man: document what happens when IFS= (and when it's not)

A question I just got from a user was "how do I make while read -r l; do
...; done < f not strip the initial tabs?". Turns out, the manual is
silent on this, and POSIX just about implies this behaviour.
(Indeed, our read is almost verbatim POSIX, and both defer to Field
 Splitting, but our Field Splitting isn't nearly as detailed,
 and thank god.)
Even POSIX spends just one line describing this pivotal behaviour
(Issue 8 Draft 2.1 line 75044-75045:
"2. If the value of IFS is null, field splitting shall have no effect,
    except that if the input is empty the result shall be zero fields.),"
and when I first encountered this it was also quite surprising to me.

Spell it out explicitly: IFS= means that input is preserved,
and the default value means whitespace is stripped from the front.
Drive it home with an example because it's esoteric (and I know from
 that user that they first tried searching for read in the manual,
 but it was not very helpful).

Reported-by: rozbrajaczpoziomow <>
Signed-off-by: Herbert Xu <>
1 file changed