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I would like to find the String __MARKER__ in someFile.txt and place a String above it (e.g. Hello World!).

How would I do that using sed?

sed -i -e "/__MARKER__//g" someFile.txt # Currently removes __MARKER__.

Contents of someFile.txt:

__MARKER__

Contents of someFile.txt after manipulation:

Hello World!
__MARKER__
4
sed -i -e 's/__MARKER__/Hello world\n&/' someFile.txt

replaces __MARKER__ with Hello world, a newline, and the match (&), i.e. the text __MARKER__.

The & just saves typing __MARKER__ again, as in

sed -i -e 's/__MARKER__/Hello world\n__MARKER__/' someFile.txt
  • After running sed -i -e 's/__MARKER__/Hello world\n&/' someFile.txt the contents of someFile.txt is Hello worldn__MARKER__. – Nicholas Adamou Jun 6 '18 at 19:46
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    That's weird because for me it works exactly as I've written and I didn't escape any backslashes (by doubling them). It just worked as is. Are you on Windows? – PerlDuck Jun 6 '18 at 19:51
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If there is only a single instance of __MARKER__ or you want to insert the new string before every instance, you can use

sed -i '/__MARKER__/ iHello World!' someFile.txt

This uses the insert command rather than the substitute command - note that on POSIX (non-GNU) sed you may need to put the insert string on a separate line:

sed -i '/__MARKER__/ i\
Hello World!
' someFile.txt

If there are multiple instances, and you only wish to insert the string before the first one, then

sed -i '0,/__MARKER__/ s//Hello World!\n&/' someFile.txt

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