On Tue, Jul 10, 2018 at 09:29:55AM +0200, Andreas Kusalananda Kähäri wrote: > On Mon, Jul 09, 2018 at 06:23:17PM -0300, Daniel Bolgheroni wrote: > > Working with some input/commands which dealt with multiline pattern > > spaces, noticed a behaviour I was not expecting. Reducing the test to a > > simple case: > > > > $ echo "foo\nbar\nbaz" | sed "1d; N" > > bar > > baz > > $ > > > > This is for 3 lines of input. However, same command but for 4 lines of > > input: > > > > $ echo "foo\nbar\nbaz\nqux" | sed "1d; N" > > bar > > baz <--- qux not here > > $ > > > > For 6 lines of input: > > > > $ echo "foo\nbar\nbaz\nqux\nquux\nquuz" | sed "1d; N" > > bar > > baz > > qux > > quux <--- quuz not here > > $ > > > > (use echo -e if on bash) > > > > When the input has an even number of lines, the last line disappears. My > > understanding is this isn't supposed to happen, but I'm unsure if it has > > something to do with POSIX gray areas of undefined behaviours. For > > instance, GNU sed from ports presents a different behaviour (second test > > case but with gsed): > > > > $ echo "foo\nbar\nbaz\nqux" | gsed "1d; N" > > bar > > baz > > qux <--- qux here > > $ > > > > Before suggesting anything, anyone with a stronger experience on sed > > historical behaviours can shed some light on if this is acceptable? > > > > Thank you. > > > > -- > > db > > > > This is really a question about what the "N" command should be doing if > there is no further input. GNU sed seems to output the current pattern > space. > > POSIX says: > > If no next line of input is available, the N command verb shall > branch to the end of the script and quit without starting a new > cycle or copying the pattern space to standard output. > > > Note: "without [...] copying the pattern space to standard output" > > So this makes me believe that OpenBSD sed is doing the correct thing > with regards to the POSIX spec. for sed. > > Also note that the input to sed should be "text files". In all your > examples, the last line is not properly terminated, which, strictly and > pedantically speaking, violates the "text file" requirement. This does > not affect the outcome of the commands though.
If you're using echo, that adds the terminating newline. I forgot about that, sorry, early morning here... I generally use printf for this sort of data though... > > Also, printf may be used to portably print strings with C escape > sequences. > > > Cheers, -- Andreas Kusalananda Kähäri, National Bioinformatics Infrastructure Sweden (NBIS), Uppsala University, Sweden. När du har kontakt med oss på Uppsala universitet med e-post så innebär det att vi behandlar dina personuppgifter. För att läsa mer om hur vi gör det kan du läsa här: http://www.uu.se/om-uu/dataskydd-personuppgifter/ E-mailing Uppsala University means that we will process your personal data. For more information on how this is performed, please read here: http://www.uu.se/om-uu/dataskydd-personuppgifter/
