Reading and Writing Mathematica Files
Storing Mathematica Expressions in External Files
You can use files on your computer system to store definitions and results from
Mathematica. The most general approach is to store everything as plain text that is appropriate for input to
Mathematica. With this approach, a version of
Mathematica running on one computer system produces files that can be read by a version running on any computer system. In addition, such files can be manipulated by other standard programs, such as text editors.
<<file or Get["file"]  read in a file of Mathematica input, and return the last expression in the file 
FilePrint["file"]  display the contents of a file 
expr>>file or Put[expr,"file"]  write an expression to a file 
expr>>>file or PutAppend[expr,"file"] 
 append an expression to a file 
Reading and writing files.
This expands (x+y)^{3}, and outputs the result to a file called tmp. 
Here are the contents of tmp. They can be used directly as input for Mathematica. 
This reads in tmp, evaluating the Mathematica input it contains.
Out[3]=  

This shows the contents of the file factors. 
This reads in the file, and returns the last expression in it.
Out[2]=  

If Mathematica cannot find the file you ask it to read, it prints a message, then returns the symbol $Failed.
Out[19]=  

When you read in a file with
<<file,
Mathematica returns the last expression it evaluates in the file. You can avoid getting any visible result from reading a file by ending the last expression in the file with a semicolon, or by explicitly adding
Null after that expression.
If
Mathematica encounters a syntax error while reading a file, it reports the error, skips the remainder of the file, then returns
$Failed. If the syntax error occurs in the middle of a package which uses
BeginPackage and other context manipulation functions, then
Mathematica tries to restore the context to what it was before the package was read.
Saving Multiple Mathematica Expressions
Mathematica input files can contain any number of expressions. Each expression, however, must start on a new line. The expressions may continue for as many lines as necessary. Just as in a standard interactive
Mathematica session, the expressions are processed as soon as they are complete. Note, that in a file, unlike an interactive session, you can insert a blank line at any point without effect.
When you use
expr>>>file,
Mathematica appends each new expression you give to the end of your file. If you use
expr>>file, however, then
Mathematica instead wipes out anything that was in the file before, and then puts
expr into the file.
This writes an expression to the file tmp. 
Here are the contents of the file. 
This appends another expression to the same file. 
Both expressions are now in the file. 
If you are familiar with commandline operating systems, you will recognize the
Mathematica redirection operators
>>,
>>> and
<< as being analogous to the commandline operators
>,
>> and
<.
Saving Mathematica Expressions in Different Formats
When you use either
>> or
>>> to write expressions to files, the expressions are usually given in
Mathematica input format, so that you can read them back into
Mathematica. Sometimes, however, you may want to save expressions in other formats. You can do this by explicitly wrapping a format directive such as
OutputForm around the expression you write out.
This writes an expression to the file tmp in output format. 
The expression in tmp is now in output format. 
Saving Definitions of Mathematica Objects
One of the most common reasons for using files is to save definitions of
Mathematica objects, to be able to read them in again in a subsequent
Mathematica session. The operators
>> and
>>> allow you to save
Mathematica expressions in files. You can use the function
Save to save complete definitions of
Mathematica objects, in a form suitable for execution in subsequent
Mathematica sessions.
Save["file",symbol]  save the complete definitions for a symbol in a file 
Save["file","form"]  save definitions for symbols whose names match the string pattern form 
Save["file","context`"]  save definitions for all symbols in the specified context 
Save["file",{object_{1},object_{2},...}]  save definitions for several objects 
Saving definitions in plain text files.
This assigns a value to the symbol a.
Out[51]=  

You can use Save to write the definition of a to a file. 
Here is the definition of a that was saved in the file. 
This defines a function f which depends on the symbol a previously defined. 
This saves the complete definition of f in a file. 
The file contains not only the definition of f itself, but also the definition of the symbol a on which f depends. 
This clears the definitions of f and a. 
You can reinstate the definitions you saved simply by reading in the file ffile.
Out[58]=  

The function
Save makes use of the output forms
Definition and
FullDefinition, which print as definitions of
Mathematica symbols. In some cases, you may find it convenient to use these output forms directly.
The output form Definition[f] prints as the sequence of definitions that have been made for f.
Out[59]=  

When you define a new object in
Mathematica, your definition will often depend on other objects that you defined before. If you are going to be able to reconstruct the definition of your new object in a subsequent
Mathematica session, it is important that you store not only its own definition, but also the definitions of other objects on which it depends. The function
Save looks through the definitions of the objects you ask it to save, and automatically also saves all definitions of other objects on which it can see that these depend. However, in order to avoid saving a large amount of unnecessary material,
Save never includes definitions for symbols that have the attribute
Protected. It assumes that the definitions for these symbols are also built in. Nevertheless, with such definitions taken care of, it should always be the case that reading the output generated by
Save back into a new
Mathematica session will set up the definitions of your objects exactly as you had them before.
Saving Mathematica Definitions in Encoded Form
When you create files for input to
Mathematica, you usually want them to contain only "plain text", which can be read or modified directly. Sometimes, however, you may want the contents of a file to be "encoded" so that they cannot be read or modified directly as plain text, but can be loaded into
Mathematica. You can create encoded files using the
Mathematica function
Encode.
Encode["source","dest"]  write an encoded version of the file source to the file dest 
<<dest  read in an encoded file 
Encode["source","dest","key"]  encode with the specified key 
Get["dest","key"]  read in a file that was encoded with a key 
Encode["source","dest",MachineID>"ID"] 
 create an encoded file which can only be read on a machine with a particular ID 
Creating and reading encoded files.
This writes an expression in plain text to the file tmp. 
This writes an encoded version of the file tmp to the file tmp.x. 
Here are the contents of the encoded file. The only recognizable part is the special Mathematica comment at the beginning. 
Even though the file is encoded, you can still read it into Mathematica using the << operator.
Out[64]=  

DumpSave["file.mx",symbol]  save definitions for a symbol in internal Mathematica format 
DumpSave["file.mx","context`"]  save definitions for all symbols in a context 
DumpSave["file.mx",{object_{1},object_{2},...}] 
 save definitions for several symbols or contexts 
DumpSave["package`",objects]  save definitions in a file with a specially chosen name 
Saving definitions in internal Mathematica format.
If you have to read in very large or complicated definitions, you will often find it more efficient to store these definitions in internal
Mathematica format, rather than as text. You can do this using
DumpSave.
This saves the definition for f in internal Mathematica format.
Out[22]=  

You can still use << to read the definition in. 
<< recognizes when a file contains definitions in internal
Mathematica format, and operates accordingly. One subtlety is that the internal
Mathematica format differs from one computer system to another. As a result,
.mx files created on one computer cannot typically be read on another.
If you use
DumpSave["package`", ...] then
Mathematica will write out definitions to a file with a name like
package.mx/system/package.mx, where
system identifies your type of computer system.
This creates a file with a name that reflects the name of the computer system being used.
Out[24]=  

<< automatically picks out the file with the appropriate name for your computer system. 