Chmod command principle and usage details [easy to understand]

Hello everyone , I meet you again , I'm your friend, Quan Jun .

Chmod The command is mainly used to modify 、 Set file permissions

chmod There are two main ways to modify file permissions ： Alphabet and number

Although the numerical method is simpler than the alphabetic method , But the number method is based on the alphabetic method , So let's introduce the alphabetic method first .

1、 Alphabet ：chmod (u g o a) (+ – =) (r w x) ( file name )

Above is chmod Usage of , Each bracket is a parameter , The first three brackets are mainly used together, that is chmod + Set the mode + file name ; Let's first introduce the meaning of these parameters ：

[u g o a]	meaning
u	user Represents the owner of the file
g	group Indicates that it belongs to the same group as the owner of the file ( group ) person , That is, user groups
o	other Represents other user groups
a	all It represents all three-is

[+ – =]	meaning
+	Increase the permissions
–	Revoke authority
=	Set permissions

[r w x]	meaning
r	read Represents read , For a directory , without r jurisdiction , Then it means not to pass ls Check out the contents of this directory .
w	write Representation can be written , For a directory , without w jurisdiction , That means you can't create new files in the directory .
x	excute For executables , For a directory , without x jurisdiction , Then it means not to pass cd Enter this directory .

usage ：chmod + Set the mode + file name Setting mode in the middle , We should treat each other separately u(user), g(group), o(other) Set the permissions .

chmod u+rwx, g+rwx, o+rwx filename  Change the command description to filename file ,  give user、group、other Both read、write、excute Authority 

Copy

Therefore, the setting mode should include permission settings for three user types , They are independent , They don't influence each other .

Illustrate with examples ：

chmod u+rw filename    State for filname,  Give only user user , read、write jurisdiction 
chmod u+rwx, g+rw  filename   Instructions for filename, give user user read、write、excute jurisdiction ; give group user read、write jurisdiction 

Copy

Be careful ：（chmod + Set the mode + file name ） The setting mode includes three types of users （u、g、o） Permission settings for , These three are independent , Mutual interference .

2、 Digital method

The number method is based on the letter method , If you understand the alphabet , Digital method is easy to understand , And easy to use .

usage ：chmod + Number combination + file name A combination of numbers usually consists of three numbers ：

The first number corresponds to the user of the alphabetic method u（user）

The second number corresponds to the user of the alphabetic method g（group）

The third number corresponds to the alphabetic user o（other）

in addition ：

r (read) —————-> 4

w (write) —————-> 2

x (excute) —————-> 1

Example details ：

Digital method ：chmod 777 file name <———– Corresponding ————–> Alphabet ： chmod u+rwx, g+rwx, o+rwx file name

The first number 7： On behalf of the user u Authority rwx, 4 (r) + 2 (w) + 1 (x) = 7

The second number 7： On behalf of the user g Authority rwx, 4 (r) + 2 (w) + 1 (x) = 7

The third number 7： On behalf of the user o Authority rwx, 4 (r) + 2 (w) + 1 (x) = 7

Illustrate with examples ：

 Digital method ：chmod  755 filename     Corresponding      Alphabet ： chmod u+rwx, g+rx, o+rx filename
 Digital method ：chmod  751 filename     Corresponding      Alphabet ： chmod u+rwx, g+rx, o+x  filename
 Digital method ：chmod  765 filename     Corresponding      Alphabet ： chmod u+rwx, g+rw, o+rx filename

Copy

After the above introduction , Should be for chmod I understand the usage of .

appendix —— chmod Another explanation of the numerical method ： The above explanation is based on the method of summation , The following uses binary method to explain, digital method to express ： r w x Permissions are represented by three binary digits ： First digit （0 or 1） Express r, by 1 Indicates valid , 0 Invalid Second digit （0 or 1) Express w, by 1 Indicates valid ,0 Invalid Third digit （0 or 1) Express x, by 1 Indicates valid , 0 Invalid 000 <———————> No authority 100 <———————> r(read) <—–> 4 010 <———————> w(write) <—–> 2 001 <———————-> x(excute) <—-> 1 101 <———————-> rx <—-> 5 110 <———————-> rw <—-> 6 111 <———————-> rwx <—–> 7

Reference material ： http://www.cnblogs.com/loaderman/p/6547609.htmlhttp://blog.csdn.net/leilu2008/article/details/8507037http://blog.chinaunix.net/uid-21880738-id-1813031.htmlhttp://www.cnblogs.com/chengJAVA/p/4319420.htmlhttps://zhidao.baidu.com/question/168415347.html

Publisher ： Full stack programmer stack length , Reprint please indicate the source ：https://javaforall.cn/148020.html Link to the original text ：https://javaforall.cn