| Message ID | rgn7yvfaitifvmsx7tsu3bicrcrqjbhkai7jdj6q5ja3ccrq3o@musfjdmv7egg (mailing list archive) |
|---|---|
| State | Accepted |
| Delegated to: | Herbert Xu |
| Headers | show |
| Series | man: document what happens when IFS= (and when it's not) | expand |
On Sat, Apr 22, 2023 at 05:16:21PM +0200, наб wrote: > 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 <rozbrajaczpoziomow@gmail.com> > --- > src/dash.1 | 13 +++++++++++++ > 1 file changed, 13 insertions(+) Patch applied. Thanks.
diff --git a/src/dash.1 b/src/dash.1 index d3893bc..bd4c662 100644 --- a/src/dash.1 +++ b/src/dash.1 @@ -1021,6 +1021,19 @@ .Ss White Space Splitting (Field Splitting) .Ev IFS as a delimiter and uses the delimiters to split the results of parameter expansion and command substitution into fields. +.Pp +If +.Ev IFS +is empty, field splitting yields no fields if the input string was empty, +and one string with the unchanged value of the input otherwise. +For example, with the default +.Ev IFS , +.Dq Ic read Fl r Ev l +will remove any initial whitespace, +but +.Dq Ev IFS Ns = Ic read Fl r Ev l +will leave the entire line in +.Ev l . .Ss Pathname Expansion (File Name Generation) Unless the .Fl f
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 <rozbrajaczpoziomow@gmail.com> --- src/dash.1 | 13 +++++++++++++ 1 file changed, 13 insertions(+)