ed prints two kinds of messages: diagnostic messages (errors and
warnings) written to standard error and help messages written to standard
output. Diagnostic messages may be suppresed with option -q. Help
messages may be enabled with option -v and may be toggled with
command ‘H’.
ed warns the first time that a command modifies a buffer loaded
from a read-only file.
When an error occurs, if ed’s standard input is a regular file or
here document, then it exits, otherwise it prints a ‘?’ and returns to
command mode. An explanation of the last error can be printed with the
‘h’ (help) command.
If the ‘u’ (undo) command occurs in a global command list, then the command list is executed only once.