| Feature | Bourne Shell | Busybox Shell | Bash |
|---|---|---|---|
| Subprocess Execution | `` (the backtick) | `` or $() | As Busybox |
| Math Evaluation | use expr (not builtin) | $(( )) | As Busybox; adds ((var=math)) |
| Constants | None | None | typeset -r |
| Integers | None | None | typeset -i |
| Evaluation | [ | [ | [ |
| Extended Evaluation | /bin/[[ (1) | /bin/[[ (1) | [[ |
(1) The [[ operator is not the same as the program /bin/[[ as a program you need to still use double quotes around the variables to be expanded; thus defeating the reason for having them in the first place.
For Example:
# file="/tmp/let me go" # touch "$file" # [[ -e $file ]] && echo "There"
Yields [Busybox]:
[[: me: unknown operand
- as $file is word split before handing it to the command [[
Yields [bash]:
There
- due to $file not being split when passed to the test -e.
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