The <#1091#><#1091#> states that ports represent input and output devices.
However, it defines only ports which are attached to files. In
<#1092#><#1092#>, ports can also be attached to strings or to a external command
input or output. String ports are similar to file ports, except that
characters are read from (or written to) a string rather than a file.
External command input or output ports are implemented with Unix pipes
and are called pipe ports. A pipe port is created by specifying the
command to execute prefixed with the string <#1093#>;SPMquot;|~;SPMquot;<#1093#>. Specification
of a pipe port can occur everywhere a file name is needed.
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