How to use ed, a line oriented text editor.

ed sample.txt
sample.txt: No such file or directory

The message above warns that the sample.txt file is newly created.

a
the quick brown fox
jumped over the lazy dog
.

That was an append command, which added text to the file.
The dot on a line by itself terminated the append.

1s/f[a-z]x/dragon/

On line 1, replace the first substring matching an f followed by a lowercase alphabetic followed by x with ‘dragon’. The substitute command accepts basic regular expressions.

1,$p

the quick brown dragon jumped over the lazy dog

Print all lines from 1 to the last.

w

51

That wrote the file to disk. The ‘q’ command ends the editing session.

q

Reference

A Tale of Five Editors