The line:

bash コードをコピーする DKIM_SELECTOR=$NODE_PATH -e 'console.log(Date().toString().substr(4, 3).toLowerCase() + new Date().getFullYear())' is dynamically setting the DKIM_SELECTOR variable using a Node.js command. Here's a breakdown of what it does:

Breakdown of the Command: $NODE_PATH: This likely refers to the path of the Node.js binary. It is executing Node.js with the -e flag, which allows you to run inline JavaScript code directly from the command line.

Date().toString().substr(4, 3):

Date() returns the current date and time as a string (e.g., "Mon Oct 2 2023 14:00:00 GMT+0000"). .toString().substr(4, 3) extracts the substring starting at index 4 and continuing for 3 characters. This corresponds to the three-letter abbreviation for the current month (e.g., "Oct"). .toLowerCase(): Converts the extracted month to lowercase (e.g., "oct").

new Date().getFullYear(): Retrieves the full year as a four-digit number (e.g., 2023).

Concatenation: The month abbreviation in lowercase and the year are concatenated, forming a string like "oct2023".

Example Output: If the command were run in October 2023, the DKIM_SELECTOR variable would be set to:

bash コードをコピーする DKIM_SELECTOR="oct2023" This creates a DKIM selector based on the current month and year, which is a common practice to ensure that the DKIM keys are easily rotated and differentiated over time.

Purpose: The DKIM selector is used in DNS to point to the public key for DKIM signing. By using a dynamic selector based on the current month and year, this setup facilitates key rotation and simplifies key management. The selector ensures that for each new period (month/year), a new DKIM key can be deployed, while older keys are still valid for verifying previous emails.