16_Linux Basics - File & Directory Permissions and Ownership

1. File/Directory Permissions#

File/Directory Permissions

Access Permissions

Readable(read)：Allows viewing file contents, displaying directory lists r
Writable(write)：Allows modifying file contents, allows creating, moving, or deleting files or subdirectories in the directory w
Executable(execute)：Allows running programs, switching directories x

Ownership

File Owner(owner)：The user account that owns the file or directory # Note: Owner
Group(group)：The group account that owns the file or directory # Note: Group
Others(others)：Other people besides the owner and group # Note: Others

Note: Distinction is made for files versus directories#

ls -al View detailed information about files or folders in the current directory, including file permissions
ls -ld View the current directory

Example 1: Check file permissions ls -al
# Note: The third column is the owner, the fourth column is the group
---------------------------------------------------------------------------------------------------------------------------------
[root@sanchuang-linux ~]# ls -al				# Note: Check file permissions
Total usage 668
-rw-r--r--.  1 root root    211 Sep  18 10:31 '!'
dr-xr-x---. 21 root root  12288 Nov  6 15:16  .
[root@sanchuang-linux ~]# ls -al zuoye.sh 		# Note: Check permissions of a single file
-rw-r--r--. 1 root root 1212 Oct 29 20:20 zuoye.sh
[root@sanchuang-linux ~]# ls -al /home			# Note: Check permissions of all files in the /home directory
Total usage 18348
drwxr-xr-x. 43 root         root            4096 Nov  6 15:36 .
dr-xr-xr-x. 38 root         root            4096 Nov  5 15:12 ..
[root@sanchuang-linux ~]# ls -al *.sh			# Note: Check file permissions
-rw-r--r--. 1 root root    0 Sep  25 09:35 backup_log.sh
-rw-r--r--. 1 root root   41 Oct  28 20:27 file_test.sh

Example 2: Check directory permissions ls -ld /home
--------------------------------------------------------------------------------------------
[root@sanchuang-linux ~]# ls -ld /home			# Note: Check permissions of the /home directory
drwxr-xr-x. 43 root root 4096 Nov  6 15:36 /home

2. File/Directory Permissions#

[root@localhost ~]# ls -ld first.py 
-rw-r--r--. 1 root root 1097 Oct 13 10:20 first.py
File Type Access Permissions Owner Group

Example: drwxr-xr-x
--------------------------------------------------------------------------------------------
The first column: indicates file type and permissions
	The first character
        -	Regular file file
        d	Indicates directory
        s	Indicates socket file			# Note: socket is a method of communication between processes.
        p	Pipe file
        c	Character device file	tty
        b	Block device file	 Disk	# Note: Block device files related to hardware are generally placed under /dev
        l	Link file				# Note: There are many link files under /usr/bin/

  #Permissions r Readable  w Writable  x Executable
  ·The 2nd to 4th characters
  	  Indicate owner permissions
  ·The 5th to 7th characters
  	  Indicate group permissions
  ·The last three
	  Indicate others' permissions
--------------------------------------------------------------------------------------------
All commands entered are searched in the PATH environment variable

Create a link file
# Note: ln -s source_file target_file		symbolic adj. Symbolic; using symbols
[root@sanchuang-linux bin]# ln -s python3.6 python			# Note: -s creates a soft link
[root@sanchuang-linux bin]# ln -s python3.6 /root/python	# Note: A link file can be created at an absolute path
# Note: The link file /root/python is placed in the /root directory
which checks the absolute path of a command
[root@sanchuang-linux bin]# which mkdir
/usr/bin/mkdir
[root@sanchuang-linux bin]# mv /usr/bin/mkdir /usr/bin/mkdirbak	# Note: After renaming, the command cannot be found
[root@sanchuang-linux bin]# mkdir
-bash: mkdir: command not found
[root@sanchuang-linux bin]# mkdirbak /tmp/mkdir
[root@sanchuang-linux bin]# mv /usr/bin/mkdirbak /usr/bin/mkdir	# Note: Rename back

There are many link files under /usr/bin/
[root@sanchuang-linux tmp]# cd /usr/bin/						# Note: There are many link files under /usr/bin/
[root@sanchuang-linux bin]# ls -al python2						# Note: Link file
lrwxrwxrwx. 1 root root 9 Jun   5 11:38 python2 -> python2.7
[root@sanchuang-linux bin]# ls -al python3						# Note: Link file
lrwxrwxrwx. 1 root root 25 Sep  18 15:47 python3 -> /etc/alternatives/python3

3. Socket#

Note: A socket is a method of communication between processes, usually used for communication between different processes on different hosts (network programming)#

The ftp service has vsftpd, which opens port 21 when the vsftpd server is running. To connect to this service, the ftp client needs to open a random port to connect to port 21 for network transmission. A specific protocol is specified through the network for the connection.

Starting a vsftpd will create a socket file.

Generally, an open port will have a socket file.

[root@localhost mail]# cd /data/mysql
[root@localhost mysql]# ls -al
srwxrwxrwx  1 mysql mysql        0 Oct  7 12:04 mysql.sock		# Note: s socket file (pink)

4. Five Major Subsystems of the Linux Kernel#

Five Major Subsystems of the Linux Kernel:

1. File System
1. Memory Management
1. Process Communication
1. Process Scheduling
1. Network Interface

5. Processes#

A process is the basic unit of resource allocation in the system, and resources are isolated between different processes.

Inter-process communication (5 common types):

Pipe
Signal (kill -9)
Shared Memory
Socket
Message Queue

Example 1: Pipe				# Note: A pipe is a method of communication between processes
Example
[root@sanchuang-linux ~]# cat /etc/passwd |grep sanchuang	# Note: Pass the output of the previous process to the input of the next process through a pipe
sanchuang:x:1005:1005::/home/sanchuang:/bin/bash
# Note: Suitable for different processes on the same host
--------------------------------------------------------------------------------------------
Example 2: Signal				
Example: kill -9 pid
[root@sanchuang-linux ~]# kill -l
 1) SIGHUP	 2) SIGINT	 3) SIGQUIT	 4) SIGILL	 5) SIGTRAP
 6) SIGABRT	 7) SIGBUS	 8) SIGFPE	 9) SIGKILL	10) SIGUSR1………………
[root@sanchuang-linux ~]# ps -ef |grep nginx
root        5868    5691  0 16:29 pts/0    00:00:00 grep --color=auto nginx
[root@sanchuang-linux ~]# kill -9 2868					# Note: Send signal 9 to process 1209
--------------------------------------------------------------------------------------------
Example 3: Shared Memory
# Note: A new memory space is created between A and B, which can be accessed together
# Note: Suitable for different processes on the same host
--------------------------------------------------------------------------------------------
Example 4: Socket
# Note: Used for network communication between different processes on different hosts
--------------------------------------------------------------------------------------------
Example 5: Message Queue
# Note: Similar to a queue of 1,2,3,4,5,6
# Note: In the shared memory space of A and B, A produces and B retrieves. A produces 1, B takes it out, 1 goes out from A

6. Under the Root Directory#

/etc Stores configuration files
/dev Stores device files # Note: Hardware-related Disk, Network Card, Socket
/root # Note: root user's home directory
/home # Note: Stores ordinary users' home directory files

[root@sanchuang-linux ~]# cd /dev
[root@sanchuang-linux dev]# ls -al
crw-rw-rw-   1 root tty       5,   0 Nov  6 11:32 tty		# Note: Character device file	tty
crw--w----   1 root tty       4,   0 Nov  5 15:12 tty0
crw--w----   1 root tty       4,   1 Nov  6 14:28 tty1
brw-rw----   1 root disk      8,   0 Nov  5 15:12 sda		# Note: Block device file	 Disk

7. Executable Permissions#

The second to tenth characters of the first column
  #Permissions r Readable  w Writable  x Executable
  The 2nd to 4th characters
  	  Indicate owner permissions
  The 5th to 7th characters
  	  Indicate group permissions
  The last three
	  Indicate others' permissions

Example
--------------------------------------------------------------------------------------------
[root@sanchuang-linux ~]# ls -al men_test.sh 
-rw-r--r--. 1 root root 0 Oct 30 16:26 men_test.sh
[root@sanchuang-linux ~]# bash men_test.sh 	# Note: Use bash interpreter to execute (creates a new bash)
[root@sanchuang-linux ~]# sh men_test.sh 	# Note: Use sh interpreter to execute (creates a new sh)
[root@sanchuang-linux ~]# . men_test.sh		# Note: Use the current bash to execute, will inherit the current environment variables
# Note: Equivalent to current bash, current environment
[root@sanchuang-linux ~]# ./men_test.sh 	# Note: Not enough permissions, lacks executable permissions
-bash: ./men_test.sh: Permission denied				 # Note: Files with executable permissions can be executed directly with ./men_test.sh

8. Shell File Execution Methods#

Shell file execution methods (3 types)

Note: There is no requirement for executable file suffixes in shell; it does not have to end with .sh#

[root@sanchuang-linux ~]# vim test_aa.sh
echo "testaa..."
echo $a
echo $b
[root@sanchuang-linux ~]# a=1
[root@sanchuang-linux ~]# b=2
[root@sanchuang-linux ~]# . test_aa.sh 	# Note: Use the current bash to execute test_aa.sh, will inherit the current shell's environment variables
testaa...								# Note: Using . will inherit all variables in the current bash
1
2
[root@sanchuang-linux ~]# bash test_aa.sh 	# Note: Creates a new bash environment to execute commands
testaa...									# Note: The new bash environment does not have defined variables a, b
											# Note: Does not output parent bash's a, b
											# Note: Does not retrieve a, b
[root@sanchuang-linux ~]# sh test_aa.sh 		# Note: Creates a new sh environment to execute commands
testaa...
											# Note: No output

[root@sanchuang-linux ~]# ./test_aa.sh 		# Note: Execute the file itself, check if the file itself has execution permissions
-bash: ./test_aa.sh: Permission denied				 # Note: Directly executing this file in the current directory requires executable permissions
# Note: Other methods do not necessarily require executable permissions

Summary
. test_aa.sh			No executable permissions required	Executed in the current bash process
bash test_aa.sh			No executable permissions required	New bash process runs
sh test_aa.sh			No executable permissions required	New bash process runs
./test_aa.sh			Requires executable permissions	New bash process runs

9. chmod Command#

chmod Command # Note: Modify file access permissions

Format 1: chmod [ugoa] [+-=] [rwx] file or directory...

u, g, o, a represent
u owner, g group, o other users, a all users
+,-,= represent
+ add, - remove, = set permissions
rwx
Corresponding permission characters

Common command options

-R: Recursively modify permissions of all files and subdirectories in the specified directory

Example
--------------------------------------------------------------------------------------------
[root@sanchuang-linux ~]# chmod u+x men_test.sh 		# Note: Add executable permission for the owner
[root@sanchuang-linux ~]# ls -al men_test.sh 
-rwxr--r--. 1 root root 0 Oct 30 16:26 men_test.sh		# Note: Executable files are all green
# Note: Executable files are all green
# Note: Socket files are all pink
[root@sanchuang-linux ~]# ./men_test.sh 				# Note: Can be executed directly now
--------------------------------------------------------------------------------------------
[root@sanchuang-linux lianxi]# chmod a-x sc -R		# Note: -R recursively removes executable permissions from the directory and all its subfiles
[root@sanchuang-linux lianxi]# chmod a+x sc -R		# Note: Adds executable permissions for all users
============================================================================================
# Note: Only the root user can use the root directory
[root@sanchuang-linux ~]# ls -ld /
dr-xr-xr-x. 38 root root 4096 Nov  5 15:12 /
============================================================================================
[root@mysql-binary ~]# chmod u+x group_member.sh 
[root@mysql-binary ~]# ./group_member.sh sanchuang5		# Note: After adding executable permissions, can run with ./
sanchuang5,sanchuang11,sanchuang13,sanchuang10,sanchuang12

10. chmod Command#

chmod Command # Note: Modify file access permissions

Format 2: chmod nnn file or directory... # Note: nnn represents a 3-digit octal number

Permission Item Read Write Execute Read Write Execute Read Write Execute

Character Representation r w x r w x r w x

Numeric Representation 4 2 1 4 2 1 4 2 1

Permission Allocation File Owner File Group Other Users

Common command options

-R: Recursively modify permissions of all files and subdirectories in the specified directory

Note: The root user can generally use it regardless of permissions#

Example
--------------------------------------------------------------------------------------------
[root@sanchuang-linux ~]# chmod 777 sc 				# Note: All have read, write, and execute permissions
[root@sanchuang-linux ~]# chmod 700 sc 				# Note: Owner has read, write, and execute permissions
--------------------------------------------------------------------------------------------
[root@sanchuang-linux home]# ls -ld sanchuang		# Note: Check directory permissions ls -ld
drwx------. 4 sanchuang sanchuang 122 Nov  6 17:10 sanchuang
[root@sanchuang-linux home]# chmod 777 /home/sanchuang		# Note: All have read, write, and execute permissions
[root@sanchuang-linux home]# ls -ld sanchuang
drwxrwxrwx. 4 sanchuang sanchuang 122 Nov  6 17:10 sanchuang
[root@sanchuang-linux home]# chmod 777 /home/sanchuang -R	# Note: -R: Recursively modify permissions of all files and subdirectories in the specified directory
[root@sanchuang-linux home]# ls -ld sanchuang
drwxrwxrwx. 4 sanchuang sanchuang 122 Nov  6 17:10 sanchuang
# Note: Granting read, write, and execute permissions to all objects in a user's home directory (e.g., /home/sanchuang) will prevent ordinary user sanchuang from logging in remotely.

Example: The root user can generally use it regardless of permissions
[root@sanchuang-linux ~]# ls -ld /etc/shadow
---------- 1 root root 4201 Nov  7 12:04 /etc/shadow
[root@sanchuang-linux ~]# vim /etc/shadow

11. Generally, ordinary users can only create files or folders in their home directory or the /tmp directory#

Generally, ordinary users can only create files or folders in their home directory or the /tmp directory.

Note: The /tmp directory is a temporary directory, generally used to store temporary items (e.g., some records needed during program execution, which are deleted after the program ends)#

[root@sanchuang-linux ~]# su - sanchuang
Last login: Fri Nov  6 17:09:41 CST 2020 pts/2
[sanchuang@sanchuang-linux ~]$ ls -ld /
dr-xr-xr-x. 38 root root 4096 Nov  5 15:12 /
[sanchuang@sanchuang-linux ~]$ ls -ld /etc
drwxr-xr-x. 90 root root 8192 Nov  7 15:00 /etc
[sanchuang@sanchuang-linux ~]$ ls -ld ./				# Note: Create files or folders in the home directory
drwxrwxrwx. 4 sanchuang sanchuang 122 Nov  6 17:10 ./
[sanchuang@sanchuang-linux ~]$ ls -ld /tmp				# Note: Create files or folders in the /tmp directory
drwxrwxrwt. 12 root root 264 Nov  7 10:36 /tmp

# Note: chmod can only modify permissions when you are the owner

12. chmod#

chmod # Note: Modify file ownership permissions

Using chmod, you must be the owner of the file to change its read, write, and execute permissions (except for root)

[root@sanchuang-linux chenpeng]# su - sanchuang11		# Note: Adding - will automatically go to the home directory
Last login: Sat Nov  7 15:10:51 CST 2020 pts/0
[sanchuang11@sanchuang-linux ~]$ exit
Logout
[root@sanchuang-linux chenpeng]# su sanchuang11			# Note: Not adding - will stay in the current path
[sanchuang11@sanchuang-linux chenpeng]$

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