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.

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:

• Standard input (fd 0) typically reads from your keyboard
  

• Standard output (fd 1) displays command results

• Standard error (fd 2) sends error messages
  

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.

# 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

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

• GNU Bash Redirections
• Linux Input/Output
• Linux Streams and Pipes

Further Reading

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.