Getting started with Lunix commands - user and file permissions (Chmod details)

summary

This article mainly introduces Lunix The user operation in the command is related Such as useradd userdel etc. . And file permission modification chmod command

Introduce

First of all, we need to clarify a core theme Lunix It is a multi-user system composed of files

Composed of documents , Naturally, there are operations on files ,Lunix There are three main operations for files read (Read) , Write (Write), perform (Excute)

Multiuser system ,Lunix Multiple users in can use the system at the same time , Then a series of permission problems will naturally arise . It can be roughly divided into user And user groups . Users are divided into administrators and ordinary users .

The user action

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Add users

Common commands are as follows , That is, a new one named zizy Users of

useradd zizy

Set the user's password

by zizy The password specified by the user is 123456

passwd zizy
New password:123456
Re-enter new password:123456

Change user groups

We want to create a project named mysql Group , Group No 5000( The group number is used to uniquely identify a group ), The users of this group manage the database

groupadd -g 5000 mysql

At the same time, we put zizy Users adding to mysql In the group As one of the database administrators

usermod -g mysql zizy

Delete user

Delete user zizy

userdel zizy 

Delete the group

groupdel mysql

File permissions

The document has attribution , Users with different identities have different permissions for different files
lunix The main documents are file type and File permissions attribute
The types of documents are Catalog d , file - , link l , Interface b , port c
The authority of documents is divided into read r , Write w , perform x

A common file attribute is expressed as

-rwxrwxrwx

first - Indicates that this is a file
The first after rwx Express The user of the file has Read write execution rights
the second rwx Indicates that the user group to which the file belongs has Read write execution rights
Third rwx Indicates other users ( Non affiliated users Non affiliated user group ) There are Read write execution rights

The type of file and the permissions of different users are calibrated through ten digits ,

Without this permission, the corresponding bit will become -
as follows

drwxr-xr--

This is a directory
The users are Read write execution rights
The user group belongs to read Executive authority
Other users can only read jurisdiction

Change file permissions

Use chmod Command to modify file permissions

Numerical assignment

This is also the most common method Using a digital 4 2 1 Represent the r w x
And add them Such as rwx by 7 rw- by 6 r-x by 5 r-- by 4

For a file a01 Use the following command

chmod 761  a01

The permission attribute of this file is

-rwxrw---x

Direct assignment method

about user (u) User group (g) Other users (o) Give permission directly

For a file a01 Use the following command

chmod u=rwx,g=rx,o=x a01

The permission attribute of this file is

-rwxr-x--x

Use + - Symbol Add delete permission

For the following permissions a01 file

-rwxr-x--x

Use the following command

chmod u-w ,o+rw a01

Permissions will change to

-r-xr-xrwx

Advanced

Okay Now friends should understand the basis of file permissions , Now let's talk about some advanced and fancy

Use umask To get file permissions

umask Set the default permission for users to create files , You can use commands umask Check it out. umask value .

simply It's through umask To get to know When we create a new file What is its default permission .

umask There are four Such as 0043 generally speaking Let's just look at the last three of it namely 043

about file , Use 666 subtract umask value Then odd digits +1( All odd digits have to be added by one )

666 - 043 = 623 Then add one to the odd number The permission of the file is 624

-rw--w-r--

about Folder Use 777 subtract umask value that will do

777 - 043 = 734 The permissions of the folder are 734

drwx-wxr--

Special file permissions

setuid about user (u) Give super permissions , use s Express

• Suppose the original permission is rwx , + s obtain rws . namely rwx + s -> rws
• Suppose the original permission is rw- , + s obtain rwS. namely rw- + s -> rwS

setgid about User group (g) Give super permissions

• Suppose the original permission is rwx , + s obtain rws . namely rwx + s -> rws
• Suppose the original permission is rw- , + s obtain rwS. namely rw- + s -> rwS

sticky Only the owner or root Can be deleted , about Other users (o) operation

• Suppose the original permission is rwx , + t obtain rwt . namely rwx + t -> rwt
• Suppose the original permission is rw- , + t obtain rwT. namely rw- + T -> rwT

notes ： Case and rwx The first two of are irrelevant Only with the third x Yes no of

For the following permissions a01 file

-rw-rwxrw-

Use this operation

chmod u+s,g+s,o+t  a01

The result is zero

-rwSrwsrwT

Special permission to use digital identification

setuid setgid sticky Use them separately 4 2 1 Express
The logo is in front of three As the first At this time, the permission of the file It becomes four
For example, the file permission bit is 741
Use chmod Join in setuid and sticky After the authority

chmod u+s,o+t a01

It becomes 5741 , 5 Marked setuid and sticky