Newlines in sed on Mac OS X

Question

I find that \n doesn't work in sed under Mac OS X. Specifically, say I want to break the words separated by a single space into lines:

# input
foo bar

I use,

echo "foo bar" | sed 's/ /\n/'

But the result is stupid, the \n is not escaped!

foonbar

After I consulted to google, I found a workaround:

echo 'foo bar' | sed -e 's/ /\'$'\n/g'

After reading the article, I still cannot understand what \'$'\n/g' means. Can some one explain it to me, or if there is any other way to do it? Thanks!

Solution 1

You can brew install gnu-sed and replace calls to sed with gsed.

---

To use it as sed instead of gsed, brew helpfully prints the following instructions after you install:

GNU "sed" has been installed as "gsed".
If you need to use it as "sed", you can add a "gnubin" directory
to your PATH from your bashrc like:

    PATH="/usr/local/opt/gnu-sed/libexec/gnubin:$PATH"

That is, append the following line to your ~/.bashrc or ~/.zshrc:

export PATH="/usr/local/opt/gnu-sed/libexec/gnubin:$PATH"

Solution 2

These would also work:

echo 'foo bar' | sed 's/ /\
/g'

echo 'foo bar' | sed $'s/ /\\\n/g'

lf=$'\n'; echo 'foo bar' | sed "s/ /\\$lf/g"

OS X's sed doesn't interpret \n in the replace pattern, but you can use a literal linefeed preceded by a line continuation character. The shell replaces $'\n' with a literal linefeed before the sed command is run.

Solution 3

The workaround you found passes a single argument string to sed -e.

That argument ends up being a string in the familiar sed s/ / /g format.

That string is created in two parts, one after the other.

The first part is quoted in '...' form.

The second part is quoted in $'...' form.

The 's/ /\' part gets the single-quotes stripped off, but otherwise passes through to sed just as it looks on the command-line. That is, the backslash isn't eaten by bash, it's passed to sed.

The $'\n/g' part gets the dollar sign and the single-quotes stripped off, and the \n gets converted to a newline character.

All together, the argument becomes

s/ /\newline/g

[That was fun. Took a while to unwrap that. +1 for an interesting question.]

Solution 4

The expression $'...' is a bash-ism which produces ... with the standard escape sequences expanded. Th \' before it just means a backslash followed by the end of the quoted section, the resulting string is s/ /\. (Yes, you can switch quoting in the middle of a string; it doesn't end the string.)

POSIX standard sed only accepts \n as part of a search pattern. OS X uses the FreeBSD sed, which is strictly POSIX compliant; GNU, as usual, adds extra stuff and then Linux users all think that is some kind of "standard" (maybe I'd be more impressed if either of them had a standards process).

Solution 5

There's a very easy to visually see what's happening. Simply echo the string!

echo 's/$/\'$'\n/g'

results in

s/$/\
/g

which is equivalent to s/$/\newline/g

If you didn't have the extra \ before the newline, the shell would interpret the newline as the end of the command prematurely.

View more solutions

Comments

[Admin]

this would probably work too: echo "foo bar" | tr ' ' '\n'

[Admin]

Thanks for the advice. But currently I just use the above case as an example, I do need to know how to escape a \n.

[Admin]

Possible duplicate: stackoverflow.com/questions/21621722/…

[Admin]

Maybe you could also use Perl instead of sed which would make it simpler. See my answer here for details. Basically, it would be echo "foo bar" | perl -pe 's/ /\n/ or perl -pe 's/ +/\n/g to replace all spaces or groups of spaces by a new line.

[Admin]

I just noticed a script running on Big Sur escaped the newline just fine, but the same script on Catalina showed your output... not a newline. I think Apple updated it to address this issue.

[Ivan Xiao]

Now I understand the $'...' part. But... what is s/ /\ ? What do you mean by switch quoting?

[Clément]

Using the same software on all platform is way easier (at last to me) than dealing with every specificities of the Mac version. Beware, the option is now --with-default-names and not --default-names. However, this option did not worked on my installation, so I had to put a alias gsed=sed in my ~/.profile to make it work.

[octosquidopus]

This is an ugly workaround. It doesn't explain why sed on OS X behaves the way it does and makes the incorrect assumption that gnu-sed is more correct. Don't be a GNU-addict and stick to POSIX standards to avoid problems in the long run.

[wolfhammer]

Why does sed $'s/ /\\\n/g' work, but not sed $'s/\\\n/ /g'?

[rascio]

It is how Apple should make its OS works by default. Why use the name of a program and then make it works differently? I waste an hour because of this...

[wisbucky]

@alec You can't use sed even in linux/unix to remove newlines because its parsed/split at each newline. If you run this in linux/unix, it won't do anything either: echo -e 'foo\nbar' | sed 's/\n//'

[iAdjunct]

@rascio - because OS X is BSD-like and Linux is Linux-like.

[Duncan Bayne]

@rascio I think you'll find that GNU sed is the newcomer; BSD sed dates back to 1979 (see en.wikipedia.org/wiki/Version_7_Unix).

[Robin Keskisarkka]

This is the preferred answer in most cases I think. If you get a compile script in your lap you don't want to spend time modifying it just to make it work with OSX. Instead, simply do brew install gnu-sed, alias sed='gsed', sh compile.sh, and remove the alias when you're done (unalias sed).

[Robin Keskisarkka]

Correction: For the alias to work correctly inside the script, the command should be source compile.sh

[k1eran]

brew install gnu-sed --with-default-names gives me Error: invalid option: --with-default-names

[grepit]

this should have been the accepted answer