======IO Redirection======

IO can be redirected as per the unix shells (sh) with "> file" , """>>""file" "< file" , "2> file".  Redirection to variables and named ports, "Here Documents" and both text and XML documents are supported.

=====Input Redirection=====
Using the syntax
	command < file
	
Input for command is taken from the file "file"  (either byte stream or Xml document).

=====Output Redirection=====
Using the syntax
	command > file
	
Output for command is passed to file "file"  (either byte stream or Xml document).
Using the syntax
	command "">>"" file
Appends the output of command to file, creating file if it does not already exist.

=====Stderr Redirection=====
Using the syntax
	command 2>  file
	
Stderr Output for command is passed to file "file"  (either byte stream or Xml document).


Using the syntax
	command 2"">>""  file
	
Stderr Output for command is appended to file "file"



=====Here Documents=====
As per the unix shells,  "Here Documents" are supported.
A "here Document" is data embedded directly in the script or from the terminal which becomes the standard input of a command.
The syntax is:

%%
	command <<EOF
Some Text
Here until the magic  "EOF" is found 
on a line by its own
EOF
%%
The tag "EOF" can be any string.   Here documents work as well for text as xml documents.
For example:

%%
xread doc <<EOF
<foo>
	bar
 </foo>
EOF
%%
====URL Input====
URL's can be used for input redirection any place a file is used.
For example

%%
$ xcat < http://test.xmlsh.org/data/books.xml
%%

Also any Internal or Builtin commands that take filenames will take URL's for input files.
For example the above can be rewritten without using redirection

%%
$ xcat  http://test.xmlsh.org/data/books.xml
%%

In both cases, the Base URI of the resulting document is preserved.

Note that URL redirection can only be used for input, not for output at this time.



====Variable Redirection====
A variable can be used for input or output redirection.  The syntax is **{variable}** wherever a file would be used.

Example the following two commands are equivalent and send the result of xls into the variable "doc".

%%
$ xls >{doc}
$ xls | xread doc
%%

And so are the following
%%
$ xcat <{doc}
$ echo $doc | xcat
%%

Variables can be //Appended//.  This will turn a non-sequenced variable into a sequenced variable.
The one exception is that if the variable is initially empty, or contains a single atomic value, AND the output of the command is text then the appending works like appending to a file, that is the strings are concatenated.

Example

%%
$ xls >{doc}
$ xls >>{doc}
%%

Now $doc is a sequence of 2 documents.  The following would produce equivilent results

%%
$ xls | xread doc
$ doc=<[$doc,$doc]>
%%

Text concatenation works when text mode commands are used.

%%
$ echo foo >{port}
$ echo bar >>{port}
$ echo $port
foobar
%%


====Port Redirection====
Input and output can be explicitly bound to a [[NamedPorts Named Port]].  Named ports are specified by preceding the redirection symbol with (port).   There are 3 predefined named ports,  "input" , "output" , "error"

For example to bind the standard input port to "file.xml" and the port named "alternate" to "alternate.xml"

%%
$ command (alternate)<alternate.xml <file.xml
%%

The syntax   2<  is synonymous with (error)<  and  2> is synonymous with (error)>
For example, the following 2 commands are equivalent.

%%
$ command 2>error.txt
$ command (error)>error.txt
%%






----
See Also [[BasicSyntax]]