Arithmetic in a bash script

This question can be split into two simple question:

1. How can I four arithmetic operations?
2. How can I know which number is less/grater?

Four arithmetic operations

You can use $((expr)) notation to do that.　You may want to see man bash for more info. Examples:

$ echo $((6 + 6))
12
$ echo $((6 * 6))
36

less/greater number

You can compare two numbers in [[ expr ]] or [ expr ] form. I would prefer [[ expr ]] form, so below is in that form. To compare number, [[ $num1 -lt $num2 ]] if $num1 is less than $num2. And -gt for check whether grater. Note that, -lt and -gt can't be written with < and >; they compare by lexicographical order, not by numeric order.

Example:

function max() {
  first=$1
  second=$2
  if [[ $first -lt $second ]]; then
    echo $second
  else
    echo $first
  fi
}

function min() {
  first=$1
  second=$2
  if [[ $first -gt $second ]]; then
    echo $second
  else
    echo $first
  fi
}

usage:

$ max 20 40
40
$ min 20 40
20

Do you mean print input and your calculated values in single line?

Let list.txt be your list of problems as such:

$ cat list.txt
1 + 2
3 + 4
5 + 10

Let calculating.sh be your script that takes string input, and print outs calculated values (here I only wrote for + operation)

$ cat calculating.sh
#!/bin/bash

varA=$(echo $1 | cut -d' ' -f1)
varB=$(echo $1 | cut -d' ' -f3)
operator=$(echo $1 | cut -d' ' -f2)

echo $(($varA + $varB))

To calculate every entry in list.txt with calculating.sh script you could do following:

$ cat list.txt | xargs -I {} sh -c "echo -n {} '= ' && ./calculating.sh '{}'"

1 + 2 = 2
3 + 4 = 6
5 + 10 = 10

xargs makes run "echo -n {} '= ' && ./calculating.sh '{}'" command every line of list.txt, where {} refers to the each line (think it as a looping each line). echo -n {} = will prints your problem-line with = character, and without newline character. and Then ./calculating.sh '{}' will prints the calculated value.

If you want a quick and dirty one, you can just shove the entered expression into the shell's arithmetic expansion:

$ cat arith.sh
#!/bin/bash
read -r expr
if [[ $expr = *\$* || $expr = *\`* ]]; then
    echo "invalid entry!" >&2
    exit 1
fi
echo "$(($expr))"

and then e.g.

$ ./arith.sh 
(10+4)*5
70

The test in the if clause rejects inputs that contain dollar signs or backticks, as those could be used to insert command substitutions. Invalid arithmetic syntax would give an error from the shell.

For comparisons, you can use the C-style comparison operators </<=/>/>= and the ternary operator, e.g. $((a > b ? a : b)) would give the maximum of a and b.

So to expand the script with max and min:

$ cat arith2.sh
#!/bin/bash

read -r expr

if [[ $expr = *\$* || $expr = *\`* ]]; then
    echo "invalid entry!" >&2
    exit 1
fi

if [[ $expr = "max "* ]]; then
    read -r op a b <<< "$expr"
    echo "$((a > b ? a : b))"
elif [[ $expr = "min "* ]]; then
    read -r op a b <<< "$expr"
    echo "$((a < b ? a : b))"
else 
    echo "$(($expr))"
fi

That'll break with inputs like max 1 2 3 due to how read works, and also floats are not supported in Bash so they'll also give errors. Depending on where the input data comes from, you may want to consider sanity checking it.