Each UNIX command is written as a general purpose program. Unless you specify differently, the default files will be opened. These default files can easily be changed when the command is invoked. Redirecting Output To redirect the output of the ls command to go into a disk file instead of going to the terminal screen. If you call up a command that expects a filename, and you don't provide one, input will come from the keyboard until a Ctrl-D is read.
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Some text is typed here. Instead of truncating the output file to length zero and directing output to it, the output is appended onto the end of the output file. Filters Most commands in UNIX are filters. Its output must always be to a printer.