Ash Vs Bash

Daniel Quinn@lemmy.ca to Linux@lemmy.ml – 34 points –

[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.

10

What have you found bad about bash arrays? I have some simple usage of those (in bash) and they work fine.

As far as I've seen, they don't provide any advantage over a string with spaces, which doesn't work well either when you've got values with spaces:

not_what_you_think=( "a b" "c" "d" )
for sneaky in ${not_what_you_think[@]}; do
  echo "This is sneaky: ${sneaky}"
done

This is sneaky: a
This is sneaky: b
This is sneaky: c
This is sneaky: d

You should put some quotes where you use the array:

not_what_you_think=( "a b" "c" "d" )
for sneaky in "${not_what_you_think[@]}"; do
  echo "This is sneaky: ${sneaky}"
done

This is sneaky: a b
This is sneaky: c
This is sneaky: d

Aaaaah! Thank you kind stranger. It never would have occurred to me to quote an array!

I too would like to know. Thank you.

ash (and its successor dash found on other distros) is a POSIX-y shell rather than a sh clone, so it has all(? most?) of the POSIX feature set, whose syntax may indeed have been 'borrowed' from shells that came later than sh.

Not sure if there's a "parent" from which both ash and bash inherit the syntax or whether bash is 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/busybox

So 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 builtin

Huh. So the link is unnecessary and Ash supports [[ out of the box? Good to know, thanks!