Detailed analysis of the chmod command to modify file permissions under Linux

Use the Linux chmod command to control who can access, read, write, or run the target file .

In Linux, who can do what with a file or directory is controlled through a set of permissions.

Permissions control the operations (read, write, or execute) that can be performed on a file or directory.

We can use the -l (long format) option to ls to list the file permissions for files and directories.

ls -l 

In each line, the first character identifies the type of entry listed. If it is a dash ( - ), it is a file. If it is the letter d , it is a directory.

The next nine characters represent three sets of permission settings.

The first three characters show the permissions of the user who owns the file ( User Rights ).

The middle three characters show the permissions of the user group membership ( group permissions ).

The last three characters show permissions for anyone outside the first two categories ( other permissions ).

There are three characters in each set of permissions. The character is an indicator of whether the permission is present or not. They are either a dash ( - ) or a letter. If the character is a dash, it means that permission is not granted. If the character is an r , w , or x , the permission is granted.

The letters represent:

r : Read permission. You can open the file and view its contents.

w : write permission. This file can be edited, modified and deleted.

x : Execute permission. If the file is a script or program, it can be run (executed).

For example:

--- means no permissions are granted at all.

rwx means full permissions are granted. Read, write, and execute indicators are present.

Syntax for setting permissions

To set permissions with chmod , we need to tell it:

who : For whom we are setting permissions.

what : What changes are we making? Are we adding or removing permissions?

which: Which permissions did we set?

We use indices to represent these values ​​and form short "permission statements" such as u+x , where "u" stands for "current user" (who), "+" stands for addition ( what ), and "x" stands for execute permission (which).

The "who" values ​​we can use are:

u : user, indicating the owner of the file.

g : Group, indicating the members of the group to which the file belongs.

o : Others, meaning people who are not managed by u and g permissions.

a : All, meaning all of the above.

The values ​​we can use for " what " include:

– : Minus sign. Remove permissions.

+ : Plus sign. Grant permissions. The permission is added to the existing permissions. If you want to have this permission and only set this permission, use the = option as described below.

= : equal sign. Set permissions and remove others.

The "which" values ​​we can use are:

r : Read permission.

w : write permission.

x : Execute permission.

Examples:

We can add execute permissions for everyone with the following command:

chmod a+x new_script.sh 

We want user dave to have read and write permissions, while group and other users have only read permissions. We can use the following command:

chmod u=rw,og=r new_file.txt 

Using the "=" operator means we clear all existing permissions and then set the specified permissions.

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