How to Make Script Executable in Linux | chmod Command

In Unix operating systems, the chmod command is used to change the access mode of a file. The name is an abbreviation of change mode. Which states that every file and directory has a set of permissions that control the permissions like who can read, write or execute the file. In this the permissions have three categories: read, write, and execute simultaneously represented by `r`, `w` and `x`. These letters combine together to form a specific permission for a group of users.

The `chmod` command is used to modify this permission so that it can grant or restrict access to directories and files. Let's have a look at the syntax and options for the `chmod` command in Linux Operating System.

Syntax of chmod command

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.

Options Available in chmod Command Linux

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

Modes in chmod Command in Linux

The "mode" helps in setting new permissions that have to be applied to files or directories.

This mode can be specified in several ways, we will discuss two modes: Symbolic and Octal mode. 

1) Symbolic mode

If we talk about symbolic mode, we can say that it is the most common method used for specifying fir permissions. In this we have to make a combination of letters and operators to set or tell what to do with permissions.

The following operators can be used with the symbolic mode:

The following letters that can be used in symbolic mode:

The following Reference that are used:

Examples of Using the Symbolic mode:

• Read, write and execute permissions to the file owner:

chmod u+rwx [file_name]

• Remove write permission for the group and others:

chmod go-w [file_name]

• Read and write for Owner, and Read-only for the group and other:

chmod u+rw,go+r [file_name]

2) Octal mode

It is also a method for specifying permissions. In this method we specify permission using three-digit number. Where..

•  First digit specify the permission for Owner.
•  Second digit specify the permission for Group. 
• Third digit specify the permission for Others. The digits 

NOTE: The digits are calculated by adding the values of the individual permissions.

Examples of Using the Octal mode:

Suppose if we to give read and write permission to the file Owner. Read, write and executable permission to the Group. Read-only permission to the Other. They our command would be.

chmod 674 [file_name]

Here.

• 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).

Practical Implementaion of How to Make script executable in Linux

Understanding Script Execution 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.

For 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.

• Determine the correct permission you want and use `chmod` command again. 
  For Example: If we want to revert the changes to "rw-r--r--" (read and write permission for owner, read-only permission for group and others), according to this our octal value would be "644" (read = 4, write=2).
• Now open the directory and write the given command :

chmod 644 [file_or_directory_name]

Here instead of "[file_or_directory_name]" use your file or directory name.

Suppose our file name is "a.txt"

Before reverting or undo changes:

After reverting or undo changes:

Conclusion

The `chmod` command in Linux is used to modify the permissions and access mode of files and directories. These are the permissions that control who can read, write and execute the file. We have discussed two types of modes for specifying permission: symbolic and octal mode. In symbolic mode, one uses letters and operators to specify the permission. Whereas octal has a three-digit number for specifying the permission. The `chmod` command also provides some options for bulk modifications, for example: `-R` for recursive and `-v` and `-c` for displaying message. The overall conclusion is that `chmod` command in Linux is a very essential tool for managing file and directories permissions.