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Each file and directory in The Linux system is configured with access permissions for different users to determine how users can access and operate files and directories. That is, only authorized users can access and operate files and directories.

Next, you’ll see how to change access to files and directories using the chmod command.

Linux File Permissions

In Linux, there are three different types of users who can access files or directories:

  • File owner
  • Group User
  • Other users

There are three types of file permissions:

  • R: read permission
  • W: write permission
  • X: execution permission

We can assign different file access permissions to three different categories of users, specifying which users are allowed to read, write, or execute files.

You can use the ls command to view file permissions:

ls -l test.sh
-rwxr-xr-x. 1 root root 2519 May 19 10:13 test.sh
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We can see something like -rwxr-xr-x with 10 bits in total:

-rwxr-xr-x
||||||||||
||||||||||
||||||||||
12345678910
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  • The first one,-Represents a file type, which can be a regular file-, directories,dSymbolic linklOr other types of files
  • The next three,rwxRepresents the owner’s permissions
  • And the next three,r-xRepresents the permissions of the same group
  • The last three,r-xRepresents the rights of other users

Note: – Indicates that no corresponding permission has been granted.

Run the chmod command to change file directory permissions

The chmod command is used to change the access permission of a file or directory.

Syntax format

Chmod [who] [+ | | =] [mode] file nameCopy the code

Mode indicates the mode state of access, usually using several specific symbols to set the status of access.

Set the user status The operator permissions
u + r
g – w
o = x
a

The permission state can be divided into three parts.

  • Set user status:

    • u: indicates the permission of the owner
    • g: Indicates the permission of the group
    • o: indicates the permission of other users
    • a: indicates the rights of all users

  • Operator:

    • +: Adds a permission to an existing permission
    • -: indicates that an existing permission is removed
    • =: indicates that the permission is set to the later value

  • Jurisdiction:

    • r: Read permission
    • w: Write permission
    • x: Execute permission

For example, add owner and group user executable permissions to the test file:

chmod ug+x test
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In addition, the chmod command also supports numeric representation of file permissions to be changed. You can use a set of 3-digit numbers to represent permissions on a file directory:

  • The first number represents the permission of the owner, i.eu
  • The second digit indicates the permission of the group, i.eg
  • The third digit indicates the permissions of other users, i.eo

Each digit is the sum of the following permission status digits (4, 2, 1, 0) :

  • 4: read permission
  • 2: write permission
  • 1: execution permission
  • 0: no corresponding permission is available

Add these numbers together to get a number between 0 and 7 that represents the status of the owner, peer group, and other user privileges.

For example, to grant read, write, and execute permissions to the test file owner, read and execute permissions to the same group, and only read permissions to all other users, you can do the following:

  • Owner:rwx = 4 + 2 + 1 = 7
  • Groups:rx = 4 + 0 + 1 = 5
  • Other:rx = 4 + 0 + 0 = 4
chmod -R 754 test
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conclusion

The chmod command can be used to change the access permission of files and directories in both symbol and digit modes.

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