Day 2 — Basic Script Writing#

Goal: Write your first shell scripts — from simple one-liners to multi-step automation scripts with variables, conditionals, and loops.

What Is a Shell Script?#

A shell script is a text file containing a series of commands that the shell (bash) executes in order. Instead of typing 10 commands one by one, you put them in a file and run it once.

# Instead of typing these every day:
cd ~/projects/tds
git pull
python3 main.py
echo "Done!"

# Put them in a script and run: ./daily.sh

Your First Script#

Step 1: Create the file#

nano ~/hello.sh

Step 2: Write the script#

#!/bin/bash
# My first shell script

echo "Hello, World!"
echo "Today is $(date)"
echo "You are: $(whoami)"
echo "You are in: $(pwd)"

Step 3: Make it executable#

chmod +x ~/hello.sh

Step 4: Run it#

~/hello.sh
# Hello, World!
# Today is Mon Jan 15 10:30:00 IST 2026
# You are: alice
# You are in: /home/alice

The Shebang Line — `#!/bin/bash`#

The first line of every script should be the shebang:

#!/bin/bash

This tells the system: “Use bash to interpret this file.” Without it, the system might use a different shell.

The name “shebang” comes from # (hash) + ! (bang).

Variables#

#!/bin/bash

# Defining variables (NO SPACES around =)
NAME="Alice"
AGE=25
GREETING="Hello, $NAME"

# Using variables
echo $NAME              # Alice
echo ${NAME}            # Alice (curly braces — safer in strings)
echo "$GREETING"        # Hello, Alice
echo "${NAME}_data"     # Alice_data (braces prevent confusion with $NAME_data)

Rules for variables#

✅ Correct	❌ Wrong	Why
`NAME="alice"`	`NAME = "alice"`	No spaces around `=`
`echo $NAME`	`echo NAME`	Missing `$` to reference
`echo "${NAME}_log"`	`echo "$NAME_log"`	Without braces, bash looks for `$NAME_log`

Command substitution — putting command output in a variable#

# Use $( ) to capture command output:
TODAY=$(date +%Y-%m-%d)
echo "Date: $TODAY"        # Date: 2026-01-15

PYTHON_VER=$(python3 --version)
echo "Python: $PYTHON_VER" # Python: Python 3.12.3

FILE_COUNT=$(ls *.csv 2>/dev/null | wc -l)
echo "CSV files: $FILE_COUNT"

Arithmetic#

A=10
B=3
SUM=$((A + B))          # 13
DIFF=$((A - B))         # 7
PRODUCT=$((A * B))      # 30
QUOTIENT=$((A / B))     # 3 (integer division)
REMAINDER=$((A % B))    # 1

echo "Sum: $SUM, Product: $PRODUCT"

🧠 Knowledge Check#

Q1: How do you correctly assign the value “Alice” to a variable named NAME in a bash script?

A) NAME = "Alice"
B) NAME="Alice"
C) $NAME="Alice"
D) set NAME to "Alice"

Answer

B — In bash, variable assignment must not have spaces around the equals sign =.

Q2: How do you capture the output of a command like date and store it in a variable?

A) TODAY=date
B) TODAY=${date}
C) TODAY=$(date)
D) TODAY="date"

Answer

C — The $(command) syntax executes the command and substitutes its output, which can then be assigned to a variable.

User Input — `read`#

#!/bin/bash
echo "What is your name?"
read USER_NAME
echo "Hello, $USER_NAME!"

# With a prompt on the same line:
read -p "Enter project name: " PROJECT
echo "Creating project: $PROJECT"

Conditionals — `if` / `elif` / `else`#

#!/bin/bash

FILE="config.json"

if [ -f "$FILE" ]; then
    echo "✓ $FILE found"
else
    echo "✗ $FILE not found"
fi

Common test conditions#

Test	Meaning
`[ -f file ]`	File exists and is a regular file
`[ -d dir ]`	Directory exists
`[ -e path ]`	Path exists (file or directory)
`[ -z "$VAR" ]`	Variable is empty
`[ -n "$VAR" ]`	Variable is non-empty
`[ "$A" = "$B" ]`	Strings are equal
`[ "$A" != "$B" ]`	Strings are not equal
`[ $NUM -gt 10 ]`	Number greater than 10
`[ $NUM -lt 5 ]`	Number less than 5
`[ $NUM -eq 0 ]`	Number equals 0

Full example#

#!/bin/bash

read -p "Enter a number: " NUM

if [ $NUM -gt 100 ]; then
    echo "That's a big number!"
elif [ $NUM -gt 0 ]; then
    echo "That's a positive number."
elif [ $NUM -eq 0 ]; then
    echo "That's zero."
else
    echo "That's a negative number."
fi

Loops#

`for` loop — iterate over a list#

#!/bin/bash

# Loop over words
for fruit in apple banana cherry; do
    echo "I like $fruit"
done

# Loop over files
for file in *.csv; do
    echo "Processing $file..."
    wc -l "$file"
done

# Loop with a number range
for i in {1..5}; do
    echo "Step $i"
done

# Loop over command output
for user in $(cat /etc/passwd | cut -d: -f1 | head -5); do
    echo "User: $user"
done

`while` loop — repeat while condition is true#

#!/bin/bash

count=0
while [ $count -lt 5 ]; do
    echo "Count: $count"
    count=$((count + 1))
done

# Read lines from a file
while read -r line; do
    echo "Line: $line"
done < input.txt

🧠 Knowledge Check#

Q1: Which loop is best suited for iterating over a specific list of items or files?

A) while loop
B) for loop
C) if statement
D) repeat loop

Answer

B — A for loop is ideal for iterating through a sequence, list, or set of files (e.g., for file in *.txt; do ...).

Pipes and Redirection in Scripts#

#!/bin/bash

# Redirect output to a file
echo "Report generated on $(date)" > report.txt

# Append to a file
echo "=== File Count ===" >> report.txt
ls -la | wc -l >> report.txt

# Redirect stderr to a separate file
python3 risky_script.py 2> errors.txt

# Use tee to both display AND save
echo "Starting process..." | tee logfile.txt

Script Arguments — `$1`, `$2`, etc.#

Scripts can accept command-line arguments:

#!/bin/bash
# Usage: ./greet.sh <name> <greeting>

NAME=$1
GREETING=${2:-"Hello"}   # default value if $2 is not provided

echo "$GREETING, $NAME!"

./greet.sh Alice
# Hello, Alice!

./greet.sh Bob "Good morning"
# Good morning, Bob!

Special variables#

Variable	Meaning
`$0`	Script name
`$1`, `$2`, …	Arguments
`$#`	Number of arguments
`$@`	All arguments (as separate words)
`$?`	Exit code of last command (0 = success)

A Real Example — Workspace Setup Script#

#!/bin/bash
# setup-workspace.sh — Create a project workspace
# Usage: ./setup-workspace.sh <project-name>

PROJECT_NAME="${1:?Usage: $0 <project-name>}"
BASE_DIR="$HOME/tds-bootcamp"
PROJECT_DIR="$BASE_DIR/$PROJECT_NAME"

# Check if it already exists
if [ -d "$PROJECT_DIR" ]; then
    echo "Error: $PROJECT_DIR already exists!"
    exit 1
fi

echo "Creating project: $PROJECT_NAME"

# Create structure
mkdir -p "$PROJECT_DIR"/{data,outputs,scripts,docs}

# Create README
cat > "$PROJECT_DIR/README.md" << EOF
# $PROJECT_NAME

Created: $(date +%Y-%m-%d)
Author: $(whoami)

## Directory Structure
- \`data/\`     — input data files
- \`outputs/\`  — generated output files
- \`scripts/\`  — shell scripts and automation
- \`docs/\`     — documentation
EOF

# Create a placeholder script
cat > "$PROJECT_DIR/scripts/run.sh" << 'EOF'
#!/bin/bash
echo "Running analysis..."
echo "Data directory: $(ls ../data/ 2>/dev/null | wc -l) files"
echo "Done!"
EOF

chmod +x "$PROJECT_DIR/scripts/run.sh"

echo ""
echo "✓ Project created at: $PROJECT_DIR"
tree "$PROJECT_DIR"

Q&A#

Q: Why does NAME = "alice" (with spaces) fail?

A: Bash interprets spaces as argument separators. With spaces, bash thinks NAME is a command, = is its first argument, and "alice" is its second argument. Without spaces, bash knows it’s a variable assignment.

NAME = "alice"    # Error: NAME: command not found
NAME="alice"      # Correct

Q: What does #!/bin/bash actually do?

A: When you run ./script.sh, the OS reads the first line. If it starts with #! (shebang), the OS uses the specified program (/bin/bash) to interpret the rest of the file. Without it, the OS might use /bin/sh (a more limited shell) or fail.

Q: What is the difference between single quotes and double quotes?

Double quotes " " — variables and commands are expanded:

NAME="Alice"
echo "Hello $NAME"     # Hello Alice
echo "Date: $(date)"   # Date: Mon Jan 15...

Single quotes ' ' — everything is literal (no expansion):

echo 'Hello $NAME'     # Hello $NAME  (literally)
echo 'Date: $(date)'   # Date: $(date)  (literally)

Q: How do I debug a script that isn't working?

A: Use bash -x to run the script in debug mode. It shows each command before executing it:

bash -x ./myscript.sh
# + NAME=alice
# + echo 'Hello, alice'
# Hello, alice

The + lines show what bash is actually executing.

Exercises#

Exercise 1: Variable practice

Write a script info.sh that prints your username, home directory, current date, and number of files in your home directory.

Solution

#!/bin/bash
# info.sh — System information

USERNAME=$(whoami)
HOME_DIR=$HOME
TODAY=$(date +%Y-%m-%d)
FILE_COUNT=$(ls ~ | wc -l)

echo "=== System Info ==="
echo "User: $USERNAME"
echo "Home: $HOME_DIR"
echo "Date: $TODAY"
echo "Files in home: $FILE_COUNT"

chmod +x info.sh
./info.sh

Exercise 2: Conditional script

Write a script check-tools.sh that checks if git, python3, and uv are installed, printing ✓ or ✗ for each.

Solution

#!/bin/bash
# check-tools.sh — Verify required tools

for tool in git python3 uv; do
    if command -v "$tool" > /dev/null 2>&1; then
        echo "✓ $tool is installed: $(command -v $tool)"
    else
        echo "✗ $tool is NOT installed"
    fi
done

chmod +x check-tools.sh
./check-tools.sh

command -v checks if a command exists. The > /dev/null 2>&1 suppresses output so we only see our custom messages.

Exercise 3: Loop script

Write a script that creates 5 files named day-1.md through day-5.md in a directory called /tmp/bootcamp/, each containing the text “Notes for Day X”.

Solution

#!/bin/bash
# create-days.sh — Create daily note files

mkdir -p /tmp/bootcamp

for i in {1..5}; do
    echo "# Notes for Day $i" > "/tmp/bootcamp/day-$i.md"
    echo "Created day-$i.md"
done

echo ""
echo "Files created:"
ls -la /tmp/bootcamp/

chmod +x create-days.sh
./create-days.sh
cat /tmp/bootcamp/day-3.md
# # Notes for Day 3

Exercise 4: Backup script

Write a script backup.sh that:

Takes a directory name as an argument ($1)
Checks if the directory exists
If yes, copies it to <name>-backup-<date>/
If no, prints an error

Solution

#!/bin/bash
# backup.sh — Backup a directory
# Usage: ./backup.sh <directory>

DIR="${1:?Usage: $0 <directory>}"
DATE=$(date +%Y%m%d)
BACKUP="${DIR}-backup-${DATE}"

if [ ! -d "$DIR" ]; then
    echo "Error: '$DIR' is not a directory or does not exist."
    exit 1
fi

if [ -d "$BACKUP" ]; then
    echo "Error: Backup '$BACKUP' already exists."
    exit 1
fi

cp -r "$DIR" "$BACKUP"
echo "✓ Backed up '$DIR' → '$BACKUP'"
echo "Contents:"
tree "$BACKUP" -L 1

Test it:

mkdir -p /tmp/myproject/{src,data}
chmod +x backup.sh
./backup.sh /tmp/myproject
# ✓ Backed up '/tmp/myproject' → '/tmp/myproject-backup-20260115'

Exercise 5: MCQ

Q1: What is the correct shebang line for a bash script?

A) # /bin/bash
B) #!/bin/bash
C) //bin/bash
D) @!/bin/bash

Answer

B — The shebang is #! followed by the path to the interpreter. #!/bin/bash tells the OS to use bash.

Q2: What does ${2:-"default"} mean?

A) The variable 2 minus "default"
B) Use $2 if provided, otherwise use "default"
C) Set variable 2 to "default"
D) Delete variable 2

Answer

B — This is bash’s default value syntax. If $2 is not provided (or empty), it uses "default" instead.

Q3: What does $? contain after running a command?

A) The command’s output
B) The command’s name
C) The exit code (0 = success, non-zero = failure)
D) The command’s process ID

Answer

C — $? holds the exit code of the last command. Convention: 0 = success, anything else = failure. You can use it in conditionals:

grep "error" logfile.txt
if [ $? -eq 0 ]; then
    echo "Errors found!"
fi

Day 2 — Basic Script Writing#

What Is a Shell Script?#

Your First Script#

Step 1: Create the file#

Step 2: Write the script#

Step 3: Make it executable#

Step 4: Run it#

The Shebang Line — #!/bin/bash#

Variables#

Rules for variables#

Command substitution — putting command output in a variable#

Arithmetic#

🧠 Knowledge Check#

User Input — read#

Conditionals — if / elif / else#

Common test conditions#

Full example#

Loops#

for loop — iterate over a list#

while loop — repeat while condition is true#

🧠 Knowledge Check#

Pipes and Redirection in Scripts#

Script Arguments — $1, $2, etc.#

Special variables#

A Real Example — Workspace Setup Script#

Q&A#

Exercises#

The Shebang Line — `#!/bin/bash`#

User Input — `read`#

Conditionals — `if` / `elif` / `else`#

`for` loop — iterate over a list#

`while` loop — repeat while condition is true#

Script Arguments — `$1`, `$2`, etc.#