Append text following each line matched by address. If text goes over more than one line, newlines must be “hidden” by preceding them with a backslash. The text is terminated by the first newline that is not hidden in this way. The text is not available in the pattern space, and subsequent commands cannot be applied to it. The results of this command are sent to standard output when the list of editing commands is finished, regardless of what happens to the current line in the pattern space.
The GNU version accepts two addresses and allows you to put the first line of text on the same line as the a command.
$ a\ This goes after the last line in the file\ (marked by $). This text is escaped at the\ end of each line, except for the last one.