Question

I’m aware that the following command can be used to recursively replace all instances of a particular string with another:

find /path/to/files -type f -print0 | xargs -0 sed -i ‘s/oldstring/newstring/g’

However, I need to do this only for lines that start with a particular string (“matchstr”).

For example, if a file contained the following lines:

This line containing oldstring should remain untouched
matchstr sometext oldstring somethingelse

I want to have this as output:

This line containing oldstring should remain untouched
matchstr sometext newstring somethingelse

Any suggestions as to how I could proceed would be appreciated.

Answer 1

You could do:

sed -i '/^matchstr/{s/oldstring/newstring/g}'

ie

find /path/to/files -type f -print0 | \
  xargs -0 sed -i '/^matchstr/{s/oldstring/newstring/g}'

The first /^matchstr/ looks for lines matching that regex, and for those lines the s/old/new/g is performed.