Insert linefeed in sed (Mac OS X)

How do I insert a newline in the replacement part of sed?

This code isn't working:

sed "s/\(1234\)/\n\1/g" input.txt > output.txt

where input.txt is:

test1234foo123bar1234

and output.txt should be:

test
1234foo123bar
1234

but insted I get this:

testn1234foo123barn1234

NOTE:

This question is specifically about the Mac OS X version of "sed", and the community has noted that it behaves differently than, say, Linux versions.

Solution 1:

Your sed version apparently does not support \n in RHS (right-hand side of substitution). You should read THE SED FAQ maintained by Eric Pement to choose one of possible solutions. I suggest trying first inserting literal newline character.

Below is the quote from it.

4.1. How do I insert a newline into the RHS of a substitution?

Several versions of sed permit \n to be typed directly into the RHS, which is then converted to a newline on output: ssed, gsed302a+, gsed103 (with the -x switch), sed15+, sedmod, and UnixDOS sed. The easiest solution is to use one of these versions.

For other versions of sed, try one of the following:

(a) If typing the sed script from a Bourne shell, use one backslash \ if the script uses 'single quotes' or two backslashes \\ if the script requires "double quotes". In the example below, note that the leading > on the 2nd line is generated by the shell to prompt the user for more input. The user types in slash, single-quote, and then ENTER to terminate the command:

 [sh-prompt]$ echo twolines | sed 's/two/& new\
 >/'
 two new
 lines
 [bash-prompt]$

(b) Use a script file with one backslash \ in the script, immediately followed by a newline. This will embed a newline into the "replace" portion. Example:

 sed -f newline.sed files

 # newline.sed
 s/twolines/two new\
 lines/g

Some versions of sed may not need the trailing backslash. If so, remove it.

(c) Insert an unused character and pipe the output through tr:

 echo twolines | sed 's/two/& new=/' | tr "=" "\n"   # produces
 two new
 lines

(d) Use the G command:

G appends a newline, plus the contents of the hold space to the end of the pattern space. If the hold space is empty, a newline is appended anyway. The newline is stored in the pattern space as \n where it can be addressed by grouping \(...\) and moved in the RHS. Thus, to change the "twolines" example used earlier, the following script will work:

 sed '/twolines/{G;s/\(two\)\(lines\)\(\n\)/\1\3\2/;}'

(e) Inserting full lines, not breaking lines up:

If one is not changing lines but only inserting complete lines before or after a pattern, the procedure is much easier. Use the i (insert) or a (append) command, making the alterations by an external script. To insert This line is new BEFORE each line matching a regex:

 /RE/i This line is new               # HHsed, sedmod, gsed 3.02a
 /RE/{x;s/$/This line is new/;G;}     # other seds

The two examples above are intended as "one-line" commands entered from the console. If using a sed script, i\ immediately followed by a literal newline will work on all versions of sed. Furthermore, the command s/$/This line is new/ will only work if the hold space is already empty (which it is by default).

To append This line is new AFTER each line matching a regex:

 /RE/a This line is new               # HHsed, sedmod, gsed 3.02a
 /RE/{G;s/$/This line is new/;}       # other seds

To append 2 blank lines after each line matching a regex:

 /RE/{G;G;}                    # assumes the hold space is empty

To replace each line matching a regex with 5 blank lines:

 /RE/{s/.*//;G;G;G;G;}         # assumes the hold space is empty

(f) Use the y/// command if possible:

On some Unix versions of sed (not GNU sed!), though the s/// command won't accept \n in the RHS, the y/// command does. If your Unix sed supports it, a newline after aaa can be inserted this way (which is not portable to GNU sed or other seds):

 s/aaa/&~/; y/~/\n/;    # assuming no other '~' is on the line!

Solution 2:

Here's a single-line solution that works with any POSIX-compatible sed (including the FreeBSD version on macOS), assuming your shell is bash or ksh or zsh:

sed 's/\(1234\)/\'$'\n''\1/g' <<<'test1234foo123bar1234'

^{Note that you could use a single ANSI C-quoted string as the entire sed script, sed $'...' <<<, but that would necessitate \-escaping all \ instances (doubling them), which is quite cumbersome and hinders readability, as evidenced by @tovk's answer).}

• $'\n' represents a newline and is an instance of ANSI C quoting, which allows you to create strings with control-character escape sequences.
• The above splices the ANSI C-quoted string into the sed script as follows:
  • The script is simply broken into 2 single-quoted strings, with the ANSI C-quoted string stuck between the two halves:
  • 's/\(1234\)/\' is the 1st half - note that it ends in \, so as to escape the newline that will be inserted as the next char. (this escaping is necessary to mark the newline as part of the replacement string rather than being interpreted as the end of the command).
  • $'\n' is the ANSI C-quoted representation of a newline character, which the shell expands to an actual newline before passing the script to sed.
  • '\1/g' is the 2nd half.

Note that this solution works analogously for other control characters, such as $'\t' to represent a tab character.

Background info:

• The POSIX sed specification: http://man.cx/sed
  • BSD sed (also used on macOS) stays close to this spec, while GNU sed offers many extensions.
• A summary of the differences between GNU sed and BSD sed can be found at https://stackoverflow.com/a/24276470/45375

Solution 3:

The solaris version of sed I could convince to work this way (in bash):

echo test1234foo123bar1234 | sed 's/\(1234\)/\
\1/g'

(you have to put the line break directly after the backslash).

In csh I had to put one more backslash:

echo test1234foo123bar1234 | sed 's/\(1234\)/\\
\1/g'

The Gnu version of sed simply worked using \n:

echo test1234foo123bar1234 | sed 's/\(1234\)/\n\1/g'

Solution 4:

Perl provides a richer "extended" regex syntax which is useful here:

perl -p -e 's/(?=1234)/\n/g'

means "substitute a newline for the zero-width match following the pattern 1234". This avoids having to capture and repeat part the expression with backreferences.

Solution 5:

Get a GNU sed.

$ brew install gnu-sed

Then your command will work as expected:

$ gsed "s/\(1234\)/\n\1/g" input.txt
test
1234foo123bar
1234

nb: you may get GNU sed thanks to mac ports too.