On Thu, 23 Feb 2023 16:26:43 +0800 (CST) "Qingsheng Huang" <huangq...@126.com> wrote:
> Here shows the behavious of empty conditional expressions: > > ------------------------------------------------------------------------ > infinite:~# [[ ]] > -bash: syntax error near unexpected token `]]' > infinite:~# [[ ]] > infinite:~# [[ "$A" ]] > -bash: syntax error near unexpected token `"$A"' > infinite:~# [[ "$A" ]] > infinite:~# [[ "$A" ]] > infinite:~# [[ ]] > infinite:~# [[ "$A" ]] > -bash: syntax error near unexpected token `"$A"' > infinite:~# echo $? > 2 > infinite:~# [[ "$A" ]] > infinite:~# echo $? > 1 > infinite:~# > infinite:~# bash --version > bash --version > GNU bash, version 5.2.15(1)-release (x86_64-pc-linux-gnu) > ------------------------------------------------------------------------ > > The variable $A has never been defined. > Double brackets surrounding 1) a few spaces or 2) an empty variable even > quoted behave illogically. I've also try different lengths of spaces, and > also an empty variable without quotes, the same illogical. > > I am curious if I am the first guy encounter such a situation. Well observed. I can confirm this strange behaviour. I just want to add that 5.1.16 behaves sensibly, with [[ ]] always raising an error in an interactive shell. -- Kerin Millar
