On Wednesday, February 06, 2013 01:44:04 PM DJ Mills wrote: > On Tue, Feb 5, 2013 at 6:39 PM, Tiwo W. <tiwoc...@gmail.com> wrote: > > > I have seen "read -d '' var" to read multi-line heredocs into > > shell variables. An empty argument to -d seemed to mean "read > > up to the end of input". And this is what it does. > > > > > In addition to all of the "don't use set -e" answers you've gotten (which i > agree with wholeheartedly), I have this to add: > > read -rd '' is synonymous to read -d $'\0'. It doesn't actually mean "read > until the end of input", but rather "read until a NUL byte is encountered". > You see this usage a lot as well when reading NUL-delimited data, say > filenames from find. For example: > > while IFS= read -rd '' file; do > some_command "$file" > done < <(find . -type f -print0) > > So what's actually happening with your here document usage case is that > read is looking for a NUL byte, but never finds one, so it stops reading > when EOF is encountered. As Greg mentioned, this then casuse read to exit > > 0. This is a perfectly acceptable usage, but now you know why that happens. > > And to reiterate, STOP USING set -e!
+1 to all of that. Note $'\0' was used here for illustration and doesn't actually expand to a nul byte. To be completely clear, the command: `read -rd '' x' is literally receiving the same arguments as the $'\0' case -- it's not only that `read' is treating them the same. Also, if it means anything, sadly ksh93 didn't perform that termination until one of the most recent alphas. I take it that this is sort of an extra special feature that most shells with -d happen to share, and not merely a necessary consequence of -d with an empty arg. Hopefully someday `mapfile' will inherit an analogous feature. -- Dan Douglas
