Course/Piping & Redirection/Useful Pipe Chains

Useful Pipe Chains

Learn common pipe combinations: cat | grep, ls | wc, and sort | uniq.

Now that you know how the pipe works, let's look at the combinations developers use every day. These patterns are so common they'll become second nature. Each one pairs a data-producing command with a data-processing command.

cat server.log | grep "ERROR"    # find error lines in a log
ls Documents/ | wc -l            # count files in a directory
cat names.txt | sort | uniq      # sort names and remove duplicates

cat | grep is the classic "search a file" pattern. cat dumps the file contents and grep filters for lines matching a pattern. ls | wc -l counts items in a directory — wc stands for "word count" and the -l flag tells it to count lines instead. Since ls outputs one item per line when piped, this effectively counts files and directories.

sort | uniq is a powerful duo for cleaning up data. sort arranges lines in alphabetical order, and uniq removes consecutive duplicate lines. Since uniq only removes adjacent duplicates, you almost always sort first. Together they answer questions like "what are the unique entries in this list?"

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The wc command has several useful flags: -l counts lines, -w counts words, and -c counts bytes (characters). The -l flag is by far the most common — counting lines is surprisingly useful for everything from checking file lengths to counting search results.

A reports directory has been set up with a bunch of files. Use ls piped to wc -l to count how many files are in the directory.

ls reports/ | wc -l

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PowerShell equivalents: Get-Content log.txt | Select-String 'ERROR' replaces cat | grep. (Get-ChildItem).Count counts files without piping. The logic is the same even though the syntax differs.

Practice