Deleting lines of a file with sed - unexpected behaviour

Question

I noticed something a bit odd while fooling around with sed. If you try to remove multiple line intervals (by number) from a file, but any interval specified later in the list is fully contained within an interval earlier in the list, then an additional single line is removed after the specified (larger) interval.

seq 10 > foo.txt

sed '2,7d;3,6d' foo.txt
1
9
10

This behaviour was behind an annoying bug for me, since in my script I generated the interval endpoints on the fly, and in some cases the intervals produced were redundant. I can clean this up, but I can't think of a good reason why sed would behave this way on purpose.

Answer 1

_{Since this question was highlighted as needing an answer in the Stack Overflow Weekly Newsletter email for 2015-02-24, I'm converting the comments above (which provide the answer) into a formal answer. Unattributed comments here were made by me in essentially equivalent form.}

Thank you for a concise, complete question. The result is interesting. I can reproduce it with your script. Intriguingly, sed '3,6d;2,7d' foo.txt (with the delete operations in the reverse order) produces the expected answer with 8 included in the output. That makes it look like it might be a reportable bug in (GNU) sed, especially as BSD sed (on Mac OS X 10.10.2 Yosemite) works correctly with the operations in either order. I tested using 'sed (GNU sed) 4.2.2' from an Ubuntu 14.04 derivative.

More data points for you/them. Both of these include 8 in the output:

sed -e '/2/,/7/d' -e '/3/,/6/d' foo.txt
sed -e '2,7d' -e '/3/,/6/d' foo.txt

By contrast, this does not:

sed -e '/2/,/7/d' -e '3,6d' foo.txt

The latter surprised me (even accepting the basic bug).

Beats me. I thought given some of sed's arcane constructs that you might be missing the batman symbol or something from the middle of your command but sed -e '2,7d' -e '3,6d' foo.txt behaves the same way and swapping the order produces the expected results (GNU sed 4.2.2 on Cygwin). /bin/sed on Solaris always produces the expected result and interestingly so does GNU sed 3.02. Ed Morton

More data: it only seems to happen with sed 4.2.2 if the 2nd range is a subset of the first: sed '2,5d;2,5d' shows the bug, sed '2,5d;1,5d' and sed '2,5d;2,6d' do not. glenn jackman

The GNU sed home page says "Please send bug reports to bug-sed at gnu.org" (except it has an @ in place of ' at '). You've got a good reproduction; be explicit about the output you expect vs the output you get (they'll get the point, but it's best to make sure they can't misunderstand). Point out that the reverse ordering of the commands works as expected, and give the various other commands as examples of working or not working. (You could even give this Q&A URL as a cross-reference, but make sure that the bug report is self-contained so that it can be understood even if no-one follows the URL.)

You can also point to BSD sed (and the Solaris version, and the older GNU 3.02 sed) as behaving as expected. With the old version GNU sed working, it means this is arguably a regression. […After a little experimentation…] The breakage occurred in the 4.1 release; the 4.0.9 release is OK. (I also checked 4.1.5 and 4.2.1; both are broken.) That will help the maintainers if they want to find the trouble by looking at what changed.

The OP noted:

• Thanks everyone for comments and additional tests. I'll submit a bug report to GNU sed and post their response. santayana