sed replace string in a first line

Question

How can I replace a string but only in the first line of the file using the program \"sed\"?

The commands s/test/blah/1 and 1s/test/blah/ d

Answer 1

This might work for you (GNU sed):

sed -i '1!b;s/test/blah/' file

will only substitute the first test for blah on the first line only.

Or if you just want to change the first line:

sed -i '1c\replacement' file