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Re: %q with truncating size loses safeness of %q


From: Robert Elz
Subject: Re: %q with truncating size loses safeness of %q
Date: Sat, 18 Apr 2020 05:37:52 +0700

    Date:        Fri, 17 Apr 2020 16:12:20 -0400
    From:        Chet Ramey <address@hidden>
    Message-ID:  <address@hidden>

  | I would say this is a programmer error.  The way precisions work with
  | string arguments is that the argument is fetched or generated (this
  | includes generating the quoted string for %q or the expanded string for
  | %b) and then printf writes number of bytes (!) from that generated string
  | specified by the precision.

This happens only because of the cheap way we (and I presume you)
implement things - in any rational scheme, it would take the precision
chars from the source string, and then quote them.

But that's hard - instead we just use printf to do the work, %q quotes
the arg string, and then the 'q' is changed to a 's' in the format, and
we just call printf(3) to do the work (padding, justification, ...)

The only excuse for this is pragmatics, no-one would deliberately set
out to design something quite that bogus.

The end result is as Greg said, "Don't do that", if precisions are
needed with %q do something like

        printf 'echo %q%q\n' "$(printf %.2s "a'b")" ';ls'

which produces

        echo a\'\;ls

which I expect is the desired result.

kre




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