Arithmetic in a bash script

Question

In a bash script, I would like to determine the maximum or minimum value depending on my input: for example if my input is max 20 40, then the output should be 40. If the input is min 20 40 then the output should be 20, and so on for any two numbers.

The same needs to be done for addition, subtraction, multiplication and division. If the input is 6 + 6 then the output is 12. If the input is 6 * 6 then the output is 36.

The problem I am having is that I am able to tell bash to echo two numbers and then display which is the min or max, or even do addition subtraction etc. For example if the input is 5 10 the output is 15 if I am running the addition file. However I do not understand how to do it all in one line, for example if the input is 5 + 10 the output is 15.

we can forget about the max/min operation for now and focus on the arithmetic part of the solution. to explain the question a bit better, the code should run as follows.

bash filename.sh
6 * 7
42

the first line is me running the file, the second line is where I type my input, and the final line is the output (the answer) followed by a new line. this is extremely easy if I need to multiply ONLY 6 * 7, however this is not the case. After I run the file bash filename.sh and click enter, I should be directed to the next line where I am able to type ANY simple arithmetic operation limited to addition subtraction multiplication and division.

more examples of how the code should run

bash filename.sh
10+ 5
15
bash filename.sh
14 / 7
2
bash filename.sh 
17 - 3
14

The script should be able to do it for any values inputted thank you

Answer 1

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

Answer 2

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.

Answer 3

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.