Linux chmod command

Linux Command Encyclopedia

Linux chmod (English full spelling: change mode) command is a command that controls user permissions to files.

chmod (change mode) is a command in Linux systems used to change the permissions of files or directories. It controls the access permissions of the file owner, the owning group, and other users to the file.

Only the file owner and the superuser can modify the permissions of a file or directory.

Linux/Unix file access permissions are divided into three levels: file owner (Owner), user group (Group), and other users (Other Users).

You can use absolute mode (octal numeric mode) and symbolic mode to specify file permissions.

Permission to use: All users

Syntax

chmod [选项] 权限模式 文件...
chmod [选项] --reference=参考文件 文件...

Common Options

• -c : Only display the change action if the file permission has actually been changed
• -f : Do not display error messages if the file permissions cannot be changed
• -v : Display detailed information about permission changes
• -R : Apply the same permission changes to all files and subdirectories under the current directory (that is, change them one by one recursively)
• --help : Display help information
• --version : Display version

mode : Permission setting string, the format is as follows:

Permission Modes

1. Symbolic mode (ugoa+/- permission)

[ugoa...][[+-=][rwxX]...][,...]

Where:

• u represents the owner of the file, g represents those who belong to the same group as the file owner, o represents others, and a represents all of the three.
• + means add permission, - means remove permission, = means set permission uniquely.
• r means readable, w means writable, x means executable, X means only when the file is a subdirectory or the file has already been set as executable.

In symbolic mode, multiple items can be set: who (user type), operator, and permission. The settings of each item can be separated by commas.

The chmod command will modify the access permissions to the file for the user types specified by who. The user type is indicated by one or more letters in the who position, as shown in the who symbolic mode table:

who	User type	Description
u	user	File owner
g	group	Group of the file owner
o	others	All other users
a	all	All users, equivalent tougo

Symbolic mode table for operator:

Operator	Description
+	Add permission to the specified user type
-	Remove permission from the specified user type
=	Set permissions for the specified user type, i.e., reset all permissions of the user type

Permission types:

Mode	Name	Description
r	read	Set as readable permission
w	write	Set as writable permission
x	Execute permission	Set as executable permission
X	Special execute permission	Only set the file permission as executable when the file is a directory file, or when other types of users have execute permission
s	setuid/gid	When the file is executed, set the file's setuid or setgid permission according to the user type specified by the who parameter
t	Sticky bit	Set the sticky bit. Only the superuser can set this bit, and only the file owner u can use this bit.

2. Octal syntax

The chmod command can use octal numbers to specify permissions.

Number	Permission
4	Read (r)
2	Write (w)
1	Execute (x)

Combination:

• Owner permission (first digit)
• Group permission (second digit)
• Other user permission (third digit)

Common combinations:

• 755: rwxr-xr-x
• 644: rw-r--r--
• 700: rwx------

The permission bits of a file or directory are controlled by 9 permission bits, each group of three bits represents the read, write, and execute permissions of the file owner (User), the read, write, and execute of the group (Group), and the read, write, and execute of other users (Other).

#	Permission	rwx	Binary
7	Read + Write + Execute	rwx	111
6	Read + Write	rw-	110
5	Read + Execute	r-x	101
4	Read-only	r--	100
3	Write + Execute	-wx	011
2	Write-only	-w-	010
1	Execute-only	--x	001
0	None	---	000

For example, 765 would be interpreted as follows:

• The owner's permission is expressed in numbers: the sum of the digits of the owner's three permission bits. For example, rwx means 4+2+1, which should be 7.
• The group's permission is expressed in numbers: the sum of the digits of the group's permission bits. For example, rw- means 4+2+0, which should be 6.
• Other users' permission is expressed in numbers: the sum of the digits of the other users' permission bits. For example, r-x means 4+0+1, which should be 5.

Examples

Set file1.txt to be readable by everyone:

chmod ugo+r file1.txt

Set file1.txt to be readable by everyone:

chmod a+r file1.txt

Set file1.txt and file2.txt so that the file owner and those in the same group can write, but others cannot write:

chmod ug+w,o-w file1.txt file2.txt

Add execute permission for the owner of the ex1.py file:

chmod u+x ex1.py

Set all files and subdirectories in the current directory to be readable by anyone:

chmod -R a+r *

In addition, chmod can also use numbers to represent permissions, e.g.:

chmod 777 file

The syntax is:

chmod abc file

Where a, b, c are each a number, representing the permissions of User, Group, and Other respectively.

r=4，w=2，x=1

• For rwx attributes, 4+2+1=7;
• For rw- attributes, 4+2=6;
• For r-x attributes, 4+1=5.

chmod a=rwx file

and

chmod 777 file

The effect is the same

chmod ug=rwx,o=x file

and

chmod 771 file

The effect is the same

If usingchmod 4755 filenameThis can give the program root privileges.

More Notes

Command	Description
chmod a+r file	Add read permission to all users for file
chmod a-x file	Remove execute permission from all users for file
chmod a+rw file	Add read and write permissions to all users for file
chmod +rwx file	Add read, write, and execute permissions to all users for file
chmod u=rw,go= file	Set read and write permission for the owner of file, and clear all permissions for the group and other users on file (a space represents no permission)
chmod -R u+r,go-r docs	Add read permission for the user to all files in the directory docs and its subdirectory hierarchy, and remove read permission for the group and other users
chmod 664 file	Set read and write permission for the owner and group of file, and set read permission for other users
chmod 0755 file	Equivalent tou=rwx (4+2+1),go=rx (4+1 & 4+1)。0No special mode.
chmod 4755 file	4The set-user-ID bit is set, and the rest is equivalent to u=rwx (4+2+1), go=rx (4+1 & 4+1).
find path/ -type d -exec chmod a-x {} \;	Remove execute permission for all users on path/ and all its directories (not including files); use '-type f' to match files.
find path/ -type d -exec chmod a+x {} \;	Allow all users to browse or pass through the directory path/

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