Pipes

Connect the output of one command to the input of another.

Basic Usage

#> ls | grep txt
file1.txt
test.txt

#> cat file.txt | grep pattern | sort | uniq

How Pipes Work

command1 | command2 | command3

• command1 stdout → command2 stdin
• command2 stdout → command3 stdin
• command3 stdout → terminal

Each command runs in its own process.

Exit Code

Pipeline exit code is from the last command:

#> false | true
#> echo $?
0  # From 'true', not 'false'

Common Patterns

Filtering

ls -la | grep txt
ps aux | grep nginx
cat log.txt | grep error

Counting

ls | wc -l                    # Count files
cat file.txt | wc -l          # Count lines
ps aux | grep python | wc -l  # Count processes

Text Processing

cat names.txt | sort | uniq
ls -la | awk '{print $5, $9}'
echo hello | tr '[:lower:]' '[:upper:]'

Data Analysis

cat data.csv | cut -d, -f2 | sort | uniq -c | sort -rn | head -10

With Other Features

With Aliases

alias count='wc -l'
ls | count

With Variables

export PATTERN=error
cat log.txt | grep $PATTERN

With Chaining

ls | grep txt && echo "Found txt files"

Difference from ||

echo hello | cat      # Pipe: passes "hello" to cat
false || echo hello   # OR: runs echo because false failed

Troubleshooting

Broken Pipe

If first command produces lots of output but second exits early:

cat huge_file.txt | head -10
# Normal behavior, handled automatically

No Output / Hanging

Command waiting for input:

cat | grep something
# Waiting forever - press Ctrl+C

Limitations

• No |& (pipe stderr)
• No process substitution <(command)