[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.
ash(and its successordashfound on other distros) is a POSIX-y shell rather than ashclone, so it has all(? most?) of the POSIX feature set, whose syntax may indeed have been ‘borrowed’ from shells that came later thansh.Not sure if there’s a “parent” from which both
ashandbashinherit the syntax or whetherbashis the true source, but that doesn’t really matter here.All that said, it’s worth checking to see if your system has a command on the PATH called
[[. That has been one way that[[support can be added to a system when the shell itself might not support it. Note that command names don’t have to be alphanumeric like functions tend to be in a programming language (or other languages if you consider that the shell can be used for programming too), so[[is perfectly valid!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!