Gotcha: Bash word splitting and globbing in variables

Unquoted variables in bash cause unexpected word splitting and filename globbing, breaking scripts with spaces or special characters.

Always quote variables in bash:

# BAD: Word splitting
FILE="my document.txt"
cp $FILE /dest/          # Runs: cp my document.txt /dest/
                          # (3 arguments instead of 2!)

# GOOD: Quoted
cp "$FILE" /dest/         # Runs: cp 'my document.txt' /dest/

# BAD: Globbing
PATTERN="*.txt"
echo $PATTERN             # Expands to matching files!

# GOOD: Quoted
echo "$PATTERN"           # Prints: *.txt

# BAD: Array iteration
FILES=("file 1.txt" "file 2.txt")
for f in $FILES; do       # Only processes "file"!
for f in ${FILES[@]}; do  # Word splits on spaces!

# GOOD: Quoted array
for f in "${FILES[@]}"; do
  echo "$f"              # Correct: 'file 1.txt', 'file 2.txt'
done

# When you WANT splitting (rare):
read -ra ARGS <<< "$INPUT"  # Split into array explicitly

# Rule: Always use "$var" unless you have a specific
# reason not to and understand the implications.

Shellcheck catches these: shellcheck myscript.sh

Bash performs word splitting and pathname expansion on unquoted variables. This is the #1 source of shell script bugs.

Any bash script that handles filenames or user input