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
