Is there an issue with sed and new line character?
I have a file test.txt with the following contents


The following does not work:
sed -r -i 's/\n/,/g' test.txt

I know that I can use tr for this but my question is why it seems not possible with sed.

If this is a side effect of processing the file line by line I would be interested in why this happens. I think grep removes new lines. Does sed do the same?

  • In this case sed might be not the best tool to use (eg. "tr"). There are tools that are more intuitive, easier to read/maintain, performing better (especially on big data) etc. ... Don't use your hammer to put the screws in (even if it works). You can find a comparison on: – omoser Feb 26 '15 at 9:51
  • tr would add a trailing , and would output an unterminated line. Best is to use paste instead: paste -sd , test.txt – Stéphane Chazelas Jan 10 '17 at 14:09
up vote 37 down vote accepted

With GNU sed and provided POSIXLY_CORRECT is not in the environment (for single-line input):

sed -i ':a;N;$!ba;s/\n/,/g' test.txt

From :

  1. create a label via :a
  2. append the current and next line to the pattern space via N
  3. if we are before the last line, branch to the created label $!ba ($! means not to do it on the last line (as there should be one final newline)).
  4. finally the substitution replaces every newline with a comma on the pattern space (which is the whole file).
  • This seems to indicate that the problem is that sed reads line by line.But I can't understand why is this an issue.It could just read the line and replace the new line character (or last character) with a , – Jim Feb 12 '14 at 20:27
  • 1
    @jim It looks like it is not in the buffer to be matched, but I am not fluent with sed, maybe someone else can shed a light on that. I think you should extend your Q with that specific info, so people are more likely to read it, and hopefully answer. – Anthon Feb 12 '14 at 20:30
  • This results in ba: Event not found – krb686 May 21 '15 at 14:07
  • @krb686 What is the "This" you are referring to? Did you run the above sed command with those exact options? On what test.txt file? With which version of sed (try sed --version)? – Anthon May 21 '15 at 14:39
  • @Anthon Sorry, I think I meant to say "the". I read another SO post that informed me that csh requires me to escape the !. Interestingly, that still did not work for me and I ended up having to double escape the ! in my .csh script. So I don't really have a problem at the moment, but you do you know why that might be? What worked for me was sed :a;N;$\\!ba;s/\n/ /g' – krb686 May 21 '15 at 17:58

From Oracle's web site:

The sed utility works by sequentially reading a file, line by line, into memory. It then performs all actions specified for the line and places the line back in memory to dump to the terminal with the requested changes made. After all actions have taken place to this one line, it reads the next line of the file and repeats the process until it is finished with the file.

Basically this means that because sed is reading line by line the newline character is not matched.

The solution from is:

sed ':a;N;$!ba;s/\n/ /g'

An explanation into how that works is provided on that page.

  • 1
    That will get you spaces between the lines, not commas – Anthon Feb 12 '14 at 20:33
  • I used a modified form of this to parse VPN logs and put the user "authenticated" and time stamp information on the same line. Cheers! – user208145 Sep 25 '15 at 1:51
  • Note that that syntax is GNU specific, and even with GNU sed, if POSIXLY_CORRECT is in the environment and the input has only one line, there will be no output. – Stéphane Chazelas Feb 12 '16 at 15:16

This works:

sed -z 's/\n/,/g' 

-z is included since 4.2.2

  • 1
    This will also replace the trailing newline which might not be what OP wants... compare the result with mikeserv's solution. – don_crissti Aug 8 '17 at 17:06

sed always removes the trailing \newline just before populating pattern space, and then appends one before writing out the results of its script. A \newline can be had in pattern-space by various means - but never if it is not the result of an edit. This is important - \newlines in sed's pattern space always reflect a change, and never occur in the input stream. \newlines are the only delimiter a sedder can count on with unknown input.

If you want to replace all \newlines with commas and your file is not very large, then you can do:

sed 'H;1h;$!d;x;y/\n/,/'

That appends every input line to hold space - except the first, which instead overwrites hold space - following a \newline character. It then deletes every line not the $!last from output. On the last line Hold and pattern spaces are exchanged and all \newline characters are y///translated to commas.

For large files this sort of thing is bound to cause problems - sed's buffer on line-boundaries, that can be easily overflowed with actions of this sort.

Alternatively, you can use a slightly simpler syntax:

sed ':a;N;s/\n/,/g;ba'

...just changing sequence order.

  • 3
    But runs the s command for each input line on a pattern space that is increasingly big. – Stéphane Chazelas Feb 12 '16 at 15:08

There's some very nice sed magic here. And some good points raised about pattern space overflow. I love to use sed even when it's not the simplest way, because it's so compact and powerful. However it has it's limitations, and for large amounts of data the pattern space would have to be mahoosive.

GNU says this:

For those who want to write portable sed scripts, be aware that some implementations have been known to limit line lengths (for the pattern and hold spaces) to be no more than 4000 bytes. The posix standard specifies that conforming sed implementations shall support at least 8192 byte line lengths. GNU sed has no built-in limit on line length; as long as it can malloc() more (virtual) memory, you can feed or construct lines as long as you like.
However, recursion is used to handle subpatterns and indefinite repetition. This means that the available stack space may limit the size of the buffer that can be processed by certain patterns.

I don't have much to add, but I would like to point you towards my go-to guide for sed. It's excellent.

and here is my solution:

for i in $(cat test.txt); do echo -n $i','; done; echo '' >> somewhere

well it works

Let's say you want to replace newlines by \n. I wanted to do that, so here's what I did:

(echo foo; echo bar; echo baz) | sed -r '$!s/$/\\n/' | tr -d '\n' 
# Output: foo\nbar\nbaz

Here's what it does: for all lines except the last, append \n. Then, delete newlines with tr.

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