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 script.sh. On a GitHub runner, this is required if you want to execute the script (e.g., ./script.sh).

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 keys have required permissions. If the permissions are too open, SSH may refuse to use the key.

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).