Pipes and Redirection

Overview

The real power of the shell comes from combining small commands into pipelines. Redirection sends a command's output to a file instead of the screen, and pipes feed one command's output into another's input. Together they let you build complex data flows out of simple tools.

Syntax / Usage

Use > and >> to redirect output, < to redirect input, and | to pipe between commands.

ls > files.txt          # write output to a file (overwrites)
ls >> files.txt         # append output to a file
sort < files.txt        # feed a file in as input
command 2> errors.txt   # redirect standard error
command > out.txt 2>&1  # redirect both stdout and stderr

cat access.log | grep "404" | wc -l   # pipe: count 404 lines

Examples

Count how many files are in a directory:

ls | wc -l

Save only error lines from a log to a separate file:

grep -i "error" app.log > errors.txt

Find the five largest processes by piping several tools together:

ps aux | sort -rk 4 | head -n 5

Common Mistakes

• Using > when you meant >>, which silently overwrites the file
• Forgetting that errors go to stderr and aren't captured by a plain >
• Piping into a command that reads arguments, not stdin (use xargs instead)
• Assuming pipeline stages run one after another — they run concurrently
• Redirecting to the same file you're reading from, which can truncate it

See Also

command-line-text-processing command-line-processes