[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.
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: ${sneaky}" doneThis is sneaky: a This is sneaky: b This is sneaky: c This is sneaky: dYou should put some quotes where you use the array:
not_what_you_think=( "a b" "c" "d" ) for sneaky in "${not_what_you_think[@]}"; do echo "This is sneaky: ${sneaky}" done This is sneaky: a b This is sneaky: c This is sneaky: dAaaaah! Thank you kind stranger. It never would have occurred to me to quote an array!
If you run your scripts through https://shellcheck.net it’ll pick up things like this. Also available as a Linux package for offline use.