chmod Command in Linux

The chmod (change mode) command in Linux/UNIX is used to modify file and directory permissions. It controls who can read, write, or execute a file by setting access rights for the owner, group, and others.

• Used to change permissions of files and directories
• Manages access for owner, group, and others
• Controls read (r), write (w), and execute (x) permissions
• Helps maintain system security and proper access control

Let’s use the chmod command to set the permission to 745.

Command:

chmod 745 newfile.txt

Output:

Breakdown of -rwxr--r-x:

• Owner (7): rwx > read, write, execute
• Group (4): r-- > read only
• Others (5): r-x > read & execute

Here, the file is:

• Fully accessible (read/write/execute) by the owner
• Read-only for the group
• Read + execute for others

Options Available in chmod Command

Note: Options in `chmod` are basically used for making changes in bulk and modifying permissions across multiple files or directories at once.

Syntax:

chmod [options] [mode] [File_name] 

Here,

• Options: Optional flags that modify the behavior of the chmod command.
• Mode: The permissions to be set, represented by a three-digit octal number or symbolic notation (e.g., u=rw,go=rx).
• File_name: The name of the file or directory for which the permissions are to be changed.

Key Permission Types:

In Linux, permissions determine who has control over a file or directory. These permissions specify who can read (access), write (edit or delete), or execute (run) a file, based on user roles: owner, group, and others.

• Read (r): Allows viewing the contents of the file.
• Write (w): Allows modifying the file or directory.
• Execute (x): Allows running the file as a program or entering the directory.

1. Examples of Using Octal mode

Suppose we want to give read and write permissions to the file owner, read, write, and execute permissions to the group, and read-only permission to others. Then the command would be:

chmod 674 [file_name]

• 6 represent permission of file Owner which are (rw-).
• 7 represent permission of Group which are (rwx).
• 4 represent permission of Other which is (r--).

Note: You can view and calculate file permissions such as who can access, read, write (delete or modify), or execute a file for the Owner, Group, and Others (Public) using a chmod calculator.

Here, we use a calculator to view the permissions. For example, the owner has read, write, and execute permissions, while the group and others (public) have only read and write permissions, but cannot execute the file.

2. Equivalent Numeric Permission

Syntax:

chmod 766 filename

Table:

3. Example of Reverting Changes Made by "chmod" Command

To undo or revert changes made by the chmod command in Linux, you simply need to run the chmod command again and specify the correct permissions you want.

Steps to revert permission changes:

1. Decide the correct permission you want to restore.
2. Use the chmod command again with the appropriate octal value.

Example:

If you want to revert the permissions to rw-r--r--

• Owner: read + write
• Group: read-only
• Others: read-only

The corresponding octal value is 644. (read = 4, write = 2).

Command:

chmod 644 file_or_directory_name

Replace file_or_directory_name with the actual name of your file or directory.

Example with a file:

If the file name is a.txt, run:

chmod 644 a.txt

Result:

• Before reverting: permissions were modified

• After reverting: permissions are restored to rw-r--r--

Practical Implementation of How to Make script executable in Linux

In Linux, scripts are typically written in languages like Bash, Python, or Perl. While the script's content is crucial, ensuring it has executable permissions is equally important. Without execution permissions, Linux won't allow the script to run.

[Step 1]: Navigate to the Script's Directory

Open the terminal and use the cd command to navigate to the directory where your script is located.

Example:

cd /path/to/your/script

[Step 2]: Check Current Permissions

Use the `ls` command with the `-l` option to list the files in the directory along with their permissions. This step helps you identify the current permissions of your script:

ls -l

[Step 3]: Make the Script Executable

To make a script executable, you need to use the `chmod` command. Assuming your script is named `example.sh`, you can make it executable with the following command:

chmod +x example.sh

This command adds the execute (+x) permission to the script.

[Step 4]: Verify Changes

Run the `ls -l` command again to verify that the script now has execute permissions. You should see an 'x' in the permission field for the script:

ls -l

[Step 5]: Execute the Script

Now that your script has executable permissions, you can run it using the `./` notation followed by the script's name. For example:

./example.sh

This notation tells Linux to look in the current directory (.) for the script named example.sh.