Pipe (`|`) in Linux

What is a pipe?

In Unix/Linux, a pipe (|) is used to connect the output of one command to the input of another. It allows command chaining, enabling small programs to work together efficiently — a core concept of the Unix philosophy.

Syntax


command1 | command2

command1: produces output (stdout)
command2: reads input (stdin) from command1’s output

Common Examples

View log and filter


cat /var/log/syslog | grep "error"

cat: displays the log
grep: filters lines containing “error”

Count files


ls -1 | wc -l

ls -1: lists one file per line
wc -l: counts number of lines (i.e., files)

Sort and remove duplicates


cat file.txt | sort | uniq

sort: sorts lines
uniq: removes consecutive duplicates

Chain multiple commands


ps aux | grep apache | awk '{print $2}' | xargs kill

Lists processes
Filters those with “apache”
Extracts PID
Kills the processes

Best Practices

Avoid using cat unnecessarily. Example:


grep "error" file.txt   # better than cat file.txt | grep "error"

Pipes can be chained for multiple processing steps.
Combine with redirection for logging output:
```
command1 | command2 > output.txt
```

Why use pipes?

Save time and typing
Avoid temporary files
Let small tools do one job each and work together
Powerful for automation and scripting

Pipe (|) in Linux