Bash Variables, Quoting, and Expansion

name='Jonathan'

# Assign a shell variable.

echo "$name"

# Expand a variable with double quotes.

export APP_ENV=production

# Export a variable to child processes.

unset name

# Remove a variable definition.

echo "${PORT:-3000}"

# Use a fallback if variable is unset or empty.

echo "${PORT:=3000}"

# Assign default if variable is unset or empty.

echo "${DATABASE_URL:?DATABASE_URL is required}"

# Abort expansion with error if variable is missing.

echo "${DEBUG:+enabled}"

# Use alternate text when variable is set and non-empty.

echo 'Path is $HOME'

# Single quotes prevent all expansion.

echo "Path is $HOME"

# Double quotes allow variable and command expansion.

echo "A quote: \""

# Backslash escapes the next character in many contexts.

printf '%s
' $'line1
line2'

# Use ANSI-C quoting for escapes like newline and tab.

echo '\$HOME'

# Print shell metacharacters literally.

today=$(date +%F)

# Capture command output into a variable.

echo $((2 + 3 * 4))

# Evaluate an arithmetic expression.

echo "${#name}"

# Get length of a variable's value.

echo "${name:0:4}"

# Extract a substring by offset and length.

echo "${file/.txt/.bak}"

# Replace first matching substring.

echo "${path//\//:}"

# Replace all matching substrings.

echo "${path#*/}"

# Strip shortest matching prefix pattern.

echo "${file%%.*}"

# Strip the longest matching suffix pattern.

printf '%s
' *.txt

# Expand a glob to matching files.

printf '%s
' file-{a,b,c}.txt

# Generate multiple words from a brace expression.

printf '%s
' {1..5}

# Generate a numeric sequence.

shopt -s nullglob

# Expand unmatched globs to nothing.

shopt -s dotglob

# Make globs match dotfiles too.