Bash Pipes and Redirection

Pipes and redirection are among the most powerful features in Bash. They connect commands together and control where data flows — from one command to another, or between commands and files. Mastering these concepts allows writing compact and efficient one-liners.

Standard Streams

Every command in Linux uses three data channels called standard streams.

┌──────────────────────────────────────────────────────┐
│                                                      │
│  stdin  (Standard Input)  → File descriptor: 0       │
│         Where a command reads input from             │
│         Default: keyboard                            │
│                                                      │
│  stdout (Standard Output) → File descriptor: 1       │
│         Where a command writes normal output         │
│         Default: terminal screen                     │
│                                                      │
│  stderr (Standard Error)  → File descriptor: 2       │
│         Where error messages appear                  │
│         Default: terminal screen                     │
│                                                      │
└──────────────────────────────────────────────────────┘

Output Redirection

Redirect stdout to a File

ls -l > filelist.txt

This saves the output of ls -l into filelist.txt. The screen shows nothing — the output went to the file.

Append stdout to a File

echo "New entry" >> filelist.txt

Redirect stderr to a File

ls /nonexistent_dir 2> error.log

Redirect Both stdout and stderr to One File

command > output.txt 2>&1

Or in a shorter modern form:

command &> output.txt

Redirection Summary Table

Input Redirection

#!/bin/bash
while read line
do
  echo "Line: $line"
done < names.txt

Pipes – Connecting Commands

A pipe | takes the output of one command and feeds it directly as the input of another command. No temporary file is needed.

┌────────────┐      ┌────────────┐      ┌────────────┐
│  Command 1 │─────►│  Command 2 │─────►│  Command 3 │
│  produces  │ pipe │  processes │ pipe │  displays  │
│  output    │  |   │  it        │  |   │  result    │
└────────────┘      └────────────┘      └────────────┘

Example – Count Files in a Directory

ls | wc -l

The output of ls (list of files) goes into wc -l (count lines). The result is the total number of files.

Example – Find and Count Errors in a Log

cat server.log | grep "ERROR" | wc -l

Example – View Long Output Page by Page

ls -la | less

Example – Sort and Remove Duplicates

cat names.txt | sort | uniq

Useful Commands Commonly Used with Pipes

Here Document (heredoc)

A heredoc sends a block of multi-line text as input to a command. Use <<LABEL to start and the same label on its own line to end.

#!/bin/bash
cat <<MESSAGE
Welcome to eStudy247.
This is a multi-line message.
Bash makes it easy.
MESSAGE

Output:

Welcome to eStudy247.
This is a multi-line message.
Bash makes it easy.

Here String

A here string passes a single string as stdin to a command using <<<.

#!/bin/bash
word="hello"
tr 'a-z' 'A-Z' <<< "$word"

Output:

HELLO

tee Command – Output to Screen AND a File

The tee command writes output to both the terminal and a file at the same time.

ls -l | tee filelist.txt

Output appears on screen and also saves to filelist.txt.

Practical Example – System Report Generator

#!/bin/bash
reportfile="system_report.txt"

{
  echo "=== System Report ==="
  echo "Date: $(date)"
  echo ""
  echo "--- Disk Usage ---"
  df -h
  echo ""
  echo "--- Memory Usage ---"
  free -h
  echo ""
  echo "--- Running Processes ---"
  ps aux | head -10
} > $reportfile

echo "Report saved to $reportfile"

Key Takeaways

• Use > to redirect output to a file and >> to append.
• Use 2> to redirect error messages to a file.
• Use 2>/dev/null to silently discard error messages.
• Use | (pipe) to connect commands and pass data between them.
• Use tee to write output to both screen and file simultaneously.
• Heredoc (<<) passes multi-line text as input to a command.