Linux--Introduction and Basic commands(Part one)

迷南。 2022-05-12 03:36 159阅读 0赞

# Welcome to Linux world! #

## Introduction and Basic commands–Part one   ![linux_PNG29.png][]            **J.C   2018.3.11**  ##

## Chapter 1 What Is Linux? ##

**Linux** is a **Unix-like** open source operating system. At the core of the operating system is the Linux kernel. It acts as the intermediary between the applications which run in the operating system and the underlying hardware.

The history of **Linux** began in **1991** with the commencement of a personal project by **Finnish** student **Linus Torvalds** to create a new free operating system kernel. Since then, the resulting Linux kernel has been marked by constant growth throughout its history. Since the initial release of its *source code* in 1991, it has grown from a small number of C files under a license prohibiting commercial distribution to the 4.15 version in 2018 with more than 23.3 million lines of *source code* without comments under the *GNU General Public License v2*.

![image][]

### Linux versions ###

*  Linux Mint
 *  Ubuntu
 *  Debian
 *  Fedora
 *  openSUSE
 *  Arch Linux
 *  CentOS
 *  Red Hat Enterprise Linux
 *  …

![image][image 1]

### [Advantages of Linux][] ###

*  **Full multitasking** — Multiple tasks can be run in the background, and multiple devices, such as a modem, printer, and hard drive, can be accessed at the same time.
 *  **Virtual memory** — Linux safely uses a portion of your hard drive as virtual memory, which increases the efficiency of your system by keeping active processes in RAM and placing less frequently used or inactive portions of memory on disk. Virtual memory also utilizes all your system’s memory and doesn’t allow memory segmentation to occur.
 *  **Hardware support** — Linux, especially Intel-based versions, supports nearly all hardware architectures and devices, with the best support for legacy hardware. This is an advantage in that new versions of the operating system will not make your older hardware obsolete.
 *  **The X Window System** — The X Window System is a graphics system for UNIX machines. This powerful interface supports many applications and is the standard interface for the industry.
 *  **Built-in networking support** — Linux uses standard TCP/IP protocols, including Network File System (NFS), Network Information Service (NIS, formerly known as YP), Session Message Block (SMB), and others. You can access the Internet by connecting your system with an Ethernet card, or a parallel-port, serial cable, or over a modem to another system.
 *  **Shared libraries** — Because each command shares a common library of subroutines it can call at runtime, Linux helps saves memory and hard drive space.
 *  **Compatibility with the IEEE POSIX.1 standard** — Because of this compatibility, Linux supports many of the standards set forth for all UNIX systems.
 *  **Open Source code** — The Linux kernel uses no code from AT&T or any other proprietary source. This allows other organizations, the GNU project, hackers, and programmers from all over the world to develop and contribute software for Linux.
 *  **Documentation** — Nearly every Linux distribution comes with more than 12,000 pages of documentation in the form of manual pages, info documents, or guides. You’ll also find extra technical documentation for software packages under the directory. Unlike operating systems offered by the monopolistic software industry, Linux is fully documented—one problem might be that there is too much information!
 *  **Lower cost than most other UNIX systems and UNIX clones** — If you have a fast Internet connection and a CDR drive, you can freely download Linux off the Internet. Many books also come with a free copy (this book includes the latest version of Intel-based Red Hat Linux on the CD-ROMs).
 *  **GNU software support** — Linux can run a wide range of free software available through the GNU project. This software includes everything from programming tools, such as compilers, assemblers, linkers, and loaders, to system administration utilities, such as stream editors, the venerable editor, and even games.

### Connecting Linux with Windows by software ###

*  [XShell][]
 *  [Putty][]

![image][image 2]![image][image 3]

### File transfer between Windows and Linux ###

*  [WinSCP][]
 *  [Xftp][]
 *  [FileZilla][]

![image][image 4]![image][image 5]![image][image 6]

### Installing Linux on your computer ###

1.  Download [Ubuntu 16.04 ISO][] or [CentOS 7 ISO][] file
2.  Installing *vitural machine* on your computer: [VirtualBox][] or [VMware Workstation][]
3.  Installing your Ubuntu or CentOS system on *vitural machine*

Ubuntu Installation Tutorial:   [http://blog.csdn.net/u013142781/article/details/50529030][http_blog.csdn.net_u013142781_article_details_50529030]

CentOS Installation Tutorial:   [http://www.osyunwei.com/archives/7829.html][http_www.osyunwei.com_archives_7829.html]

### Learning Linux Online ###

1.  鸟哥的Linux私房菜:  [http://linux.vbird.org/][http_linux.vbird.org]
2.  Linux中国:  [https://linux.cn/][https_linux.cn]
3.  Linux公社:  [http://www.linuxidc.com/][http_www.linuxidc.com]
4.  Learn Shell Programming:  [http://www.learnshell.org/][http_www.learnshell.org]
5.  Unix Terminal Online(CentOS):   [https://www.tutorialspoint.com/unix\_terminal\_online.php][https_www.tutorialspoint.com_unix_terminal_online.php]
6.  Linux Terminal Online:  [http://cb.vu/][http_cb.vu]
7.  Ubuntu online tour:  [http://tour.ubuntu.com/en/\#surf-the-web][http_tour.ubuntu.com_en_surf-the-web]
8.  Execute Bash Shell Online (GNU Bash v4.4):  [http://www.compileonline.com/execute\_bash\_online.php][http_www.compileonline.com_execute_bash_online.php]
9.  Overview of Unix and Linux commands:  [https://www.computerhope.com/unix/overview.htm][https_www.computerhope.com_unix_overview.htm]
10. JSLinux:  [https://bellard.org/jslinux/][https_bellard.org_jslinux]

### Free ebooks for Linux ###

1.  [http://www.allitebooks.com/][http_www.allitebooks.com]
2.  [https://doc.lagout.org/programmation/][https_doc.lagout.org_programmation]
3.  [https://www.syncfusion.com/ebooks][https_www.syncfusion.com_ebooks]
4.  [http://www.onlineprogrammingbooks.com/][http_www.onlineprogrammingbooks.com]
5.  [http://www.ebook777.com/it-ebooks][http_www.ebook777.com_it-ebooks]

### Linux online practice ###

1.  [https://linuxsurvival.com/testimonials/][https_linuxsurvival.com_testimonials]

## Chapter 2 Linux Directory Structure ##

The Linux directory structure is like a tree. The base of the Linux file system hierarchy begins at the **root**, or trunk, and directories branch off from there. Each one of these directories, called folders on other operating systems, can and often do contain other directories. The directories on a Linux system are separated by a forward slash.

### Common Top-Level Directories: ###

*  /      The Root Directory
 *  /bin   Binaries
 *  /etc   System Configuration Files
 *  /home  Home Directories
 *  /opt   Optional or Third-Party Software
 *  /tmp   Temporary Space
 *  /usr   User-Related Data, Read-Only
 *  /var   Variable Data

![image][image 7]

## Chapter 3 Command Line Interface(CLI) ##

![image][image 8]

A **shell** is a program that accepts commands and instructs the operating system to execute those commands. When you connect to a Linux system over the network, for example, a shell is started and acts as your interface to the system. The shell in this particular case is a command line interpreter. The command-line interface is often referred to by its abbreviation, **CLI**.

Ubuntu 14.04 LTS linuxsvr tty1 linuxsvr login:
    jason Password: Welcome to Ubuntu 14.04 LTS
    jason@linuxsvr:~$

The tilde represents the home directory of the current user which is **/home/jason** in this example.  
\- **$**: super user  
\- **\#**: common user

### Basic Commands ###

#### Usually, CLI commands are constructed by: basic command + options + arguements. ####

*  pwd            current working directory
 *  cd \[directory\]       changes the current directory to the supplied directory
 *  ls            lists directory contents
 *  cat \[file\]         concatenates, or displays files
 *  exit,logout,Ctrl-d      exits the shell or your current session
 *  Ctrl-C          stops your current command
 *  man\[command\]     displays the online manual for a given command
 *  cp \[source\] \[directory\]  makes copies of files and directories
 *  touch \[file\]        creates, changes and modify timestamps of a file
 *  date           prints out, or changes the value of, the system’s time and date information
 *  cal           displays a simple, formatted calendar in your terminal
 *  shutdown         shutdown the system
 *  reboot         reboot the system
 *  tar \[file\]          create, maintain, modify, and extract files that are archived in the tar format
 *  zip/unzip \[file\]       squash a bunch of files together into a zip file or extract the zip files
 *  head/tail \[file\]      outputs the first/last part, or “head”/”tail”, of files

For more details anout the above basic commands, please refer to the website: [https://www.computerhope.com/unix.htm\#commands][https_www.computerhope.com_unix.htm_commands].

## Chapter 4 Directories ##

In addition to referencing directories by their full or absolute paths, you can reference directories  
by their relative paths.

e.g.  
\- cd /home/vagrant/Music      change directory by absolute path  
\- cd ./Music             change directory by relative path

### Basic Commands ###

*  **.**             current directory
 *  **..**             parent directory
 *  **-**             quickly return to your previous working directory
 *  mkdir \[-p\] directory    create a directory. -p: intermediate directories are created.
 *  rmdir \[-p\] directory    remove a directory. -p: all specified directories are removed.The rmdir command only removes empty  
    directories. To remove directories and their contents, use **rm**.
 *  rm -rf directory     removes files, directories, or both. -r:recursive and -f: force. Order does not matter. The commands **rm -r -f dir, rm -rf dir**, and **rm -fr dir**  
    are all equivalent.

e.g.

$ mkdir newdir
    $ mkdir newdir/one/two
    mkdir: cannot create directory ‘newdir/one/two’: No such file or directory
    $ mkdir -p newdir/one/two
    $ rmdir newdir
    rmdir: directory "newdir": Directory not empty
    $ rm -rf newdir
    $ ls newdir
    ls: newdir: No such file or directory
    $ mkdir newerdir
    $ rmdir newerdir
    $ ls newerdir
    ls: cannot access newerdir: No such file or directory

## Chapter 5 Viewing File and Directory Details ##

The **ls** command only lists files and directories, it can  
provide important details about those files and directories.

##### Remember: CLI commands are constructed by: basic command + options + arguements #####

<table> 
 <thead> 
  <tr> 
   <th>option</th> 
   <th>meaning</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td><strong>-l</strong></td> 
   <td>displays a long listing format</td> 
  </tr> 
  <tr> 
   <td><strong>-a</strong></td> 
   <td>display hidden files with ls</td> 
  </tr> 
  <tr> 
   <td><strong>-F</strong></td> 
   <td>append a file type indicator to the name of the file or directory</td> 
  </tr> 
  <tr> 
   <td><strong>-t</strong></td> 
   <td>sort the output by time, displays the most recently modified items first</td> 
  </tr> 
  <tr> 
   <td><strong>-r</strong></td> 
   <td>reverse the order of <strong>-t</strong></td> 
  </tr> 
  <tr> 
   <td><strong>-R</strong></td> 
   <td>perform a recursive listing</td> 
  </tr> 
  <tr> 
   <td><strong>-d</strong></td> 
   <td>have <strong>ls</strong> operate on just the directory</td> 
  </tr> 
  <tr> 
   <td><strong>–color</strong></td> 
   <td>colorize the output of the ls command</td> 
  </tr> 
  <tr> 
   <td><strong>-b</strong></td> 
   <td>Quoting and escaping to space,&nbsp;&nbsp;|,&nbsp;&nbsp;\&amp;,&nbsp;&nbsp;\’,&nbsp;&nbsp;\;,&nbsp;&nbsp;(, &nbsp;&nbsp;),&nbsp;&nbsp;&lt;,&nbsp;&nbsp;&gt;,&nbsp;&nbsp;space,&nbsp;&nbsp;tab</td> 
  </tr> 
 </tbody> 
</table>

##### Options can be used separately or combinatively.Following are some examples: #####

**ls -l**

$ ls
    Desktop Documents Downloads Music to-do.txt
    $ ls -l
    total 20
    drwxrwxr-x 2 jason users 4096 May 3 08:33 Desktop
    drwxrwxr-x 2 jason users 4096 May 3 08:35 Documents
    drwxrwxr-x 2 jason users 4096 May 3 08:38 Downloads
    drwxrwxr-x 3 jason users 4096 Jun 21 21:16 Music
    -rw-r--r-- 1 jason users 73 Jun 22 19:34 to-do.txt

**ls -l** starts with a series of characters that represent the  
**permissions** of the file or directory. The number that  
follows the permissions string represents the number of links to the file or directory. Next, the owner is displayed followed by the group name. The file size is then displayed. The timestamp provided represents the modification time. The last item is the name of the file or directory itself.

**ls -a**

$ ls
    Desktop Documents Downloads Music to-do.txt
    $ ls -a
    . .. .bash_history .bash_logout .bashrc Desktop Documents Downloads
    Music .profile .ssh to-do.txt
    $ ls -a -l
    total 48
    drwxr-xr-x 7 jason users 4096 Jun 22 20:36 .
    drwxr-xr-x 6 root root 4096 May 4 10:26 ..
    -rw------- 1 jason users 3738 Jun 22 19:37 .bash_history
    -rw-r--r-- 1 jason users 220 Mar 30 2013 .bash_logout
    -rw-r--r-- 1 jason users 3650 Jun 22 19:41 .bashrc
    drwxrwxr-x 2 jason users 4096 May 3 08:33 Desktop
    drwxrwxr-x 2 jason users 4096 May 3 08:35 Documents
    drwxrwxr-x 2 jason users 4096 May 3 08:38 Downloads
    drwxrwxr-x 3 jason users 4096 Jun 21 21:16 Music
    -rw-r--r-- 1 jason users 675 Mar 30 2013 .profile
    drwx------ 2 jason users 4096 May 3 12:44 .ssh
    -rw-r--r-- 1 jason users 73 Jun 22 19:34 to-do.txt

**ls -a -l** is equivalent to **ls -al** and **ls -la**.

**ls -F**

$ ls
    Desktop Documents Downloads link-to-to-do Music program to-do.txt
    $ ls -F
    Desktop/ Documents/ Downloads/ link-to-to-do@ Music/ program* todo.txt
    $ ls -lF
    total 24
    drwxrwxr-x 2 jason users 4096 May 3 08:33 Desktop/
    drwxrwxr-x 2 jason users 4096 May 3 08:35 Documents/
    drwxrwxr-x 2 jason users 4096 May 3 08:38 Downloads/
    lrwxrwxrwx 1 jason users 9 Jun 22 21:01 link-to-to-do -> to-do.txt
    drwxrwxr-x 3 jason users 4096 Jun 21 21:16 Music/
    -rwxr-xr-x 1 jason users 13 Jun 22 21:02 program*
    -rw-r--r-- 1 jason users 73 Jun 22 19:34 to-do.txt

<table> 
 <thead> 
  <tr> 
   <th>Symbol</th> 
   <th>File Type</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td>/</td> 
   <td>Directory</td> 
  </tr> 
  <tr> 
   <td>@</td> 
   <td>Symlink. The file that follows the -&gt; symbol is the target of the link.</td> 
  </tr> 
  <tr> 
   <td>*</td> 
   <td>Executable script or program</td> 
  </tr> 
  <tr> 
   <td>=</td> 
   <td>Socket</td> 
  </tr> 
  <tr> 
   <td>&gt;</td> 
   <td>Door</td> 
  </tr> 
  <tr> 
   <td>|</td> 
   <td>Named pipe</td> 
  </tr> 
 </tbody> 
</table>

**ls -t**

$ ls -t
    program link-to-to-do to-do.txt Music Downloads Documents Desktop
    $ ls -lt
    total 24
    -rwxr-xr-x 1 jason users 13 Jun 22 21:02 program
    lrwxrwxrwx 1 jason users 9 Jun 22 21:01 link-to-to-do -> to-do.txt
    -rw-r--r-- 1 jason users 73 Jun 22 19:34 to-do.txt
    drwxrwxr-x 3 jason users 4096 Jun 21 21:16 Music
    drwxrwxr-x 2 jason users 4096 May 3 08:38 Downloads
    drwxrwxr-x 2 jason users 4096 May 3 08:35 Documents
    drwxrwxr-x 2 jason users 4096 May 3 08:33 Desktop
    $ ls -lrt
    total 24
    drwxrwxr-x 2 jason users 4096 May 3 08:33 Desktop
    drwxrwxr-x 2 jason users 4096 May 3 08:35 Documents
    drwxrwxr-x 2 jason users 4096 May 3 08:38 Downloads
    drwxrwxr-x 3 jason users 4096 Jun 21 21:16 Music
    -rw-r--r-- 1 jason users 73 Jun 22 19:34 to-do.txt
    lrwxrwxrwx 1 jason users 9 Jun 22 21:01 link-to-to-do -> to-do.txt
    -rwxr-xr-x 1 jason users 13 Jun 22 21:02 program

**ls -R**

$ ls -R
    .:
    Desktop Documents Downloads link-to-to-do Music program to-do.txt
    ./Desktop:
    ./Documents:
    cat.jpg report.txt
    ./Downloads:
    ./Music:
    JohnColtrane
    ./Music/JohnColtrane:
    giant-steps.mp3

To accomplish the same goal, but in a more visually appealing way, use the **tree** command. To view only the directory structure, use **tree -d**. For colorized output, use **tree -C**. The **tree** command is not always installed by default so you may have to rely on the ls command.

**ls -d**

$ tree Music/
    Music/
    |-- JohnColtrane
    |-- giant-steps.mp3
    1 directory, 1 file
    $ ls Music/
    JohnColtrane
    $ ls -l Music/
    total 4
    drwxrwxr-x 2 jason users 4096 Jun 22 21:39 JohnColtrane
    $ ls -d Music/
    Music/
    $ ls -ld Music/
    drwxrwxr-x 3 jason users 4096 Jun 21 21:16 Music/

**ls –color**

$ ls --color
    Desktop Documents Downloads link-to-to-do Music program to-do.txt

**ls -b**

$ ls
    my to do list
    $ ls -l
    total 4
    -rw-r--r-- 1 jason users 73 Jun 22 22:16 my to do list
    $ ls -l my to do list
    ls: cannot access my: No such file or directory
    ls: cannot access to: No such file or directory
    ls: cannot access do: No such file or directory
    ls: cannot access list: No such file or directory
    $ ls -l "my to do list"
    -rw-r--r-- 1 jason users 73 Jun 22 22:16 my to do list
    $ ls -l my\ to\ do\ list
    -rw-r--r-- 1 jason users 73 Jun 22 22:16 my to do list
    $ ls -lb
    total 4
    -rw-r--r-- 1 jason users 73 Jun 22 22:16 my\ to\ do\ list

## Chapter 6 Permissions ##

#### File System Permissions ####

##### File Types #####

<table> 
 <thead> 
  <tr> 
   <th>File Type</th> 
   <th>Symbol</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td>Regular file</td> 
   <td><strong>-</strong></td> 
  </tr> 
  <tr> 
   <td>Block special file</td> 
   <td>b</td> 
  </tr> 
  <tr> 
   <td>Character special file</td> 
   <td>c</td> 
  </tr> 
  <tr> 
   <td>Directory</td> 
   <td>d</td> 
  </tr> 
  <tr> 
   <td>Symbolic link</td> 
   <td>l</td> 
  </tr> 
  <tr> 
   <td>FIFO (named pipe)</td> 
   <td>p</td> 
  </tr> 
  <tr> 
   <td>Socket</td> 
   <td>s</td> 
  </tr> 
  <tr> 
   <td>Some other file type</td> 
   <td>?</td> 
  </tr> 
 </tbody> 
</table>

##### Permissions #####

<table> 
 <thead> 
  <tr> 
   <th>Permission</th> 
   <th>Symbol</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td>Read</td> 
   <td>r</td> 
  </tr> 
  <tr> 
   <td>Write</td> 
   <td>w</td> 
  </tr> 
  <tr> 
   <td>Execute</td> 
   <td>x</td> 
  </tr> 
 </tbody> 
</table>

##### File and Directory Permissions #####

<table> 
 <thead> 
  <tr> 
   <th>Permission</th> 
   <th>File Meaning</th> 
   <th>Directory Meaning</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td>Read</td> 
   <td>Read Allows a file to be read.</td> 
   <td>Allows file names in the directory to be read.</td> 
  </tr> 
  <tr> 
   <td>Write</td> 
   <td>Allows a file to be modified.</td> 
   <td>Allows entries within the directory to be modified.</td> 
  </tr> 
  <tr> 
   <td>Execute</td> 
   <td>Allows the execution of a file.</td> 
   <td>Allows access to the contents and metadata of entries within the directory.</td> 
  </tr> 
 </tbody> 
</table>

#### User Permissions ####

All files in Linux are owned by a **user** and a **group**. This allows for unique permissions to be applied across three sets of users: **the user owner**, **the group owner**, and **others**. When modifying permissions, these sets can be represented by a single letter: **u** for the user owner, **g** for the group owner, and **o** for others. In addition, the letter **a** can represent all three of these permissions groups.

##### Categories #####

<table> 
 <thead> 
  <tr> 
   <th>Category</th> 
   <th>Symbol</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td>User</td> 
   <td>u</td> 
  </tr> 
  <tr> 
   <td>Group</td> 
   <td>g</td> 
  </tr> 
  <tr> 
   <td>Other</td> 
   <td>o</td> 
  </tr> 
  <tr> 
   <td>All</td> 
   <td>a</td> 
  </tr> 
 </tbody> 
</table>

#### Groups ####

Every user is a member of at least one group, called their primary group. However, users can be members of many groups. Groups are used to organize users into logical sets.

To determine what groups you are a member of, run the **groups** command. If you supply another user’s ID as an argument to the groups command, you will see the list of groups to which that user belongs. You can also run **id -Gn \[user\]** to get the same result.

$ groups
    users sales
    $ id -Gn
    users sales
    $ groups tracy
    users projectx dba
    $ groups john
    users sales manager

#### Decoding Permissions ####

$ ls -l sales.data
    -rw-r--r-- 1 jason users 10400 Jun 14 09:31 sales.data

The first  
character in the permissions string is the type. The next three characters represent the permissions available to the user, also known as the owner of the file. The next three characters represent the permissions available to the group. The last three characters represent the permissions available to all others.

In this case, order has meaning. Permission groups will always be displayed in this order: user, group, and others. Within these three permission groups, permission types will always be in this order: read, write, and execute. If a particular permission type is not granted, then a hyphen (-) will take its place

$ ls -l sales.data.selinux
    -rw-r--r--. 1 jason users 1040 Jun 14 09:31 sales.data.selinux
    $ ls -l sales.data.acl
    -rw-r--r--+ 1 jason users 1040 Jun 14 09:31 sales.data.acl

If there happens to be an additional character at the end of the permissions string, an alternative access control method has been applied. A trailing period (**.**) means that a **SELinux (SecurityEnhanced Linux)** security context has been applied to the file or directory. A trailing plus sign (**+**) means that **ACLs (Access Control Lists)** are in use. SELinux and ACLs are beyond the scope of this book. However, you will be pleased to know that the use of these is rare.

#### Changing Permissions ####

Permissions are also known as modes. The command **chmod**, which is short for “change mode,” is used to change permissions. The format of the chmod command is chmod mode file. There are two ways to specify the mode. The first way is called **symbolic mode**. The symbolic mode  
format is **chmod user\_category operator permission**. Here is a table view of the chmod command using the symbolic mode format.The second way to specify modes with the chmod command is called **octal mode**, which will discussed later.

##### Change Mode Command Symbols #####

<table> 
 <thead> 
  <tr> 
   <th>Symbol</th> 
   <th>Description</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td><strong>chmod</strong></td> 
   <td>The change mode command itself.</td> 
  </tr> 
  <tr> 
   <td><strong>ugoa</strong></td> 
   <td>The user category. Use one or more of <strong>u</strong> for user, <strong>g</strong> for group, <strong>o</strong> for other, a for all.</td> 
  </tr> 
  <tr> 
   <td><strong>+-=</strong></td> 
   <td>One of <strong>+</strong>, <strong>-</strong>, or <strong>=</strong>. Use <strong>+</strong> to add permissions, <strong>-</strong> to subtract them, or <strong>=</strong> to explicitly set them.</td> 
  </tr> 
  <tr> 
   <td><strong>rwx</strong></td> 
   <td>The actual permissions. Use one or more of <strong>r</strong> for read, <strong>w</strong> for write, and <strong>x</strong> for execute.</td> 
  </tr> 
 </tbody> 
</table>

$ ls -l sales.data
    -rw-r--r-- 1 jason users 10400 Jun 14 09:31 sales.data
    $ chmod g+w sales.data
    $ ls -l sales.data
    -rw-rw-r-- 1 jason users 10400 Jun 14 09:31 sales.data

$ ls -l sales.data
    -rw-rw-r-- 1 jason users 10400 Jun 14 09:31 sales.data
    $ chmod g-w sales.data
    $ ls -l sales.data
    -rw-r--r-- 1 jason users 10400 Jun 14 09:31 sales.data

$ ls -l sales.data
    -rw-r--r-- 1 jason users 10400 Jun 14 09:31 sales.data
    $ chmod g+wx sales.data
    $ ls -l sales.data
    -rw-rwxr-- 1 jason users 10400 Jun 14 09:31 sales.data

$ ls -l sales.data
    -rw-r--r-- 1 jason users 10400 Jun 14 09:31 sales.data
    $ chmod ug+wx sales.data
    $ ls -l sales.data
    -rwxrwxr-- 1 jason users 10400 Jun 14 09:31 sales.data

$ ls -l sales.data
    -rw-r--r-- 1 jason users 10400 Jun 14 09:31 sales.data
    $ chmod u=rwx,g+x sales.data37
    $ ls -l sales.data
    -rwxr-xr-- 1 jason users 10400 Jun 14 09:31 sales.data

$ ls -l sales.data
    -rw-r--r-- 1 jason users 10400 Jun 14 09:31 sales.data
    $ chmod a=r sales.data
    $ ls -l sales.data
    -r--r--r-- 1 jason users 10400 Jun 14 09:31 sales.data

$ ls -l sales.data
    -rw-r--r-- 1 jason users 10400 Jun 14 09:31 sales.data
    $ chmod u=rwx,g=rx,o= sales.data
    $ ls -l sales.data
    -rwxr-x--- 1 jason users 10400 Jun 14 09:31 sales.data

#### Numeric Based Permissions ####

The second way to specify modes with the chmod command is called **octal mode**.Some Linux users never move beyond symbolic permissions. However, experienced Linux users find using octal mode quicker and easier in the long term because there are only a few commonly used permissions which can be readily memorized and recalled.

Octal mode permissions are based on the binary numeral system, also known as the base-2 numeral system. Each permission type is treated as a bit that is either set to off, represented by a zero (**0**), or on, represented by a one (**1**). In permissions, order has meaning. Permissions are always in read, write, and execute order. If **r**, **w**, and **x** are all set to off, the binary representation is **000**. If they are all set to on, the binary representation is **111**. To represent read and write permissions while omitting execute permissions, the binary number is **110**.

##### Base-2 and Base-10 Representations of Permissions #####

<table> 
 <thead> 
  <tr> 
   <th>/</th> 
   <th>Read</th> 
   <th>Write</th> 
   <th>Execute</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td>Binary and decimal value for off</td> 
   <td>0</td> 
   <td>0</td> 
   <td>0</td> 
  </tr> 
  <tr> 
   <td>Binary value for on.</td> 
   <td>1</td> 
   <td>1</td> 
   <td>1</td> 
  </tr> 
  <tr> 
   <td>Decimal value for on.</td> 
   <td>4</td> 
   <td>2</td> 
   <td>1</td> 
  </tr> 
 </tbody> 
</table>

##### Octal Permissions #####

<table> 
 <thead> 
  <tr> 
   <th>Permissions</th> 
   <th>String</th> 
   <th>Binary</th> 
   <th>Octal</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td>No permissions</td> 
   <td></td> 
   <td>000</td> 
   <td>0</td> 
  </tr> 
  <tr> 
   <td>Execute only</td> 
   <td>–x</td> 
   <td>001</td> 
   <td>1</td> 
  </tr> 
  <tr> 
   <td>Write only</td> 
   <td>-w-</td> 
   <td>010</td> 
   <td>2</td> 
  </tr> 
  <tr> 
   <td>Write and Execute</td> 
   <td>-wx</td> 
   <td>011</td> 
   <td>3</td> 
  </tr> 
  <tr> 
   <td>Read only</td> 
   <td>r–</td> 
   <td>100</td> 
   <td>4</td> 
  </tr> 
  <tr> 
   <td>Read and Execute</td> 
   <td>r-x</td> 
   <td>101</td> 
   <td>5</td> 
  </tr> 
  <tr> 
   <td>Read and Write</td> 
   <td>rw-</td> 
   <td>110</td> 
   <td>6</td> 
  </tr> 
  <tr> 
   <td>Read, Write, and Execute</td> 
   <td>rwx</td> 
   <td>111</td> 
   <td>7</td> 
  </tr> 
 </tbody> 
</table>

##### Symbolic, Binary, and Decimal Representations of a Given Permission #####

<table> 
 <thead> 
  <tr> 
   <th>/</th> 
   <th>User</th> 
   <th>Group</th> 
   <th>Other</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td>Symbolic</td> 
   <td>rwx</td> 
   <td>rw-</td> 
   <td>r–</td> 
  </tr> 
  <tr> 
   <td>Binary</td> 
   <td>111</td> 
   <td>110</td> 
   <td>100</td> 
  </tr> 
  <tr> 
   <td>Decimal</td> 
   <td>7</td> 
   <td>6</td> 
   <td>4</td> 
  </tr> 
 </tbody> 
</table>

#### Commonly Used Permissions ####

##### Commonly Used Permissions #####

<table> 
 <thead> 
  <tr> 
   <th>Meaning</th> 
   <th>Octal</th> 
   <th>Symbolic</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td>Allows the file’s owner full control over the file. No others on the system have access.</td> 
   <td>700</td> 
   <td>-rwx——</td> 
  </tr> 
  <tr> 
   <td>Allows everyone on the system to execute the file but only the owner can edit it.</td> 
   <td>755</td> 
   <td>-rwxr-xr-x</td> 
  </tr> 
  <tr> 
   <td>Allows a group of people to modify the file and let others read it.</td> 
   <td>664</td> 
   <td>-rw-rw-r–</td> 
  </tr> 
  <tr> 
   <td>Allows a group of people to modify the file and not let others read it.</td> 
   <td>660</td> 
   <td>-rw-rw—-</td> 
  </tr> 
  <tr> 
   <td>Allows everyone on the system to read the file but only the owner can edit it.</td> 
   <td>644</td> 
   <td>-rw-r–r–</td> 
  </tr> 
 </tbody> 
</table>

#### Working with Groups ####

When a file is created, it is set to the current user’s primary group. You can override this behavior by using the **newgrp** command, but remember by default a new file will inherit your default group. In the following example, Jason’s primary group is **users**. The group owner of the file could be set to the Linux group named **sales** by using the **chgrp** command. The format of the **chgrp** command is **chgrp GROUP FILE**.

$ nano sales.report
    $ ls -l sales.report
    -rw-r--r-- 1 jason users 6 Jun 15 20:41 sales.report 
    $ chgrp sales sales.report 
    $ ls -l sales.report 
    -rw-r--r-- 1 jason sales 6 Jun 15 20:41 sales.report 
    $ chmod 664 sales.report 
    $ ls -l sales.report 
    -rw-rw-r-- 1 jason sales 6 Jun 15 20:41 sales.report

#### Directory Permissions ####

Directory permissions usually only contain **0\*\*s, \*\*5\*\*s, and \*\*7\*\*s. Common directory permissions include \*\*755**, **700**, **770**, and **750**. Incorrect directory permissions can prevent file access and file execution. If you determine that a file’s permissions have been set correctly, look at the parent directory’s permissions. Work your way toward the root of the file system by running **ls - ld .** in the current directory, moving up to the parent directory with **cd ..**, and repeating those two steps until you find the problem.

$ ls -ld directory/ 
    drwxr-xr-x 2 jason users 4096 Sep 29 22:02 directory/ 
    $ ls -l directory/ 
    total 0 
    -rwxr--r-- 1 jason users 0 Sep 29 22:02 testprog 
    $ chmod 400 directory 
    $ ls -ld directory/ 
    dr-------- 2 jason users 4096 Sep 29 22:02 directory/ 
    $ ls -l directory/ 
    ls: cannot access directory/testprog: Permission denied 
    total 0 
    -????????? ? ? ? ? ? testprog 
    $ directory/testprog 
    -su: directory/testprog: Permission denied 
    $ chmod 500 directory/ 
    $ ls -ld directory/ 
    dr-x------ 2 jason users 4096 Sep 29 22:02 directory/ 
    $ ls -l directory/ 
    total 0 
    -rwxr--r-- 1 jason users 0 Sep 29 22:02 testprog 
    $ directory/testprog 
    This program ran successfully.

#### Default Permissions and the File Creation Mask ####

The file creation mask, also known as the **umask**, determines the default permissions of new files and directories. The **umask** is typically set by the system administrator; however, an individual user may override the setting by including a umask statement in his or her account’s initialization files.

If no mask is applied, new directories receive **777 (rwxrwxrwx)** permissions and new files receive **666 (rw-rw-rw-)** permissions. When the **umask** is applied to these base permissions, it disables, or masks, certain permissions. For example, a umask of **000** will disable, or mask, zero bits. In this case, new directories receive **777** permissions and new files receive **666** permissions. At the other extreme, a umask of **777** disables all permissions bits. New files and directories receive **000** permissions in this instance.

##### Creation Permission Estimation #####

<table> 
 <thead> 
  <tr> 
   <th>/</th> 
   <th>Directory</th> 
   <th>File</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td>Base Permission</td> 
   <td>777</td> 
   <td>666</td> 
  </tr> 
  <tr> 
   <td>Subtract the umask</td> 
   <td>-022</td> 
   <td>-022</td> 
  </tr> 
  <tr> 
   <td>Creation Permission</td> 
   <td>755</td> 
   <td>644</td> 
  </tr> 
 </tbody> 
</table>

Using a **umask** of **002** is ideal for working with members of your group. When files or directories are created, the permissions allow members of the group to manipulate those files and directories.

By using **007**, no permissions are granted to users outside of the group.

[linux_PNG29.png]: /images/20220512/ebd95459c2664b579e0ff54230c00cc9.png
[image]: http://media.bestofmicro.com/Linus-Torvalds,F-7-16099-13.jpg
[image 1]: https://tse3.mm.bing.net/th?id=OIP.1BLkFfm4S3IdkRyxMumrtAHaFC&pid=Api
[Advantages of Linux]: http://www.linkedin.com/pulse/advantages-linux-justin-kennedy
[XShell]: http://rj.baidu.com/soft/detail/15201.html?ald
[Putty]: http://rj.baidu.com/soft/detail/15699.html?ald
[image 2]: https://timgsa.baidu.com/timg?image&quality=80&size=b9999_10000&sec=1519740833335&di=ffd8ea2b75bd73e21915118a7daa0fcd&imgtype=0&src=http%3A%2F%2Fsrc.onlinedown.net%2Fsupply%2Fsup_logo%2Flogo-1122%2F36382_g.jpg
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[WinSCP]: http://rj.baidu.com/soft/detail/15150.html?ald
[Xftp]: http://rj.baidu.com/soft/detail/18249.html?ald
[FileZilla]: http://rj.baidu.com/soft/detail/13432.html?ald
[image 4]: https://ss3.bdstatic.com/70cFv8Sh_Q1YnxGkpoWK1HF6hhy/it/u=1102947755,2775506422&fm=11&gp=0.jpg
[image 5]: https://timgsa.baidu.com/timg?image&quality=80&size=b9999_10000&sec=1519741346099&di=78ff8c1121f041df23b854c1c85aa7bc&imgtype=0&src=http%3A%2F%2Fimg3.orsoon.com%2Fico%2F201709%2F05183804_5320b2a7fb.png
[image 6]: /images/20220512/369c00baf54b48fb8a7deadf4a118d98.png
[Ubuntu 16.04 ISO]: https://www.ubuntu.com/download/desktop
[CentOS 7 ISO]: https://www.centos.org/download/
[VirtualBox]: http://rj.baidu.com/soft/detail/15321.html?ald
[VMware Workstation]: http://rj.baidu.com/soft/detail/13808.html?ald
[http_blog.csdn.net_u013142781_article_details_50529030]: http://blog.csdn.net/u013142781/article/details/50529030
[http_www.osyunwei.com_archives_7829.html]: http://www.osyunwei.com/archives/7829.html
[http_linux.vbird.org]: http://linux.vbird.org/
[https_linux.cn]: https://linux.cn/
[http_www.linuxidc.com]: http://www.linuxidc.com/
[http_www.learnshell.org]: http://www.learnshell.org/
[https_www.tutorialspoint.com_unix_terminal_online.php]: https://www.tutorialspoint.com/unix_terminal_online.php
[http_cb.vu]: http://cb.vu/
[http_tour.ubuntu.com_en_surf-the-web]: http://tour.ubuntu.com/en/#surf-the-web
[http_www.compileonline.com_execute_bash_online.php]: http://www.compileonline.com/execute_bash_online.php
[https_www.computerhope.com_unix_overview.htm]: https://www.computerhope.com/unix/overview.htm
[https_bellard.org_jslinux]: https://bellard.org/jslinux/
[http_www.allitebooks.com]: http://www.allitebooks.com/
[https_doc.lagout.org_programmation]: https://doc.lagout.org/programmation/
[https_www.syncfusion.com_ebooks]: https://www.syncfusion.com/ebooks
[http_www.onlineprogrammingbooks.com]: http://www.onlineprogrammingbooks.com/
[http_www.ebook777.com_it-ebooks]: http://www.ebook777.com/it-ebooks
[https_linuxsurvival.com_testimonials]: https://linuxsurvival.com/testimonials/
[image 7]: /images/20220512/5e0dd82b9c2143598aed7869ee6c221b.png
[image 8]: http://cdn.makeuseof.com/wp-content/uploads/2014/08/ubuntu-command-line.png?f411a4
[https_www.computerhope.com_unix.htm_commands]: https://www.computerhope.com/unix.htm#commands