OpenAppend

opens a file to append output to it, and returns an OutputStream object.

Details and Options

BinaryFormat	False	whether to use binary format for the file
CharacterEncoding	Automatic	what raw character encoding to use
FormatType	InputForm	default format for printing expressions
NumberMarks	$NumberMarks	when to use ` marks in approximate numbers
PageWidth	78	number of character widths per line
TotalWidth	Infinity	maximum number of character widths for a single expression
Method	Automatic	stream method to use

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 OpenAppend 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 "Files and Streams".
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 Wolfram System 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.
With the default setting BinaryFormat->False, the setting for CharacterEncoding is taken to be "ASCII".
With the Method option, the stream is opened using the given input stream method. This overrides the default way that OpenAppend resolves file names.
OpenAppend[File["file"]] is also supported.

open allclose all

Create a file and write an expression to it:

Read the file:

Open the file using OpenAppend:

Write an expression and close the stream:

Read the file:

Delete the temporary file:

Write an expression to the file specified by the File object:

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