On 2021-08-31 at 11:04 +1000, Shineru wrote:
> zsh 5.8.1. Arch GNU/Linux
>
> zsh: echo "hello" | tr [:lower:] [:upper:]
> zsh: no matches found: [:lower:]

setopt nonomatch
or:  unsetopt nomatch
but: both of these are dangerous, because there's a real quoting issue
here.

% touch l p
% echo "hello" | tr [:lower:] [:upper:]
heppo
bash$ echo "hello" | tr [:lower:] [:upper:]
heppo

Without quoting,
  [:lower:] matches any of: l o w e r :
  [:upper:] matches any of: u p e r :
so the globbing turns the pipeline into:
  echo "hello" | tr "l" "p"

By default, zsh complains about unmatched patterns, rather than letting
them fall through silently.  Falling through leads to this sort of
"foot-gun" construct, where shells encourage you to do something which
"only works as long as X is not true", instead of having reliable code.

So you want, for safety, in any shell which resembles POSIX at all:

  echo 'hello' | tr '[:lower:]' '[:upper:]'

(This probably belongs on zsh-users.)

-Phil