Substitute entire line while removing newline of that particular line

Question

I'm trying to substitute (with sed) an entire line containing a specific word and the newline at the end. Here the testfile:

this # target for substitution
this is a test
another test?

Now, I already posted here, and from the linked post, I understand how to do this in some way:

sed 's/^this$/test/g' testfile

That works, or at least it seems so, because the newline at the end of the word this is still there:

test # target for substitution but newline is still there
this is a test
another test?

Given the above, I'm also fully aware sed can't match the newline directly (although I do recall that I could use '\n' in certain version of sed, but that's beside the point).

I do know how to at least delete the entire word/line and the newline:

sed '/^this$/d' testfile

Except I need to substitute it instead.

How can I do this? (with sed preferably)

Answer 1

As I understand you, you want to replace a line consisting only of the word this and the following newline by test, so

foo
this
this is a test

should become

foo
testthis is a test

In sed you can do simply join the next line with N and replace everything up to the newline:

sed '/^this$/{N;s/.*\n/test/;}'

Answer 2

I'd suggest to use perl here, syntax isn't that different from sed for this case:

$ cat ip.txt
this
this is a test
another test?

$ perl -pe 's/^this\n/XYZ/' ip.txt
XYZthis is a test
another test?

Answer 3

With GNU sed, you can read all lines into memory with -z and do the matching from there, e.g.:

sed -z 's/this\n/test/'

Answer 4

Turn out doing this, or as other have pointed out "joining" lines in sed is possible:

sed ':a;/^this$/{N;s/\n//;ba}' testfile

result:

thisthis is a test
another test?

To also do substitution:

sed 's/^this$/test/g;:a;/^test$/{N;s/\n//;ba}' testfile

testthis is a test
another test?

Taken from this answer.

Answer 5

One way using awk by manipulating the output record separator:

$ awk '{ ORS = sub(/^this$/,"FOO") ? "" : RS }1' file

---

$ sed -e '
    $!N
    s/^this\n/FOO/;t
    P;D
' file

Answer 6

Using awk:

awk '/^this$/{$0="test"}1'  file

In this command, if pattern is found then this is replaced by replacement.

The remaining work is to remove newline after replacement because newline is still there. printf would do that work because unlike print this doesn’t add newline at the end by default.

This is:

awk '/^this$/{printf "%s", "test"; next} 1'   file

And this works.