CIS241

System-Level Programming and Utilities

Redirection and Piping

Erik Fredericks, frederer@gvsu.edu
Fall 2025

Based on material provided by Erin Carrier, Austin Ferguson, and Katherine Bowers

First, command chaining

Or, running multiple commands in a single line

Seperate with ; or &&

• date ; who ; whoami ; echo "Hi there!"
• date && who && whoami && echo "Hi there!"
  • Don't use a single & by mistake - that means run in the background!

A reference

https://dev.to/bowmanjd/bash-execution-tips-for-shell-jockeys-and-script-fabricators-5dan

Use || to execute one command only when the previous one fails.
Combine the above for conditional branching.
Use ; to join two commands when you want the second to execute no matter the result of the first one.
Use & to run the first job in the background while the next executes. Follow all with wait for a clean return to the command prompt

IO redirection

Standard streams

• 0: stdin - Standard input (user typing)
• 1: stdout - Standard output (print to terminal // output of program)
• 2: stderr - Standard error (also prints to terminal)

IO redirection

Redirect stdout to file:

• ls -la > dir_contents

Redirect stderr to file:

• ls -la 2> errors

Redirect file to stdin:

• sort < my_file.txt

Example:

Redirect stdout to file and stderr to stdout:

• command >output 2>&1

Redirect stdout to file and stderr to another file:

• command >output 2>output.err

Redirect stderr to the void:

• command 2> /dev/null

command = echo "Hi" or eccccho "Hi"

Piping

Piping

Send output of one command as input to another

command1 args | command2

• cat file | sort
• cat file | sort | uniq
• cat file | sort | uniq > output.txt