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I know bash tries very hard to keep new versions compatible with previous ones.

However they sometime introduced (minor, but still) breaking changes:

for exemple when we could no longer have : { foo } # which now hangs

but instead had to have : { foo ;} # ie have a ; or a newline before the closing }

(not sure which version introduced that, it was more than 20 years ago ^^)

My question is : how to find out which other "breaking" changes occured along the whole history of bash updates?

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  • side question: how to add a simple "newline" in the question without having to do 2 newlines (which introduce a paragraph separator) ? ^^ Commented Apr 25 at 7:12
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    Add two spaces at the end of the line to force a line break. Commented Apr 25 at 7:32
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    { foo } is (and AFAIK always has been in bash) the { keyword (which introduces a command group) followed by the foo command with } as argument (and you'll need a } closing keyword (in command position, so for instance following a ; or & or newline) later to close the command group). Can you clarify what you mean by it hangs? Commented Apr 25 at 7:34
  • Correction, looks like before bash 2.0 (1996), bash did indeed not treat the } in { foo } as an argument to foo as the Bourne or Korn shell did and as POSIX requires. That's mentioned in the COMPAT file from 2.0. Commented Apr 25 at 7:45

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Breaking changes are listed (tersely) in the COMPAT file in the source distribution. I imagine most distributions provide this in their packages too — for example, /usr/share/doc/bash/COMPAT.gz in Debian and derivatives.

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    See also info bash compat for the section about its compatibility modes which lists some of the changes it is possible to revert by setting $BASH_COMPAT. It doesn't go as far back as bash 1.x which you'd need to revert to for that handling of { foo }. Commented Apr 25 at 7:47
  • thanks a lot ! I'm having a look (I tried to search a way to comb through "all" the READMEs in the git history (and didn't find how to) but didn't notice "COMPAT") Commented Apr 25 at 7:53
  • The COMPAT file is perfectly what I wanted, thank you (it mentions my exemple as number 2., lines 31 to 48) Commented Apr 25 at 11:42

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