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When an error occurs,
if ed's input is from 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.
Attempting to quit ed or edit
another file before writing a modified buffer results in an error. If
the command is entered a second time, it succeeds, but any changes to
the buffer are lost.
ed exits with 0 if no errors occurred; otherwise >0.
Go to the previous section.