COMP 10024 – Fundamentals of UNIX (Week6)

February 23, 2026 8:40 pm Published by

The Unix Shell: Expansions & Quoting

Demystifying how bash transforms your commands before execution

 

What is the Shell?

The shell is a program, a command interpreter. It reads input from the keyboard (or a file), decides what to do, and runs other programs. Think of it as the wrapper around the operating system’s core.

We’re focusing on bash (Bourne Again SHell), the default on most Linux distributions but keep in mind that there are other shells that can be installed like ZSH or Oh My ZSH

Why “shell”? Just as a kernel is the core of an OS, the shell is the outer layer that interacts with users. The kernel manages hardware; the shell manages you.

Quick Review: The Unix Kernel

The kernel is the heart of the OS and is loaded at boot, it controls all hardware resources: CPU, memory, disks, network, etc. The shell talks to the kernel on your behalf.

Shell → Kernel → Hardware

The Nine Expansions

Before executing a command, bash processes it in this exact order:

1. Brace
2. Tilde
3. Parameter
4. Command
5. Arithmetic
6. Process
7. Word Splitting
8. Filename
9. Quote Removal

Let’s explore the most important ones in detail.

Tilde Expansion ~

The tilde ~ is a shortcut for home directories.

You type Shell expands to
~ your home directory (value of $HOME)
~bob Bob’s home directory
~/Documents /home/you/Documents
$ echo ~ # /home/alice (if you’re alice)
$ ls ~bob/*.txt # list all txt files in bob’s home
$ cp ~/music/*.mp3 . # copy your mp3s to current dir

Filename (Globbing) Expansion

Special pattern characters * ? [] are replaced with matching filenames.

Pattern Meaning
* any string of characters (including none)
? any single character
[abc] any one character from the set

Example: Files in /tmp/car_files:

  • m3.jpg
  • m3.mp3
  • m5.jpg
  • m5.mp3
  • race_video.mv4
  • vette.jpg
  • vette.mp3
$ ls *.mp3 # m3.mp3 m5.mp3 vette.mp3
$ ls m?.jpg # m3.jpg m5.jpg
$ ls m[35].jpg # same: m3.jpg m5.jpg
$ ls *.[mj]* # tricky: matches any file with .m or .j
WARNING! Be careful: ls *m* matched everything containing ‘m’ — including .jpg files! Why? Because ‘m’ appears in the name. To narrow, use patterns like *.m?? or *.[mp][34]*.

A Note on Dot Files (.)

Files starting with a dot (.bashrc, .profile) are hidden files, meaning they don’t show in the finder. By design, filename expansion ignores leading dots unless the pattern itself begins with a dot.

$ ls * # shows only non-hidden files
$ ls .* # shows hidden files (and . and ..)

This prevents accidental matches on . and .. — imagine if rm -rf * tried to delete parent directories!

Brace { }

Generate arbitrary strings — not limited to existing filenames.

Expression Expands to
a{1,2,3}b a1b a2b a3b
{one,two}.txt one.txt two.txt
file{A,B}.bak fileA.bak fileB.bak

Great for quick backups or creating multiple directories:

$ mkdir -p project/src/{main,test}/java
$ touch img_{small,large}.png

Parameter (Variable)

Variables store values. Use $VAR or ${VAR} to expand.

$ NAME=alice
$ echo $NAME # alice
$ echo ${NAME} # alice

Why ${VAR}? It clearly delimits the variable name:

$ echo ${NAME}_the_fast_runner # alice_the_fast_runner
$ echo $NAME_the_fast_runner # expands $NAME_the (probably empty)

Quoting: Preserving Literals

Sometimes we want to remove the special meaning of characters.

Problem: Print She said “Hi Fred”

$ echo She said “Hi Fred”
She said Hi Fred # quotes are gone!

Solutions:

Method Example Output
Backslash escape echo She said \”Hi Fred\” She said “Hi Fred”
Single quotes echo ‘She said “Hi Fred”‘ She said “Hi Fred”

Quote types:

  • \ — escapes the next character only.
  • ‘…’ — preserve literal value of every character inside.
  • “…” — preserve most literals, but $, `, and \ still have special meaning.
$ echo “Home: $HOME” # Home: /home/alice (expanded)
$ echo ‘Home: $HOME’ # Home: $HOME (literal)

Other Expansions (briefly)

  • Command substitution: $(date) or `date` — runs command and inserts output.
  • Arithmetic expansion: $((2+2)) → 4.
  • Process substitution: <(cmd) — treat command output as a file (advanced).
  • Word splitting: after expansions, bash splits results into words (based on IFS).
$ echo “Today is $(date +%A)” # Today is Monday
$ echo “The answer is $((6*7))” # The answer is 42

Environment Variables – A Preview

Every Unix process has a block of memory for environment variables. In the shell, they work like parameters, but are inherited by child processes.

$ export VAR=value # makes it an environment variable
$ echo $PATH # typical environment variable

To truly understand environment variables, we need to discuss processes: forking, parents, children, and (oh my) zombies. That’s a lecture for another day.

Putting It All Together

When you hit Enter, bash follows these steps:

  1. Reads input (from keyboard/script)
  2. Breaks into words and operators
  3. Performs expansions (brace → tilde → parameter → command → arithmetic → process → word splitting → filename)
  4. Removes quotes
  5. Executes the command

Understanding this order helps you predict how any command will behave.

Shell Expansions — Practice Lab

Hands-on exercises to tame tilde, brace, filename globbing,
variables, and quoting.
Open a terminal and follow each step. (Assumes bash)

📁 Lab Setup: Create Practice Files

First, let’s build a playground with different file types.

$ cd ~
$ mkdir -p lab_files/car_files
$ cd lab_files/car_files
$ touch m3.jpg m3.mp3 m5.jpg m5.mp3 race_video.mv4 vette.jpg vette.mp3
$ mkdir hidden stuff
$ touch .secret.txt .hidden.jpg stuff/regular.txt
$ cd ../.. # back to home area
$ echo “Ready.”

This gives us a mix of files to test expansions.

1. Tilde Expansion

~ is your home sweet home.

🔹 Task 1: Display your home directory path using echo.
$ echo ~
/home/your_username (or /Users/your_username on Mac)
🔹 Task 2: List contents of your home directory using ls ~. Then list only the lab_files folder with ls ~/lab_files.
$ ls ~
$ ls ~/lab_files
🔹 Task 3: Try echo ~root (if you have permission). This shows root’s home. Also try a user that doesn’t exist: echo ~notreal.
$ echo ~root
/root
$ echo ~notreal
~notreal (no expansion)
Remember: ~ alone = $HOME. ~user = that user’s home (if known).

2. Filename (Globbing) Expansion

Patterns with *, ?, [] match files.

First, go to our test directory:

$ cd ~/lab_files/car_files
🔹 Task 4: List all .jpg files.
$ ls *.jpg
m3.jpg m5.jpg vette.jpg
🔹 Task 5: List all files with exactly 4 characters before extension (e.g., m3.jpg, m5.jpg) using ?.
$ ls ??.???
m3.jpg m3.mp3 m5.jpg m5.mp3
(that’s 2 chars + dot + 3 chars)
🔹 Task 6: Use [ ] to list only m3.jpg and m5.jpg (two ways).
$ ls m[35].jpg
m3.jpg m5.jpg
$ ls m?.jpg # same effect here
🔹 Task 7: List all mp3 files plus the video (.mv4) in one pattern.
$ ls *.mp3 *.mv4
🔹 Task 8: List files that have “m” followed by anything, then a dot, then anything starting with ‘m’ or ‘j’ — see what happens.
$ ls *m*.[mj]*
WARNING! Observation: That command matched more than you expected? Because *m* matches any name containing ‘m’, and .[mj]* means a dot, then m or j, then anything. Pattern matching can be tricky.

3. Hidden Files (dot files)

🔹 Task 9: List all files (including hidden) in the current directory (car_files).
$ ls -a
🔹 Task 10: Try to match hidden .secret.txt using *.txt. Does it appear? Why?
$ ls *.txt
(no .secret.txt — because leading dot is not matched by *)
🔹 Task 11: Match hidden files explicitly.
$ ls .* # lists ., .., and hidden files (be careful)
$ ls .secret.txt
Rule: Filename expansion ignores leading dots unless the pattern starts with a dot.

4. Brace Expansion

Generate arbitrary strings — not limited to existing files.

🔹 Task 12: Echo combinations of colors and sizes.
$ echo {red,green,blue}-{large,small}
red-large red-small green-large green-small blue-large blue-small
🔹 Task 13: Create several new files at once with brace expansion.
$ touch report-{2024,2025}-{jan,feb,mar}.txt
$ ls report* # see what appeared
🔹 Task 14: Use brace expansion to create a backup of a single file (simulate).
$ cp vette.jpg{,.bak} # expands to “cp vette.jpg vette.jpg.bak”
$ ls vette.jpg*
{,.bak} is a clever trick: first empty string, then .bak → expands to original and backup name.

5. Parameter (Variable) Expansion

🔹 Task 15: Assign a variable and display it.
$ NAME=alice
$ echo $NAME
$ echo ${NAME}
🔹 Task 16: See why ${} matters.
$ echo $NAME_the_fast_runner # likely empty — tries variable NAME_the
$ echo ${NAME}_the_fast_runner # alice_the_fast_runner
🔹 Task 17: Use variable to navigate.
$ MYDIR=~/lab_files/car_files
$ ls $MYDIR

“ 6. Quoting – Taming Special Characters

🔹 Task 18: Print the string She said “Hello” using escape and quotes.
$ echo She said \”Hello\”
$ echo ‘She said “Hello”‘
🔹 Task 19: Demonstrate the difference between single and double quotes with a variable.
$ NAME=alice
$ echo ‘My name is $NAME’ # My name is $NAME
$ echo “My name is $NAME” # My name is alice
🔹 Task 20: Use a backslash to echo a $ sign literally.
$ echo “The price is \$5.00”
🔹 Task 21: Try to list a file with a space in its name (create one first).
$ touch “my important file.txt”
$ ls my important file.txt # fails: three arguments
$ ls “my important file.txt” # works
$ ls my\ important\ file.txt # also works (backslash escapes space)
Always quote or escape spaces, otherwise the shell splits them into separate arguments.

7. Mixed & Challenge Tasks

Combine what you’ve learned. Work in ~/lab_files/car_files.

🔹 Challenge 1: List all files that have exactly one character between ‘m’ and ‘.’ (like m3.jpg, m5.jpg) but not others.
$ ls m?.??? # works here because all extensions are 3 chars
🔹 Challenge 2: Copy all files that start with ‘vette’ to a new folder ~/lab_files/vette_backup using brace expansion to create the folder.
$ mkdir -p ~/lab_files/vette_backup
$ cp vette.* ~/lab_files/vette_backup/
🔹 Challenge 3: Use a single echo with brace expansion to print the following three words: apple-pie banana-pie cherry-pie.
$ echo {apple,banana,cherry}-pie
🔹 Challenge 4: Display the message: Your home is $HOME but with $HOME expanded (using double quotes). Then again with single quotes so $HOME is not expanded.
$ echo “Your home is $HOME”
$ echo ‘Your home is $HOME’
🔹 Challenge 5: List all files that start with a dot (hidden) in your real home directory, but not . or .. (use a pattern like .[!.]* — advanced).
$ ls -d ~/.[!.]* # lists hidden files but not . or ..

This uses a negated character class [!.] meaning “not a dot”.

8. Cleanup (Optional)

When you’re done experimenting, remove the lab folder:

$ rm -rf ~/lab_files
WARNING! Be absolutely sure you want to delete before running rm -rf.
Shell Fu grows with practice. Run these tasks, then invent your own patterns.
Remember: order matters — brace, tilde, parameter, filename, quote removal.

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This post was written by amax

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