How to use pipes and redirection in Bash

The Pipe: Unix's Most Powerful Idea

Every command in Linux reads from stdin and writes to stdout. The pipe | connects the stdout of one command directly to the stdin of the next. This lets you chain simple tools into complex processing pipelines without writing intermediate files.

Pipes in Practice

# Count how many processes are running
ps aux | wc -l

# Find the 5 largest files in current directory
du -sh * | sort -rh | head -5

# See only unique error messages in a log
grep "ERROR" app.log | sort | uniq -c | sort -rn

Redirection Operators

> redirects stdout to a file, overwriting it. >> appends instead of overwriting. 2> redirects stderr. 2>&1 merges stderr into stdout — useful when you want all output in one log file.

# Save command output to file
ls -lah > directory_listing.txt

# Append to existing file
echo "Build completed" >> build.log

# Discard all errors (silence stderr)
command 2> /dev/null

# Save both stdout and stderr
build_script.sh > build.log 2>&1

# Write to file AND see it on screen
build_script.sh | tee build.log

tee splits output — it writes to stdout (your terminal) and to a file simultaneously. Invaluable when you want to watch and record output at the same time.