How can I add numbers in a Bash script?

For integers:

• Use arithmetic expansion: $((EXPR))

  num=$((num1 + num2))
  num=$(($num1 + $num2))       # Also works
  num=$((num1 + 2 + 3))        # ...
  num=$[num1+num2]             # Old, deprecated arithmetic expression syntax
  

• Using the external expr utility. Note that this is only needed for really old systems.

  num=`expr $num1 + $num2`     # Whitespace for expr is important
  

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For floating point:

Bash doesn’t directly support this, but there are a couple of external tools you can use:

num=$(awk "BEGIN {print $num1+$num2; exit}")
num=$(python -c "print $num1+$num2")
num=$(perl -e "print $num1+$num2")
num=$(echo $num1 + $num2 | bc)   # Whitespace for echo is important

You can also use scientific notation (for example, 2.5e+2).

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Common pitfalls:

• When setting a variable, you cannot have whitespace on either side of =, otherwise it will force the shell to interpret the first word as the name of the application to run (for example, num= or num)

  num= 1 num =2

• bc and expr expect each number and operator as a separate argument, so whitespace is important. They cannot process arguments like 3+ +4.

  num=`expr $num1+ $num2`