How can I replace each newline (\n) with a space using sed?

In order to replace all newlines with spaces using awk, without reading the whole file into memory:

awk '{printf "%s ", $0}' inputfile

If you want a final newline:

awk '{printf "%s ", $0} END {printf "\n"}' inputfile

You can use a character other than space:

awk '{printf "%s|", $0} END {printf "\n"}' inputfile

tr '\n' ' ' 

is the command.

Simple and easy to use.

Three things.

tr (or cat, etc.) is absolutely not needed. (GNU) sed and (GNU) awk, when combined, can do 99.9% of any text processing you need. stream != line based. ed is a line-based editor. sed is not. See sed lecture for more information on the difference. Most people confuse sed to be line-based because it is, by default, not very greedy in its pattern matching for SIMPLE matches - for instance, when doing pattern searching and replacing by one or two characters, it by default only replaces on the first match it finds (unless specified otherwise by the global command). There would not even be a global command if it were line-based rather than STREAM-based, because it would evaluate only lines at a time. Try running ed; you'll notice the difference. ed is pretty useful if you want to iterate over specific lines (such as in a for-loop), but most of the times you'll just want sed. That being said, sed -e '{:q;N;s/\n/ /g;t q}' file works just fine in GNU sed version 4.2.1. The above command will replace all newlines with spaces. It's ugly and a bit cumbersome to type in, but it works just fine. The {}'s can be left out, as they're only included for sanity reasons.

Why didn't I find a simple solution with awk?

awk '{printf $0}' file

printf will print the every line without newlines, if you want to separate the original lines with a space or other:

awk '{printf $0 " "}' file

The answer with the :a label ...

How can I replace a newline (\n) using sed?

... does not work in freebsd 7.2 on the command line:

( echo foo ; echo bar ) | sed ':a;N;$!ba;s/\n/ /g'
sed: 1: ":a;N;$!ba;s/\n/ /g": unused label 'a;N;$!ba;s/\n/ /g'
foo
bar

But does if you put the sed script in a file or use -e to "build" the sed script...

> (echo foo; echo bar) | sed -e :a -e N -e '$!ba' -e 's/\n/ /g'
foo bar

or ...

> cat > x.sed << eof
:a
N
$!ba
s/\n/ /g
eof

> (echo foo; echo bar) | sed -f x.sed
foo bar

Maybe the sed in OS X is similar.

Easy-to-understand Solution

I had this problem. The kicker was that I needed the solution to work on BSD's (Mac OS X) and GNU's (Linux and Cygwin) sed and tr:

$ echo 'foo
bar
baz


foo2
bar2
baz2' \
| tr '\n' '\000' \
| sed 's:\x00\x00.*:\n:g' \
| tr '\000' '\n'

Output:

foo
bar
baz

(has trailing newline)

It works on Linux, OS X, and BSD - even without UTF-8 support or with a crappy terminal.

Use tr to swap the newline with another character. NULL (\000 or \x00) is nice because it doesn't need UTF-8 support and it's not likely to be used. Use sed to match the NULL Use tr to swap back extra newlines if you need them

You can use xargs:

seq 10 | xargs

or

seq 10 | xargs echo -n

If you are unfortunate enough to have to deal with Windows line endings, you need to remove the \r and the \n:

tr '\r\n' ' ' < $input > $output

I'm not an expert, but I guess in sed you'd first need to append the next line into the pattern space, bij using "N". From the section "Multiline Pattern Space" in "Advanced sed Commands" of the book sed & awk (Dale Dougherty and Arnold Robbins; O'Reilly 1997; page 107 in the preview):

The multiline Next (N) command creates a multiline pattern space by reading a new line of input and appending it to the contents of the pattern space. The original contents of pattern space and the new input line are separated by a newline. The embedded newline character can be matched in patterns by the escape sequence "\n". In a multiline pattern space, the metacharacter "^" matches the very first character of the pattern space, and not the character(s) following any embedded newline(s). Similarly, "$" matches only the final newline in the pattern space, and not any embedded newline(s). After the Next command is executed, control is then passed to subsequent commands in the script.

From man sed:

[2addr]N Append the next line of input to the pattern space, using an embedded newline character to separate the appended material from the original contents. Note that the current line number changes.

I've used this to search (multiple) badly formatted log files, in which the search string may be found on an "orphaned" next line.

In response to the "tr" solution above, on Windows (probably using the Gnuwin32 version of tr), the proposed solution:

tr '\n' ' ' < input

was not working for me, it'd either error or actually replace the \n w/ '' for some reason.

Using another feature of tr, the "delete" option -d did work though:

tr -d '\n' < input

or '\r\n' instead of '\n'

I used a hybrid approach to get around the newline thing by using tr to replace newlines with tabs, then replacing tabs with whatever I want. In this case, " " since I'm trying to generate HTML breaks.

echo -e "a\nb\nc\n" |tr '\n' '\t' | sed 's/\t/ <br> /g'`

You can also use this method:

sed 'x;G;1!h;s/\n/ /g;$!d'

Explanation

x   - which is used to exchange the data from both space (pattern and hold).
G   - which is used to append the data from hold space to pattern space.
h   - which is used to copy the pattern space to hold space.
1!h - During first line won't copy pattern space to hold space due to \n is
      available in pattern space.
$!d - Clear the pattern space every time before getting the next line until the
      the last line.

Flow

When the first line get from the input, an exchange is made, so 1 goes to hold space and \n comes to pattern space, appending the hold space to pattern space, and a substitution is performed and deletes the pattern space.

During the second line, an exchange is made, 2 goes to hold space and 1 comes to the pattern space, G append the hold space into the pattern space, h copy the pattern to it, the substitution is made and deleted. This operation is continued until EOF is reached and prints the exact result.

cat file | xargs

for the sake of completeness

Bullet-proof solution. Binary-data-safe and POSIX-compliant, but slow.

POSIX sed requires input according to the POSIX text file and POSIX line definitions, so NULL-bytes and too long lines are not allowed and each line must end with a newline (including the last line). This makes it hard to use sed for processing arbitrary input data.

The following solution avoids sed and instead converts the input bytes to octal codes and then to bytes again, but intercepts octal code 012 (newline) and outputs the replacement string in place of it. As far as I can tell the solution is POSIX-compliant, so it should work on a wide variety of platforms.

od -A n -t o1 -v | tr ' \t' '\n\n' | grep . |
  while read x; do [ "0$x" -eq 012 ] && printf '<br>\n' || printf "\\$x"; done

POSIX reference documentation: sh, shell command language, od, tr, grep, read, [, printf.

Both read, [, and printf are built-ins in at least bash, but that is probably not guaranteed by POSIX, so on some platforms it could be that each input byte will start one or more new processes, which will slow things down. Even in bash this solution only reaches about 50 kB/s, so it's not suited for large files.

Tested on Ubuntu (bash, dash, and busybox), FreeBSD, and OpenBSD.

In some situations maybe you can change RS to some other string or character. This way, \n is available for sub/gsub:

$ gawk 'BEGIN {RS="dn" } {gsub("\n"," ") ;print $0 }' file

The power of shell scripting is that if you do not know how to do it in one way you can do it in another way. And many times you have more things to take into account than make a complex solution on a simple problem.

Regarding the thing that gawk is slow... and reads the file into memory, I do not know this, but to me gawk seems to work with one line at the time and is very very fast (not that fast as some of the others, but the time to write and test also counts).

I process MB and even GB of data, and the only limit I found is line size.

Finds and replaces using allowing \n

sed -ie -z 's/Marker\n/# Marker Comment\nMarker\n/g' myfile.txt

Marker

Becomes

# Marker Comment Marker

You could use xargs — it will replace \n with a space by default.

However, it would have problems if your input has any case of an unterminated quote, e.g. if the quote signs on a given line don't match.

On Mac OS X (using FreeBSD sed):

# replace each newline with a space
printf "a\nb\nc\nd\ne\nf" | sed -E -e :a -e '$!N; s/\n/ /g; ta'
printf "a\nb\nc\nd\ne\nf" | sed -E -e :a -e '$!N; s/\n/ /g' -e ta

To remove empty lines:

sed -n "s/^$//;t;p;"

Using Awk:

awk "BEGIN { o=\"\" }  { o=o \" \" \$0 }  END { print o; }"

A solution I particularly like is to append all the file in the hold space and replace all newlines at the end of file:

$ (echo foo; echo bar) | sed -n 'H;${x;s/\n//g;p;}'
foobar

However, someone said me the hold space can be finite in some sed implementations.

Replace newlines with any string, and replace the last newline too

The pure tr solutions can only replace with a single character, and the pure sed solutions don't replace the last newline of the input. The following solution fixes these problems, and seems to be safe for binary data (even with a UTF-8 locale):

printf '1\n2\n3\n' |
  sed 's/%/%p/g;s/@/%a/g' | tr '\n' @ | sed 's/@/<br>/g;s/%a/@/g;s/%p/%/g'

Result:

1<br>2<br>3<br>

It is sed that introduces the new-lines after "normal" substitution. First, it trims the new-line char, then it processes according to your instructions, then it introduces a new-line.

Using sed you can replace "the end" of a line (not the new-line char) after being trimmed, with a string of your choice, for each input line; but, sed will output different lines. For example, suppose you wanted to replace the "end of line" with "===" (more general than a replacing with a single space):

PROMPT~$ cat <<EOF |sed 's/$/===/g'
first line
second line
3rd line
EOF

first line===
second line===
3rd line===
PROMPT~$

To replace the new-line char with the string, you can, inefficiently though, use tr , as pointed before, to replace the newline-chars with a "special char" and then use sed to replace that special char with the string you want.

For example:

PROMPT~$ cat <<EOF | tr '\n' $'\x01'|sed -e 's/\x01/===/g'
first line
second line
3rd line
EOF

first line===second line===3rd line===PROMPT~$