How to use sed to replace only the first occurrence in a file?

Question

I would like to update a large number of C++ source files with an extra include directive before any existing #includes. For this sort of task, I normally use a small bash s

Answer 1

If anyone came here to replace a character for the first occurrence in all lines (like myself), use this:

sed '/old/s/old/new/1' file

-bash-4.2$ cat file
123a456a789a
12a34a56
a12
-bash-4.2$ sed '/a/s/a/b/1' file
123b456a789a
12b34a56
b12

By changing 1 to 2 for example, you can replace all the second a's only instead.