chmod Command Examples:

Here are practical examples of how to use the chmod command to change file permissions:

1. Making a File Executable:

**Single file:
** chmod +x script.sh

  • [This command adds execute permission to the file script.sh. Required in order to run your bash script if you were to write it on a GitHub runner.Now you can run the script by typing ./script.sh.]{.mark}

**Recursive on a directory:
** chmod -R +x bin/

  • This command recursively adds execute permission to all files and directories within the bin/ directory.

2. Setting Specific Permissions:

[SSH public keys and private keys each have a certain permissions. Otherwise it won't be possible to use these keys when you use SSH within your runner.]{.mark}

**Setting read and write permissions for the owner:
** chmod 600 ~/.ssh/id_rsa

  • This sets the permissions of the file ~/.ssh/id_rsa to 600, which translates to:

    • 6 - owner (user) has read and write permissions

    • 0 - group has no permissions

    • 0 - others have no permissions

**Setting read permissions for the owner, group, and others:
** chmod 644 ~/.ssh/id_rsa.pub

  • This sets the permissions of the file ~/.ssh/id_rsa.pub to 644, which translates to:

    • 6 - owner (user) has read and write permissions

    • 4 - group has read permissions

    • 4 - others have read permissions

3. Understanding Permission Modes:

  • Octal Notation: chmod uses octal notation to represent permissions:

    • The first digit (hundreds place) represents the owner's permissions.

    • The second digit (tens place) represents the group's permissions.

    • The third digit (ones place) represents the others' permissions.

  • Permissions:

    • 4: Read permission

    • 2: Write permission

    • 1: Execute permission

4. Using umask:

  • Setting the default permissions: The umask command sets the default permissions for newly created files and directories. For example, umask 022 sets the default permissions to 644 for files and 755 for directories.

**Setting a specific umask:
** umask 022

  • This sets the umask to 022, ensuring that newly created files will have read/write permissions for the owner and read permissions for the group and others.

Key Points:

  • Safety: Be cautious when using chmod. Incorrectly setting permissions can make files inaccessible or grant unintended access to others.

  • Best Practices: It's often good practice to restrict permissions on sensitive files like SSH keys (~/.ssh/id_rsa) to the owner only (using chmod 600).

  • Reference: You can use ls -l to view the current permissions of a file. The first character in the output represents the file type (e.g., - for a regular file, d for a directory), and the next nine characters represent the permissions (three sets of three characters for owner, group, and others).