Variable Types and Quoting

While bash variables are technically untyped (everything is a string), understanding how to properly quote and type variables is essential for writing robust scripts.

The Truth About Bash Variables

In bash, all variables are stored as strings:

NUMBER=42       # This is actually the string "42"
TEXT="hello"    # This is the string "hello"

# Bash interprets strings as numbers in arithmetic contexts
RESULT=$((NUMBER + 8))   # Works: 50

Declaring Variable Types

Use declare to give variables specific attributes:

# Integer variable (only stores integers)
declare -i count=0
count=5+3           # Arithmetic is automatic: 8
count="hello"       # Becomes 0 (invalid integer)

# Read-only variable
declare -r CONSTANT="unchangeable"

# Array variable
declare -a fruits=("apple" "banana")

# Associative array (dictionary)
declare -A colors
colors[sky]="blue"

Why Quoting Matters

Quoting controls how bash interprets special characters and whitespace:

FILE="my file.txt"

# Without quotes - WRONG (tries to cat "my" and "file.txt")
cat $FILE

# With quotes - CORRECT
cat "$FILE"

Three Types of Quoting

1. Double Quotes " "

Variables and commands are expanded, but whitespace is preserved:

NAME="Alice"
echo "Hello, $NAME"     # Hello, Alice
echo "Today: $(date)"   # Today: Mon Jan 10...
echo "Path: $HOME"      # Path: /home/user

2. Single Quotes ' '

Nothing is expanded - everything is literal:

NAME="Alice"
echo 'Hello, $NAME'     # Hello, $NAME (literal)
echo 'Today: $(date)'   # Today: $(date) (literal)

3. No Quotes

Variables expand, but whitespace causes word splitting:

FILES="file1.txt file2.txt"
# Without quotes - loops twice (word splitting)
for f in $FILES; do echo $f; done

# With quotes - loops once
for f in "$FILES"; do echo $f; done

Exercise: Practice Quoting

Understand the difference between quote types:

Escaping Characters

Use backslash \ to escape special characters:

# Escape dollar sign
echo "Price: \$100"     # Price: $100

# Escape double quote inside double quotes
echo "She said \"Hi\""  # She said "Hi"

# Escape newline (continues on next line)
echo "This is a \
long line"

# Common escapes
echo "Tab:\tNewline:\nBackslash:\\"

The $'...' Syntax

For special escape sequences:

# ANSI-C quoting
echo $'Line1\nLine2'    # Actual newline
echo $'Tab\there'       # Actual tab
echo $'Single\'quote'   # Single quote inside

Word Splitting

Understanding word splitting is crucial:

# Word splitting occurs on unquoted variables
TEXT="one two three"
for word in $TEXT; do     # Splits into 3 words
    echo "Word: $word"
done
# Output:
# Word: one
# Word: two
# Word: three

# Quotes prevent word splitting
for word in "$TEXT"; do   # Treated as 1 word
    echo "Word: $word"
done
# Output:
# Word: one two three

Glob Expansion

Unquoted variables can trigger glob (wildcard) expansion:

PATTERN="*.txt"

# Without quotes - expands to matching files
echo $PATTERN     # file1.txt file2.txt ...

# With quotes - literal asterisk
echo "$PATTERN"   # *.txt

Best Practices for Quoting

Always quote variables unless you specifically want word splitting or glob expansion:

# GOOD - always quote
filename="my file.txt"
cp "$filename" "$backup_dir/"

# BAD - will break on spaces
cp $filename $backup_dir/

# Exception: arithmetic context doesn't need quotes
count=$((count + 1))

# Exception: [[ ]] doesn't need quotes (but it doesn't hurt)
if [[ $name == "admin" ]]; then
    echo "Welcome admin"
fi

Exercise: Handle Spaces in Filenames

Work with variables containing spaces:

Empty Variables

Handle empty or unset variables carefully:

# This can be dangerous if VAR is empty
rm -rf $VAR/       # If VAR is empty, this becomes: rm -rf /

# Safe: Quote and use default
rm -rf "${VAR:?'VAR must be set'}/"

# Safe: Always quote
if [ -n "$VAR" ]; then
    rm -rf "$VAR/"
fi

Here Documents and Here Strings

For multi-line strings:

# Here document - variables are expanded
cat << EOF
Hello $NAME,
Welcome to the system.
Today is $(date).
EOF

# Here document - no expansion (quoted delimiter)
cat << 'EOF'
Hello $NAME,
The variable is not expanded.
EOF

# Here string
grep "pattern" <<< "$variable"

Key Takeaways

• Bash variables are untyped (strings that can be used as numbers)
• Use declare -i for integer-only variables
• Double quotes "$VAR" expand variables but preserve whitespace
• Single quotes '$VAR' treat everything literally
• Always quote variables to prevent word splitting and glob expansion
• Use \ to escape special characters
• Quoting is one of the most important aspects of bash scripting