ed is the standard text editor.
ed has two modes: command and input. By default it is in command mode. Use i or a to insert or append text and when finished input a . on a line all by itself to return to command mode.
A summary of ed commands
|a||append text to the buffer|
|i||insert text at the current line in the buffer|
|p||print the current line - try 1p to print the first line|
|q||quit ed - to quit without saving, type q!|
|s||search/replace - try s/old/new|
|w||write out the buffer to disk - w filename|
|.||return to command mode from append or input mode|
If you have made an error, ed will only respond with ?