/etc/cron.d相关使用咨询：Ubuntu16.04下cron.daily文件命名与语法合规性

Let's walk through each of your concerns with your Ubuntu 16.04 /etc/cron.daily setup:

1. Is the filename cron_daily safe/valid? #

Absolutely. Ubuntu uses run-parts to execute scripts in /etc/cron.daily, and run-parts has strict rules for which files it will run:

• Filenames can only contain letters, numbers, underscores (_), and hyphens (-)
• No file extensions allowed (extensions will cause run-parts to skip the file)

Your cron_daily name fits these rules perfectly—run-parts will recognize it as a valid script to execute, so this naming is safe and compliant.

2. Are no file extension and no shebang line compliant? #

No file extension: #

As noted above, this is actually required for run-parts to process the file, so that's totally fine.

No shebang line: #

This is a potential gotcha. By default, run-parts on Ubuntu 16.04 executes scripts using /bin/sh, which is a symlink to dash (a POSIX-compliant shell, not Bash). If your script uses Bash-specific syntax (like arrays, [[ ]] conditionals, or shopt settings), running it with dash will cause errors.

Even if you're sourcing a file that sets up a Bash environment, the lack of a shebang means the parent shell executing your script is still dash. To ensure your script runs in Bash as intended, add the shebang line at the very top:

#!/bin/bash

3. Is variable expansion for drt and rse compliant? #

If those variables are properly exported (via export drt rse) in the file you're sourcing, then using them directly in your cron_daily script is fully compliant with Bash syntax.

A critical note here: Cron runs with a very minimal environment. When you source a file, make sure to use an absolute path to it (e.g., /home/youruser/env_vars.sh instead of ./env_vars.sh or ~/env_vars.sh). Relative paths won't work reliably in cron's restricted environment.

Extra Tips for Cron Daily Scripts #

• Ensure your script has executable permissions: Run sudo chmod +x /etc/cron.daily/cron_daily—run-parts won't execute non-executable files.
• Check cron logs if you run into issues: Look at /var/log/syslog for entries tagged with CRON to debug execution problems.

内容的提问来源于stack exchange，提问作者Arcticooling