When to use [ ] instead of [[ ]]?

That's a very practical follow-up question, Labby here to help!

While [[ ]] are generally preferred in modern Bash for their enhanced features and robustness, there are indeed situations where you might need or want to use [ ] (single square brackets):

1. Portability to other shells (POSIX compliance):

   • If you're writing a script that needs to run not just in Bash, but also in other shells like sh, dash (often linked to sh), or ksh, then you must use [ ]. The [[ ]] syntax is a Bash extension (and also available in zsh, ksh), but it's not part of the POSIX standard.
   • Many system initialization scripts or simpler utility scripts are written to be POSIX compliant to ensure they run everywhere.

2. When clarity of test command behavior is desired:

   • [ ] is essentially an alias for the test command. While [[ ]] is a special shell keyword. Sometimes, explicitly using test or [ ] can make it clear that you are invoking a command, especially when dealing with command-line arguments.

3. Older systems:

   • On very old Unix-like systems or very minimalist environments, Bash might not be available, or the version of Bash might be so old that it doesn't fully support [[ ]]. This is rare in modern Linux distributions, but it's a consideration for highly specialized or embedded systems.

In summary:

• Use [[ ]] (double brackets): For most of your everyday Bash scripts where you want the enhanced features, better string handling, and direct logical operators (&&, ||). It's the recommended default for Bash.
• Use [ ] (single brackets): When your script needs to be highly portable and run reliably on any POSIX-compliant sh shell, or if you're on a system where [[ ]] might not be available.

It's good to understand both so you can choose the right tool for the job! Does that clarify things for you?