Overview

In this lesson, we’ll dive into Linux streams and learn how to manipulate them in shell scripting. Linux streams are the flow of data between processes—much like water flowing through a pipe. Just as you can redirect a river with gates, you can reroute streams in the shell using redirection operators and pipes.

The image is a "Streams Overview" diagram comparing general streams, represented by wavy lines and gates or valves, to Linux streams, represented by code symbols and redirection operators or programming constructs.

Understanding streams is essential for robust shell scripts. You’ll see how to capture command output, handle errors, and chain commands with pipes to build powerful one-liners.

Standard Streams in Linux

Every Linux process is born with three standard file descriptors:

File Descriptor	Stream Name	Description	Default Source / Destination
0	stdin	Reads input (keyboard or another stream)	Keyboard or pipe
1	stdout	Writes normal output	Terminal or redirected file
2	stderr	Writes error messages and diagnostics	Terminal or redirected file

Standard input (fd 0) typically reads from your keyboard

The image is a "Streams Overview" slide showing "0: Stdin" with an icon of a keyboard, indicating standard input.

Standard output (fd 1) displays command results
Standard error (fd 2) sends error messages

stdout and stderr both default to your terminal. Redirecting one does not affect the other unless you explicitly combine them.

Redirecting Streams

You can reroute streams using <, >, and pipes (|). This allows you to save output to files, read input from files, or chain commands together.

The image is a "Streams Overview" slide showing three concepts: output redirection, input redirection, and pipes, each represented by a symbol.

# Redirect stdout to a file
grep "error" logfile.txt > results.txt

# Redirect stderr to a file
gcc program.c 2> compile_errors.log

# Send both stdout and stderr to the same file
./run_tests.sh > all_output.log 2>&1

# Read stdin from a file
sort < unsorted_list.txt

# Pipe stdout of one command into stdin of another
ps aux | grep sshd

When combining redirections, order matters. Always place 2>&1 after > file to capture both streams.

Examples

Capture command output and errors separately:

ls -l /some/path > listing.txt 2> errors.txt

Chain commands using pipes to filter data:

cat server.log | grep "WARN" | sort | uniq -c

Use input redirection to feed a script:

bash < setup_script.sh

Links and References

Topic	Description	Link
Bash Scripting	Comprehensive guide to Bash syntax	https://www.gnu.org/software/bash/manual/
Advanced Pipelines	Building complex command pipelines	https://www.linuxjournal.com/content/beauty-pipelines
File Descriptors	Low-level I/O in Unix/Linux	https://opensource.com/article/18/4/introduction-file-descriptors

Mastering streams and redirection will elevate your shell scripting from basic commands to automation powerhouses. Practice these techniques to handle data flows confidently in your scripts.

Introduction

Conventions

Refresher

Streams

Expansions - Part One

Globs

Expansions - Part Two

Special Shell Variables

Arrays

Good Practices applied

awk

sed

Standard Streams in Linux

Redirecting Streams

Examples

Links and References

Further Reading

Watch Video

​Standard Streams in Linux

​Redirecting Streams

​Examples

​Links and References

​Further Reading

Watch Video

Standard Streams in Linux

Redirecting Streams

Examples

Links and References

Further Reading