read -n ignores -d

[stephane-chazelas]

$ printf 'x\ny:' | ksh -c 'IFS= read -rd : -n2 a; printf "<%s>\n" "$a"'
<x>

That still reads 2 bytes off a line, a newline-delimited record, not a : delimited record here.

I know that's how it's currently documented, but that's less useful and I don't expect changing that would break backward compatibility (you wouldn't use -d and still expect line-oriented input).

A few shells have copied ksh93's -n and -d and all read those bytes (characters in the case of bash) up to the delimiter specified with -d:

$ printf 'x\ny:' | bash -c 'IFS= read -rd : -n2 a; printf "<%s>\n" "$a"'
<x
>
$ printf 'x\ny:' | mksh -c 'IFS= read -rd : -n2 a; printf "<%s>\n" "$a"'
<x
>
$ printf 'x\ny:' | busybox sh -c 'IFS= read -rd : -n2 a; printf "<%s>\n" "$a"'
<x
>

[stephane-chazelas]

A few shells have copied ksh93's -n and -d and all read those bytes (characters in the case of bash) up to the delimiter specified with -d:

BTW, changelog for ksh93t had:

08-10-10 read -n and -N fixed to count characters in multibyte locales.

(though man page still said byte for -n)

[McDutchie]

That still reads 2 bytes off a line, a newline-delimited record, not a : delimited record here.

I know that's how it's currently documented,

Hmm, I'm not seeing that documented actually.

sh.1:

              -d      Causes the read to continue to the first character of
                      delim instead of the newline control character.
                      Multibyte characters for delim are not supported.
              -n      Causes at most n bytes to be read instead of a full
                      line, but will return when reading from a slow device as
                      soon as any characters have been read.

read --man:

  -d delim        Read until delimiter delim instead of to the end of line.
  -n count        Read at most count characters or (for binary fields) bytes.
                  When reading from a slow device, will return as soon as any
                  characters have been read, unless the posix shell option is
                  on.

None of this mentions -n disabling the effect of -d, so I'm going to call this a bug.

[McDutchie]

The problem is the hacky way in which the option-argument for d is included in the flags variable, which necessitated clearing it if -n is also given.

Solution: pass the delimiter as a separate argument to sh_readline() instead. It's way more complicated than that.

Patch removed; broken.

[McDutchie]

Sorry, that one's still broken, working on a correct one.