Hi, Lawrence,

I hold the wrong impression that zsh is compatible with bash. I just assume
it because I see both Ubuntu and MacOS have replaced bash with it.
Thank you for your explanation. Indeed, it uses letter l as a word modifier
(14.1.4 Modifiers). I will read the zsh manual!

thanks,
--lx




On Fri, Aug 19, 2022 at 5:40 PM Lawrence Velázquez <larryv@zsh.org> wrote:

> On Fri, Aug 19, 2022, at 6:32 PM, Liu Xin wrote:
> > I think zsh is compatible with bash
>
> It is only partially compatible, and compatibility is not a development
> priority.  The notion that zsh is a fancy superset of bash is false;
> zsh will only run simple bash scripts correctly.  (Whether it errs
> loudly or quietly depends on the feature used.  Using sh emulation
> may also help.)
>
> You are better off assuming that an arbitrary bash feature *does
> not* work with zsh unless proven otherwise, rather than assuming
> that it *does* work.
>
> > but I found one different behavior
> > in parameter expansion.  In the following example, I guess zsh
> > interprets "$1:l" as a whole. Is it intentional?
>
> Yes.  From the documentation you linked:
>
>         In addition to the following operations, the colon modifiers
>         described in "Modifiers" in "History Expansion" can be
>         applied: for example, ${i:s/foo/bar/} performs string
>         substitution on the expansion of parameter $i.
>
>         [...]
>
>         ${name}
>                 The value, if any, of the parameter _name_ is
>                 substituted.  [...]  In addition, more complicated
>                 forms of substitution usually require the braces
>                 to be present; exceptions, which only apply if the
>                 option KSH_ARRAYS is not set, are a single subscript
>                 or any colon modifiers appearing after the name
>                 [...].
>
> Your example applies the "l" history modifier, which converts the
> expansion to lowercase.  This is more obvious with a different
> choice of value:
>
>         % export VAR=HELLO
>         % zsh -c 'echo "$VAR:l"'
>         hello
>
> Braces are required if KSH_ARRAYS is set (either explicitly or via
> sh/ksh emulation).
>
>         % zsh --emulate sh -c 'echo "$VAR:l"'
>         HELLO:l
>         % zsh --emulate sh -c 'echo "${VAR:l}"'
>         hello
>
> --
> vq
>