Basic Input/Output Redirection

Input and Output redirection allows Linux users to:

1. Save command output to files
2. Read input from files
3. Combine commands
4. Redirect errors
5. Automate workflows

Standard Streams in Linux

Linux processes use three default streams.

Stream	Number	Purpose
STDIN	0	Input
STDOUT	1	Normal output
STDERR	2	Error output

1. Output Redirection >

Purpose

Redirect command output into a file.

Syntax

command > file

Example

ls > files.txt

Instead of showing output on terminal:

• Output is saved in:

files.txt

Important Behavior

overwrites file content.

Example

echo "Hello" > test.txt

File content becomes:

Hello

Real-World Usage

Save Process List

ps aux > processes.txt

Export Kubernetes Pods

kubectl get pods > pods.txt

2. Append Output >>

Purpose

Append output to existing file.

Syntax

command >> file

Example

echo "New Log" >> app.log

Adds text to end of file.

Difference Between > and >>

Operator	Behavior
>	Overwrite
>>	Append

Real-World Usage

Append Logs

date >> backup.log

Store Deployment History

kubectl get nodes >> cluster_status.log

3. Input Redirection <

Purpose

Provide file content as input to command.

Syntax

command < file

Example

wc -l < users.txt

Counts lines in file.

Real-World Usage

SQL Script Execution

mysql < database.sql

4. Error Redirection 2>

Purpose

Redirect errors separately.

Syntax

command 2> error.log

Example

ls invalidfile 2> errors.txt

Error saved into:

errors.txt

Real-World Usage

Save Deployment Errors

kubectl apply -f app.yaml 2> deploy_errors.log

5. Redirect STDOUT and STDERR Together

Syntax

command > output.log 2>&1

Meaning

Part	Purpose
>	Redirect stdout
2>&1	Redirect stderr to stdout

Example

docker ps > docker.log 2>&1

Both:

1. Normal output
2. Errors

go into same file.

Modern Shortcut

command &> file.log

6. Pipe |

Purpose

Send output of one command as input to another.

Syntax

command1 | command2

Example

ls -l | less

Another Example

ps aux | grep nginx

Process:

1. ps aux shows processes
2. Output passed to grep
3. grep filters nginx processes

Real-World Usage

Kubernetes

kubectl get pods | grep running

Docker

docker ps | grep nginx

7. Tee Command

Purpose

Display output and save to file simultaneously.

Syntax

command | tee file

Example

ls | tee output.txt

Output:

1. Displays on terminal
2. Saves in file

Real-World Usage

Save Deployment Logs

kubectl apply -f app.yaml | tee deploy.log

Practical DevOps Examples

Monitor Logs

tail -f app.log | grep ERROR

Save System Information

df -h > disk_usage.txt

Store Failed Services

systemctl status nginx 2> nginx_errors.log

Backup Kubernetes Output

kubectl get all > cluster_backup.txt

Common Redirection Symbols

Symbol	Purpose
>	Redirect output
>>	Append output
<	Redirect input
2>	Redirect errors
tee	Save and display output

Dangerous Mistake

Accidentally Overwriting File

echo "test" > important.txt

This removes old content.

Use:

>>

if appending is needed.