Command Substitution and Arithmetic

$( ) — capture command output

now="$(date +%Y-%m-%d)"
files="$(find . -name '*.py' | wc -l)"
echo "$files Python files as of $now"

$(...) runs the command, captures stdout, and substitutes the value (with trailing newlines trimmed). Always quote the result: "$(cmd)" — without quotes the value is word-split.

Backticks (legacy)

You'll see `cmd` in old scripts. Same effect, but harder to nest and easier to confuse with single quotes. Use $( ) in new code; backticks are deprecated in style guides.

(( )) — arithmetic

i=0
((i++))
((sum = a + b * 2))
((i < 10)) && echo 'still small'

Inside (( )) you don't need $ on variable names. Operators: + - * / % ** and C-style ++ -- && ||. The exit code is 0 if the result is non-zero, 1 if zero — opposite of typical C, but consistent with shell truthiness.

$(( )) — arithmetic substitution

echo "$((1 + 2))"           # 3
echo "file_$((i + 1)).txt"

Computes the value and substitutes the result. Common in filename generation and counters.

Bash 5 doesn't do floating-point

Shell arithmetic is integer only. For floats, pipe to bc -l or awk 'BEGIN{print 3.14*2}'. Most scripts don't need floats; if yours does, it's probably time to switch to Python.