OpenAppend["file"] opens a file to append output to it, and returns an OutputStream object.
The following options can be given:
On computer systems that support pipes, OpenAppend["!command"] runs the external program specified by command, and opens a pipe to send input to it.
If OpenRead does not succeed in opening a particular file or pipe, it generates a message, and returns $Failed.
OpenAppend resolves file names according to the procedure described in Section A.6.1.
OpenAppend returns OutputStream["name", n], where name is the full name of a file or command, and n is a serial number that is unique across all streams opened in the current Mathematica session.
SetOptions can be used to change the properties of an output stream, after it is already open.
Functions like Put and Write automatically open the files or pipes they need, if they are not already open.
Setting the option DOSTextFormat->True causes newlines specified by \n to be output as \r\n pairs, suitable for text-mode files on MS-DOS and related systems.
See The Mathematica Book: Section 2.11.3.
See also: Close, Put, Streams, LinkCreate.