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I came across this weird parameter expansion for bash/dash which I don't find any reference for. What is this sort of expansion called? Where is it documented?

$ /???
bash: /bin: Is a directory
$ /????
bash: /boot: Is a directory
$ /?????
bash: /cdrom: Is a directory
$ /??????
bash: /libx32: Is a directory
  • The ? behaves as . does in regular expressions; it matches one character. This is documented in your shell's manual. – Kusalananda Aug 1 '18 at 8:30
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This is pathname expansion using shell pattern matching: ? matches any single character, so /??? expands to every three-character filename in / in alphabetical order.

That makes /bin first on most systems, which is a directory and not an executable, so you get the error you posted.

This is the same sort of expansion as *, and might be useful for cases like cp file??.txt dest where a set of files have a common structure to their names with only a few fixed characters changing.

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