[For reference, I’m talking about Ash in Alpine Linux here, which is part of BusyBox.]
I thought I knew the big differences, but it turns out I’ve had false assumptions for years. Ash does support [[ double square brackets ]] and (as best I can tell) all of Bash’s logical trickery inside them. It also supports ${VARIABLE_SUBSTRINGS:5:12}` which was another surprise.
At this stage, the only things I’ve found that Bash can do that Ash can’t are:
- Arrays, which Bash doesn’t seem to do well anyway
- Brace expansion, which is awesome but I can live without it.
What else is there? Did Ash used to be more limited? The double square bracket thing really surprised me.
Yup, that looks like exactly what was done in Alpine:
$ docker run --rm -it alpine ls -l /usr/bin/[[ lrwxrwxrwx 1 root root 12 Sep 28 11:18 /usr/bin/[[ -> /bin/busyboxSo while the Ash itself doesn’t support the
[[extension, this work-around produces the same effect. Nifty.Although that link exists, that’s not what is being used by default.
[[is a shell builtin in ash/busybox, so that takes precedence.On Alpine:
❯ which [[ /usr/bin/[[ ❯ command -V [[ [[ is a shell builtinHuh. So the link is unnecessary and Ash supports [[ out of the box? Good to know, thanks!