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   Haelwenn /элвэн/ :triskell: (lanodan@queer.hacktivis.me)'s status on Friday, 26-Apr-2024 16:55:24 JST Haelwenn /элвэн/ :triskell:

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   • Simon Tatham

   @simontatham That's part of POSIX btw.
   And ${parameter:?} allows to also throw an error when it's null rather than only unset, plus ${parameter:?word} would throw word to standard error, which can allow for better errors.

   (posix draft note, in the special case of ${@?} and${*?}` it's undefined behavior)

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     Simon Tatham (simontatham@hachyderm.io)'s status on Friday, 26-Apr-2024 16:55:25 JST Simon Tatham

     In bash, writing ${var?} instead of just ${var} or $var means if var isn't defined then bash will throw an error and _not_ execute your command, instead of expanding it to "" and carrying on.

     mv file1 file2 $subdir # oops, I overwrote file2
     mv file1 file2 ${subdir?} # error message instead of disaster

     My favourite use of this is for example commands in documentation, with placeholders for the user to fill in. Then it's OK if a user accidentally copy-pastes it _without_ filling them in!