how to let script recognize parenthesis input on the command line?

bsmile · 04-26-2021, 03:09 PM

I am writing a simple script to let bc evaluate simple math expressions and output on the screen. I write down a file called mybc as

Code:

#!/bin/sh

express=$1

echo "scale=6; $express" | bc

and run it on the command line as

mybc sqrt(6.0)

and get the following error message

bash: syntax error near unexpected token `('

If I run the following on the command line as

mybc sqrt\(6.0\)

It can work correctly. But I hope mybc can just run as the first way. I searched a bit and landed on the following link

https://stackoverflow.com/questions/...-the-command-l

but it seems to help with parenthesis used within the script, and thus does not help.

Ser Olmy · 04-26-2021, 03:50 PM

The "problem" is not with the script but with the command interpreter.

bash interprets parentheses as instructions to run commands in a subshell. There's nothing your script can do to prevent this.

JeremyBoden · 04-26-2021, 07:58 PM

Unless you use quotes:-

Code:

#!/bin/bash
echo "scale=6; $1" | bc

Code:

./mybc "sqrt(6.0)"
2.449489

bsmile · 04-29-2021, 12:25 AM

Thanks! I think this works. I can give every expression a quotation mark, a bit more convenient than giving a back slash on each parenthesis.

Quote:

Originally Posted by JeremyBoden

Unless you use quotes:-

Code:

#!/bin/bash
echo "scale=6; $1" | bc

Code:

./mybc "sqrt(6.0)"
2.449489

boughtonp · 04-29-2021, 07:06 AM

I guess at some point you'll want to learn a bit more about quoting, specifically single quotes vs double quotes.

Here's the relevant Bash docs: https://www.gnu.org/software/bash/manual/html_node/Quoting.html