For example, if we have a file:
$ cat foo This is a sample hello world file.
And want to change 'hello world' to 'hello shell-fu' we need to replace across lines. This can be done with the following command:
:~$ cat foo | sed '/hello$/N;s/hello\nworld/hello\nshell-fu/' This is a sample hello shell-fu file.
Here sed first looks for lines which end with 'hello' then reads the next line, finally replacing 'hello\nworld' with 'hello\nshell-fu'.
This also has a lot of other uses, for example converting double line spaced files to single:
cat doublespace | sed '/^$/N;s/\n$//g'
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