CIS241

System-Level Programming and Utilities

bash Basics

Erik Fredericks, frederer@gvsu.edu
Fall 2025

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

What is a bash script?

Easy mode: a series of bash commands run in order

• Or, everything you've done so far!

However, bash is a full programming language!

• Control structures (loops, conditionals)
• Variables
• Functions
• Arguments (parameters)
• Arrays
• ...

But why?

• Automate all the things!
  • Way more efficient to script out repetitive tasks!
  • Even better if you can customize them

Examples!

• Sysadmins checking server status and running regular updates
  • Or auto-creating accounts and giving permissions
• Build and deploy personal website!
  • All the git actions for building/deploying, for example

OK LET'S GO

Basically, we're going to put all the bash commands we know into a file

• And then running it!

Convention is to use a .sh extension

• Run it with bash file.sh

Make it executable?

• chmod u+x filename
• chmod +x filename
• ./filename

The pound-bang

Or hashbang, shebang, hashpling

(The top line telling your script where to find bash)

#!/bin/bash

• Q: Why bother adding it? It isn't strictly required...
  • A: Portability! Users don't need to necessarily know what to use to call the script
  • Also, easily run bash scripts from other shells!

Examples

• #!/bin/bash
• #!/usr/bin/env bash

Can use with others like Python:

• #!/usr/bin/env python3

/usr/bin/env vs. /bin/bash

• env uses whatever version of the executable comes first in $PATH
  • env - users can have different behavior!

Mac users (from Carrier)

• Use brew install bash to get updated version
• Different path from previous slide!
• Hers was: #!/usr/local/bin/bash

Comments!

# begins a comment until the end of the line

Exception:

• Hashbang on first line of script

Ex:

#!/bin/bash

echo "Hello there" # this is also a comment
# this is a comment