Command substitution and arithmetic in Bash

Command Substitution: Use Output as a Value

Command substitution captures the stdout of a command and injects it as a value. Always use $() — it nests cleanly. Backticks `` ` `` are legacy and don't nest.

today=$(date +%Y-%m-%d)
logfile="app-${today}.log"
echo "Writing to $logfile"

# Nesting works with $()
files=$(ls $(pwd))

# Count lines in a file
lines=$(wc -l < /etc/hosts)
echo "Hosts file has $lines lines"

Arithmetic Expansion

Bash handles only integer math natively. Use $(( )) for arithmetic — it's faster than calling external tools.

x=10
y=3

echo $(( x + y ))   # 13
echo $(( x * y ))   # 30
echo $(( x / y ))   # 3  (integer division)
echo $(( x % y ))   # 1  (remainder)

# Increment a counter
count=0
(( count++ ))
echo $count  # 1

Float Math with bc

# Bash can't do floats natively — pipe to bc
result=$(echo "scale=2; 10 / 3" | bc)
echo $result  # 3.33

# Percentage calculation
percent=$(echo "scale=1; (42 / 100) * 100" | bc)
echo "${percent}%"

For serious numeric work in scripts, awk is faster than repeated bc calls and handles formatted output in one step.