pseudo Device File: Simulate physical devices. Access UNIX via network or virtual

terminal in X Window System.


Introduction UNIX/Linux Course Device Files

Name Type Major Minor Description

/dev/fd0 Block 2 0 Floppy disk.

/dev/hda Block 3 0 First IDE disk.

/dev/hda2 Block 3 2 Second primary partition of first IDE disk.

/dev/hdb Block 3 64 Second IDE disk.

/dev/hdb3 Block 3 67 Third primary partition of second IDE disk.

/dev/ttyp0 Char 3 0 Terminal

/dev/console Char 5 1 Console

/dev/lp1 Char 6 1 Parallel printer

/dev/ttyS0 Char 4 64 First serial port

/dev/rtc Char 10 135 Real-time clock

/dev/null Char 1 3 Null device

Not all device files are real hardware devices. Pseudo devices (fictitious logical devices) are /dev/null, /dev/zero.



Virtual File System (VFS)

To achieve the abstraction (i.e. “black box operation) to the user, common API to the user through glibc library and common callback function signature to the I/O functions.

Inode represents an object in the file system with a unique identifier (translating filename).

struct file_operations abstractions (i.e. read/write/open ) allow all I/O operations to have common interface. The indirect calls (i.e. callback functions) are APIs specific to the file system.



Introduction UNIX/Linux Course The UNIX File System InterProcess Communication Mechanisms:

Pipes, Named Pipes (FiFO), and Sockets. Pipe

Two related processes communicate with each other on same system: UNIX auto-creates stdin, stdout, stderr. Tied to terminal: stdin = keyboard, stdout/stderr = console.

A pipe allows output (stdout) of a command (process) to be sent to input (stdin) of another command (process).

Does not use disk, implemented in main memory.

Named Pipe (FIFO) File of type Named Pipe.

Process reading FIFO blocks waiting for data. Two un-related processes to communicate with each other on same system.

Independently executing processes on a system.




The UNIX File System Socket

InterProcess Communication through Socket. Socket established on both hosts. Client / Server Model

Client connects to Server to request information. Two processes communicate with same socket type and in same

domain. UNIX Domain – Communicate through a common File System on same

host. Address: Character string (entry in the File System). Address Family: AF_UNIX

Internet Domain – Communicate through Internet on different hosts. Address: Internet Protocol (IP) address of the host and port number. Address Family: AF_INET



Introduction UNIX/Linux Course The UNIX File System Socket

Client Side Socket:System Calls

1)socket(): Create a socket. 2)connect(): Connect the socket to the address of the server.

AF_INET: Port number on host machine.

AF_UNIX: Character string (file).

. . . 3)read(): Receive data.4)write(): Send data.

Server Client Side Socket:System Calls

1)socket(): Create a socket. 2)bind(): Bind a socket to an address.

AF_INET: Port number on host machine.

AF_UNIX: Character string (file).3)listen(): List for connections.

. . . 4)accept(): Accept a connection.5)read(): Receive data.6)write(): Send data.




The UNIX File System File System Structure Issues: How are files …

Organized from User’s View Stored on Disk Files Manipulated

and Permissions Structured Hierarchy

Upside-Down Tree Base: /root

Specified in three ways: Absolute Path: / Relative to Present Working Directory Relative to User’s Home Directory




The UNIX File System File System Structure Pathnames: Absolute and Relative

Absolute pathname starting from root. cd /home/sau/buildArmLinux

Relative pathname starting at Present Working Directory.

cd buildArmLinux Relative pathname starting at “~” (tilda) or Home

Directory. cd ~/buildArmLinux cd ~




The UNIX File System File System Structure Home and Present Working Directories

$HOME, ~ (tilda), . (dot), pwd User’s Home Directory. Configured when User account created.

. (dot), pwd Present Working Directory: Point where Relative Pathname is

derived. Login Present Working Directory is the User’s Home Directory. . / <filename>: File in Present Working Directory.

. . (dot dot) Parent directory of the Present Working Directory. .. / <filename>: File in parent directory.



Introduction UNIX/Linux Course The UNIX File System File System Structure Executing Commands

$ <filename> : Finds <filename> in $PATH or Shell Built-In. $ . / <filename>: Finds <filename> in CWD. $ / home / sau / <filename>: finds <filename> in path preceeding

<filename>.

Executing Shell Scripts $ <script>: Finds <script> in $PATH, uses “#!/bin/sh” as the Shell

Interpreter. $ . / <script>: Finds <script> in CWD, uses “#!/bin/sh” as the Shell

Interpreter. $ source <script>: Finds <script> in CWD, ignores “#!/bin/sh” and

uses current Shell Interpreter. $ . <script>: Same as “source <script>”.




The UNIX File SystemSystem Administration Users and Groups

Specify user’s Login Shell and Home Directory.




The UNIX File System /etc/passwd

/etc/shadow

. . .sau:x:500:500:Simon Au:/home/sau:/bin/bash

student1:x:501:501:student1:/home/student1:/bin/bash

student2:x:502:502:student2:/home/student2:/bin/bash

claruspon:x:503:503:claruspon:/home/claruspon:/bin/bash

cs206student1:x:504:504:cs206student1:/user/cs206student1:/bin/bash

. . .sau:$1$xklKmTjR$7cOSRZv2IidNQgeUV/8UZ1:14930:0:99999:7:::

student1:$1$ElaqELFc$7/GVVYEG/YwKyVDxEyL7R.:15371:0:99999:7:::

student2:$1$BUXC1RaH$7aeZSMERxGSNLVRuH9CyX.:15598:0:99999:7:::

claruspon:$1$p9TJrbar$HO8iVTNieMTvGldXBJaZa1:15760:0:99999:7:::

cs206student1:$1$oyuB45f9$ceSvmEmF9s4vav3eqCPAA0:15979:0:99999:7:::

Username to userid and groupid mapping.



Linux Directory Structure

vmlinux

inittabfstab

passwd

bin mount

usr

/boot

etc

bingrep

varlog

devtty0

mntcdrom

home student1

/boot – Linux bootup files (i.e. Linux Kernel)

/etc – Linux system configuration files.

/etc/inittab – Processes started at bootup (i.e. Runlevel)

/etc/fstab – File systems and mount points.

/etc/passwd – Users are defined and user accounts.

/bin – Linux system binaries (i.e. cat, cp, ls, mkdir, pwd, rm, rmdir)

/sbin/init – Process runned during boot process. System Administration.

/usr/bin – Applications for the users.

/lib – The shared libraries for dynamically linked modules.

/var – Data changes when the Linux system is running.

/var/log – The running Linux system updated log files.

/dev – Devices that are available to Linux system. Devices are treated like files and devices can be read/written as files.

/mnt – Storage devices (i.e. hard disk, CD-ROMs) must be attached to some directory before accessing. Directores are the mount points.

/home – Each users have own directory and only place normal users are allowed to write.

/proc – Special directory containing information about the kernel.

/proc/devices – List of devices configured into current kernel.proc

devices

lib

sbin init



Introduction UNIX/Linux Course The UNIX File System File System Structure

Navigating the File System Structure Log on: System places user in home directory.

Determining Absolute Pathname of Home Directory echo [ string ]

string: “string” sent to the console. sau@buildbed-vm:~> echo $HOME

/home/sau sau@buildbed-vm:~> echo $PWD

/home/sau Browsing File System

cd [ directory ] directory: Change Present Working Directory to absolute or relative

directory pathname. ls [ options ] [ pathname-list ]

-F: Display “/” after directories, * after binaries, @ after symbolic links. -a: Display name of all files, including hidden files. -l: Display long list, including permissions, owner, group, size, time. pathname-list: List of files to display.



The UNIX File System File System Structure

Browsing File System sau@buildbed-vm:~/class> cd $HOME/class sau@buildbed-vm:~/class> pwd /home/sau/class sau@buildbed-vm:~/class> ls -F file1 file2 file_dir/ power* power.c sau@buildbed-vm:~/class> ls -a . .. file1 file2 file_dir .hidden_file power power.c sau@buildbed-vm:~/class> ls -l total 28 -rw-r--r-- 1 sau users 10 2012-09-21 01:50 file1 -rw-r--r-- 1 sau users 11 2012-09-21 01:50 file2 drwxr-xr-x 2 sau users 4096 2012-10-01 18:08 file_dir -rwxr-xr-x 1 sau users 10042 2012-09-21 01:37 power -rw-r--r-- 1 sau users 288 2012-09-21 01:37 power.c sau@buildbed-vm:~/class> ls -l .hidden_file -rw-r--r-- 1 sau users 0 2012-10-01 18:10 .hidden_file





Summary of the “ls –l” Command (Fields listed left to right).





Some Important Hidden Files and Their Purpose.





Creating and Removing Directories mkdir [ options ] dirnames

dirnames: Create “dirname” directories. -m MODE: Create with given access permissions. -p: Create parent directories that do not exist in “dirname”.

home/ sau/

usrdir1/

usrdir2/

usrdir1_1/

mkdir –p usrdir1/usrdir1_1

mkdir usrdir2/

mkdir /tmp/tmp1

tmp/ tmp1/

Current Working Directory





Removing Directories rmdir [ options ] dirnames

dirnames: Remove “dirname” directories. -p: Remove parent directories.

home/ sau/

usrdir1/

usrdir2/

usrdir1_1/

rmdir –p usrdir1/usrdir1_1

rmdir usrdir2/

rmdir /tmp/tmp1

tmp/ tmp1/

Current Working Directory



Introduction UNIX/Linux Course File System Structure

File Attributes: Using ls command[student2@unknown001320aa6702 ~]$ lsDesktop Download mbox Pictures Templates VideosDocuments link_mbox Music Public testTools[student2@unknown001320aa6702 ~]$ ls -a. .bashrc Download .gnome2_private link_mbox Pictures .Trash.. .config .gconf .gstreamer-0.10 mbox Public Videos.bash_history Desktop .gconfd .gtk-bookmarks .metacity .redhat .viminfo.bash_logout .dmrc .gnome .gtkrc-1.2-gnome2 Music Templates .xsession-errors.bash_profile Documents .gnome2 .ICEauthority .nautilus testTools .zshrc[student2@unknown001320aa6702 ~]$ ls -FDesktop/ Download/ mbox Pictures/ Templates/ Videos/Documents/ link_mbox@ Music/ Public/ testTools*[student2@unknown001320aa6702 ~]$ ls -ltotal 592drwxr-xr-x 2 student2 student2 4096 2012-09-14 18:26 Desktopdrwxr-xr-x 2 student2 student2 4096 2012-09-14 18:26 Documentsdrwxr-xr-x 2 student2 student2 4096 2012-09-14 18:26 Downloadlrwxrwxrwx 1 student2 student2 4 2013-10-02 01:47 link_mbox -> mbox-rw------- 1 student2 student2 714 2012-09-23 14:26 mbox. . .-rwxr-xr-x 1 root root 517659 2013-10-02 01:48 testTools



Introduction UNIX/Linux Course File System Structure

File Attributes: Using ls command

$ ls –ldrwxr-xr-x 2 student2 student2 4096 2012-09-14 18:26 Desktop





File Content Type. File [ option ] file-list

-f FILE: Use FILE as a file of “file-list”

sau@buildbed-vm:~/class> file * file1: ASCII text file1~: ASCII text file1_link: symbolic link to `file1' file2: ASCII text file_dir: directory power: ELF 32-bit LSB executable, Intel 80386, version 1 (SYSV), dynamically linked (uses shared libs), for GNU/Linux 2.6.4, not stripped power.c: ASCII C program text




The UNIX File System File Representation

Inode Table Entry

Block number = Disk Sector

Sector = Disk No, Cylinder No, Track No, Sector No

File Space allocated in clusters of two, four, or eight

512-Byte Disk Block.




The UNIX File System File Representation

1) Directory contains array of entries <inode #, filename>.

2) Entry placed in Inode Table in RAM when file is opened.

3) Indexing into Inode Table returns the entry of the Inode containing the block location of file on disk.




The UNIX File System Standard Files and File Descriptors File Descriptor for every open file in UNIX. Three Standard Files when command executed:

Stdin (0): Terminal Keyboard. Stdout (1): Monitor Screen. Stderr (2): Monitor Screen.

Redirect Operations: < Input Redirect > Output and Error RedirectFile Descriptor

File Descriptor Table

Systemwide File Table

Systemwide Inode Table

File Contents On Disk




The UNIX File System Standard Files and File Descriptors




The UNIX File Security Time Sharing System Allows Multiple Users

Access. Protect Shared Hardware/Software Resources.

Storage Device. I/O Devices. CPU. Main Memory.

File Protection From Unauthorized Access: UNIX provides three mechanisms to protect files. User Login Name and Password. Encrypt File. Access Privileges to Users.



Introduction UNIX/Linux Course The UNIX File Security Password-Based Protection. Password Discovered by:

Telling Someone. Guessing “weak” Passwords. Brute Force Method.

Change Password. sau@buildbed-vm:~> passwd Changing password for sau. Old Password: New Password: Bad password: it is based on a dictionary word New Password: Bad password: too simple New Password: Reenter New Password: Password changed.



Introduction UNIX/Linux Course The UNIX File Security Encryption-Based Protection.

GNU Privacy Guard$ gpg -c minicom.logEnter passphrase:<password>

Repeat passphrase: <password>

$ ls… minicom.log.gpg …

$ gpg minicom.log.gpggpg: CAST5 encrypted dataEnter passphrase: <password>gpg: WARNING: message was not integrity protected

GNU Privacy Guard



Introduction UNIX/Linux Course The UNIX File Security Encryption-Based Protection.

[student1@localhost ~]$ gpg --gen-key

gpg (GnuPG) 1.4.7; Copyright (C) 2006 Free Software …

…

Please select what kind of key you want:

(1) DSA and Elgamal (default)

(2) DSA (sign only)

(5) RSA (sign only)

Your selection?

…

What keysize do you want? (2048)

…

Please specify how long the key should be valid.

…

Key is valid for? (0)

Key does not expire at all

Is this correct? (y/N)

You need a user ID to identify your key; the software constructs the user ID …

Real name: Simon Au

Email address: [email protected]

Comment: Lecturer

You selected this USER-ID:

"Simon Au (Lecturer) <[email protected]>"

…

…

You need a Passphrase to protect your secret key.

…

gpg: /home/student1/.gnupg/trustdb.gpg: trustdb created

…

[student1@localhost ~]$ gpg --encrypt --recipient simon

minicom.log

[student1@localhost ~]$ ls

…

envSetup.bash minicom.log.gpg test2

…

[student1@localhost ~]$ gpg --output minicom_3.log –

decrypt minicom.log.gpg

You need a passphrase to unlock the secret key for

user: "Simon Au (Lecturer) <[email protected]>"

…



Introduction UNIX/Linux Course The UNIX File Security Protection Based on Access Permission

Prevents users from accessing each other’s files when not logged on as the file’s owner.

File Owner: Assign Access Rights to Files. Dictates how other users can access them (i.e. Read, Write,

Execute).

Without this protection scheme: UNIX Filesystem is easy to access, has single root, from

which all files are derived. Users can access each other’s files.



Introduction UNIX/Linux Course The UNIX File Security Access Permission Protection. Types of Users

User (owner) : Group : Other User: Owner of the file. Group: Group of Users. Other: Other Users Not in Group.

$ more /etc/group…root:x:0:…video:x:33:sauusers:x:100:

$ groups sausau : video users $ groups rootroot : root

$ more /etc/passwd…root:x:0:0:root:/root:/bin/bash…sau:x:1001:100:sau:/home/sau:/bin/bash

Group Name: Info: Group ID: UsersUser : Pass: UserID: GroupID: UserInfo: Home: Shell

Name: Word: : : : :



Introduction UNIX/Linux Course The UNIX File Security Access Permission Protection. Types of Users (Cont)

$ id sauuid = 1001 (sau) gid = 100 (users) groups = 100 (users), 33 (video)

Primary Group: Files created by owner (sau) will have primary group.

Supplementary Group: Access to additional resources (files).

Commands:

Add new user to primary ( -g ) and supplementary ( -G ) group.

useradd -G <group> <new user>

useradd -g <group> <new user>

Add existing user to primary ( -g ) and supplementary ( -G ) group.

usermod -G <group> <existing user>

usermod -g <group> <existing user>



Introduction UNIX/Linux Course The UNIX File Security Types of Access Permissions

Read: Write: Execute




Read: Write: Execute File has nine types of permissions: 3 for User, 3 for Group, 3 for Others

3 Bits For File Permission of Each Type. User = 7 (Read / Write / Execute) Group = 4 (Read / NA / NA) Others = 4 (Read / NA / NA)

1 1 1

1 0 0

1 0 0



Introduction UNIX/Linux Course The UNIX File Security Types of Access Permissions View Access Permission of Files / Directories

ls - l [ file-list ] Display long list of files in ‘file-list’ or all files in Present

Working Directory. ls – l : List all files in Present Working Directory. ls – l /etc/passwd : List file /etc/passwd. ls – l /etc : List all files in /etc.

ls - ld [ directory-list ] Display long list of directories in ‘directory-list’ or all

directories in Present Working Directory. ls - ld : List Present Working Directory. ls - ld /etc/passwd: List file /etc/passwd. ls –ld /etc : List directory /etc.




View Access Permissions of Files / Directories ls - l , ls - ld

File Other Access Permission.

File Owner Access Permission.

File Group Access Permission.

File Type: “d” indicates Directory, “ – “ indicates File.




View Access Permissions of Directories r: Read the contents of the directory ( use “ls” command ). w: Create, remove entries in the directory. x: Searching the directory ( use “ls -l” command with files).

Group CANNOT write.Group CAN use “ls” command.Group CAN use “ls –l” command.

Other CANNOT write.Other CANNOT use “ls” command.Other CANNOT use “ls –l” command.



Introduction UNIX/Linux Course The UNIX File Security Changing File Access Privileges chmod [ options ] octal-mode file-list

octal-mode: Using Octal Value to Represent Read / Write / Execute Access Permission. 7 = Read/Write/Execute, 4 = Read-Only, 6 = Read/Write.

chmod [ options ] symbolic-mode file-list symbolic-mode: < who > < operator > < privilege >

“=“ Operator



Introduction UNIX/Linux Course The UNIX File Security Changing File Access Privileges



Introduction UNIX/Linux Course The UNIX File Security Changing File Access Privileges$ ls -ltotal 60drwxr-xr-x 2 sau users 4096 2012-10-03 15:58 file_dir$ ls -l file_dirtotal 0-rw-r--r-- 1 sau users 0 2012-10-03 15:54 temp-rw-r--r-- 1 sau users 0 2012-10-03 15:58 temp2$ chmod -R 711 file_dir$ chmod -R 700 file_dir/temp2$ ls -l total 60...drwx--x--x 2 sau users 4096 2012-10-03 15:58 file_dir...$ ls -l file_dirtotal 0-rwx--x--x 1 sau users 0 2012-10-03 15:54 temp-rwx------ 1 sau users 0 2012-10-03 15:58 temp2$ chmod 7 example$ chmod 70 file_dir$ ls -ltotal 60d------rwx 2 sau users 4096 2012-10-03 17:23 example...d---rwx--- 2 sau users 4096 2012-10-03 15:58 file_dir...

Octal-mode privileges positional.



Introduction UNIX/Linux Course The UNIX File Security Changing Directory Access Privileges

Read: Allows Reading the Directory’s Contents. Write: Allows Creating / Removing Files or Directories. Execute: Searching the Directory.

NOTE: Read / Write Privilege MUST Have Execute Privilege Set.



Introduction UNIX/Linux Course The UNIX File Security Default File or Directory Access Privileges Set Access Privilege For New File or Directory. umask [ mask ]

mask: Set access permissions on new files and directories EXCEPT for “mask’ bits.

umask 027 (Prohibit non-group members from accessing files and directories) New Files: umask: 027 ; --- -w- rwx Initial File Permission: 666 ; rw- rw- rw- Complement of mask: NOT(027) = 750 ; rwx r-x --- Resultant File Permission: 750 AND 666 = 640 ; rw- r-- --- New Directories: Initial Directory Permission: 777 ; rwx rwx rwx Complement of umask: NOT(027) = 750; rwx r-x --- Resultant Dir Permission: 750 AND 777 =750; rwx r-x ---



Introduction UNIX/Linux Course The UNIX File Security Special Access Bits The Set-User-ID (SUID) Bit

If the SUID Bit is set for an executable file (i.e. command or shell script), the process takes on the User privilege of the owner of the file when it executes.

If the SUID Bit is NOT set for an executable file, the process takes on the privilege of the user executing the file.

File /etc/passwd is owned by root. Command passwd run by users that change /etc/passwd file. Allow /etc/passwd to be changed by passwd command, but not by other

users accessing /etc/passwd.

sau@buildbed-vm:/usr/bin> ls -l /etc/passwd -rw-r--r-- 1 root root 2029 2012-10-02 15:43 /etc/passwd sau@buildbed-vm:/usr/bin> ls -l /usr/bin/passwd -rwsr-xr-x 1 root shadow 80268 2011-07-29 12:55 /usr/bin/passwd

“s” = Execute and SUID set.“S” = SUID set.



Introduction UNIX/Linux Course The UNIX File Security Special Access Bits The Set-Group-ID (SGID) Bit

If the SGID Bit is set for an executable file, the process takes on the Group privilege of the owner of the file when it executes.

If the SGID Bit is NOT set for an executable file, the process takes on the privilege of the user executing the file.

sau@buildbed-vm:~/class> ls -ld file_dir d---rws--- 2 sau users 4096 2012-10-03 15:58 file_dir

The Sticky Bit If Sticky Bit is set, only the file or directory’s owner or SuperUser

can rename or delete the file or directory. If Sticky Bit is NOT set, any user with write and execute

permissions can rename or delete a file or directory. sau@buildbed-vm:/> ls -ld /tmp drwxrwxrwt 189 root root 12288 2012-10-04 03:15 /tmp

“t” = Execute and Sticky Bit set.“T” = Sticky Bit set.

“s” = Execute and SGID Bit set.“S” = SGID Bit set.



Introduction UNIX/Linux Course The UNIX File Security Special Access Bits

The Set-User-ID (SUID) Bit chmod 4xxx file-list chmod u+s file-list

The Set-Group-ID (SGID) Bit chmod 2xxx file-list chmod g+s file-list

The Sticky Bit chmod 1xxx file-list chmod +t file-list




[student3@unknown001320aa6702 ~]$ umask 0002 -rw-rw-r-- 1 student3 student3 0 2013-10-04 16:42 sau_file [student3@unknown001320aa6702 ~]$ ls -ld sau drwxrwxr-x 2 student3 student3 4096 2013-10-04 16:41 sau [student3@unknown001320aa6702 ~]$ chmod 2775 sau [student3@unknown001320aa6702 ~]$ ls -ld sau drwxrwsr-x 2 student3 student3 4096 2013-10-04 16:41 sau [student3@unknown001320aa6702 ~]$ ls -l sau_file [student3@unknown001320aa6702 ~]$ chmod 4664 sau_file [student3@unknown001320aa6702 ~]$ ls -ld sau_file -rwSrw-r-- 1 student3 student3 0 2013-10-04 16:42 sau_file [student3@unknown001320aa6702 ~]$




[student3@unknown001320aa6702 ~]$ [student3@unknown001320aa6702 ~]$ ls -ld sau drwxrwsr-x 2 student3 student3 4096 2013-10-04 16:41 sau [student3@unknown001320aa6702 ~]$ chmod 1775 sau [student3@unknown001320aa6702 ~]$ ls -ld sau drwxrwsr-t 2 student3 student3 4096 2013-10-04 16:41 sau

summer 2015 silicon valley university confidential 1 introduction to unix / linux - 4 dr. jerry...

Documents

file storage

file command

file operations

file extensionsmeanings

sharing file

block device file

character device file

pseudo device file