Score:2

Ubuntu

Evaluate string ```$((expression))```-like and save in a variable

Bruno Henrique Peixoto

6/8/23, 3:29 PM

This will sound pretty awkward for some of you. Suppose I have a string which represents some $((expression)), arithmetic expression mathematical like '$((1+1))'. I wish to perform it and deliver its result to a variable.

Example:

$ expression='$((5+5))'
$ echo $expression 
'$((5+5))'

$ # Expected result: 10

How can I evaluate that string and store its result?

0 + 0

command-line

bash

Bruno Henrique Peixoto

6/8/23, 4:57 PM

Oh, I may cook a great pasta with all that macarronic mistakes, thanks!

Score:1

Ubuntu

Arctic Kona

6/8/23, 6:59 PM

If you're evaluating a string try eval.

str="$[ 5 + 5 ]"
eval "numb=$str"    # Turns into `numb=$[ 5 + 5 ]`
echo $numb    # Outputs 10

eval can execute strings as if they were shell code. Be careful! If $str can contain special characters it may break your script or even create a security hole.

0 + 0

Score:0

Ubuntu

Liso

6/8/23, 3:31 PM

Just store it in variable like you would normally do.

$ result=$((5+5))
$ echo $result 
10

If this is not what you meant, edit your post and post additional information.

0 + 0

Bruno Henrique Peixoto

6/8/23, 4:15 PM

I updated the question for better undestannding of my case. But I appreciate the answers sofar! :-)

Dan

6/8/23, 4:25 PM

@BrunoHenriquePeixoto In your edit, you used almost the same code. This answer is exactly what you want in this case. The only error you have is that use the quotes on the `$((5+5))` part, you need to remove that.

Bruno Henrique Peixoto

6/8/23, 4:29 PM

Well, the operation I need to evaluate has quotes. Then, I am afraid I cannot evaluate them.

Score:0

Ubuntu

terdon

6/8/23, 3:34 PM

If it is actually mathematical, specifically if it involves simple integer arithmetic, you can use exactly the format you have:

$ var=$((1+1))
$ echo "$var"
2

The same goes if it is a command. For example, slightly more complicated math can be done by piping to bc:

## simple integer math works
$ echo $((3 * 2))
6
## fractions fail
$ echo $((1.3 * 2))
bash: 1.3 * 2: syntax error: invalid arithmetic operator (error token is ".3 * 2")
## bc works
$ echo "1.3 * 2" | bc
2.6

So, to store that in a variable, you just use standard command substitution:

$ var=$(echo "1.3 * 2" | bc)
$ echo "$var"
2.6

Finally, you aso have the eval builtin (see help eval in bash):

$ help eval
eval: eval [arg ...]
    Execute arguments as a shell command.
    
    Combine ARGs into a single string, use the result as input to the shell,
    and execute the resulting commands.
    
    Exit Status:
    Returns exit status of command or success if command is null.

0 + 0

Elon Musk

I sit in a Tesla and translated this thread with Ai:

EN: Evaluate string ```$((expression))```-like and save in a variable

TH: ประเมินสตริง ```$((expression))```-like และบันทึกในตัวแปร

RO: Evaluați șirul ```$((expresie))```-like și salvați într-o variabilă

RU: Вычислить строку ```$((выражение))```-подобно и сохранить в переменной

VI: Đánh giá chuỗi ```$((expression))```-like và lưu vào một biến

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