[mulle_nat]

When I benchmarked it, I saw no difference between [ and [[ when using bash, so I assume [ is handled (now) by the shell as well.

The syntax inside [[ is different though.

[chasil]

The dash shell is supposedly four times faster than bash, and dramatically reduced Ubuntu's boot time.

The [[ test syntax is not implemented in dash. It is listed as a future reserved word in the POSIX shell standard, but is not (yet?) specified.

(Google: POSIX shell)

https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V...

If you pull the HTML file off the above link, you will get a directory listing of all of the POSIX.2 utilities (tar, sed, awk, etc.). The options that you find in the specifications are maximally portable.