4. Pipes and Filters

Questions

• How can I combine existing commands to do new things?

Objectives

• Redirect a command’s output to a file.
• Construct command pipelines with two or more stages.
• Explain what usually happens if a program or pipeline isn’t given any input to process.
• Explain the advantage of linking commands with pipes and filters.

Key points

• wc counts lines, words, and characters in its inputs.
• cat displays the contents of its inputs.
• sort sorts its inputs.
• head displays the first 10 lines of its input.
• tail displays the last 10 lines of its input.
• command > [file] redirects a command’s output to a file (overwriting any existing content).
• command >> [file] appends a command’s output to a file.
• [first] | [second] is a pipeline: the output of the first command is used as the input to the second.
• The best way to use the shell is to use pipes to combine simple single-purpose programs (filters).