Why does bc work with 'here string' and not via pipe in this example?

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Why does bc work with 'here string' and not via pipe in this example?

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01-22-2014

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Why does bc work with 'here string' and not via pipe in this example?

Hi!

I actually got it running, but I still would like to understand, why and how, since I am a beginner in bash scripting. I Need floating numbers and thus use bc in my bash script. Here it is:

Code:

#!/bin/bash
num1="10^-15" | bc -l #power function piped to bc - DOES NOT WORK
echo $num1
fact="10^-15" | bc -l #power function piped to bc - DOES ONLY WORK WHEN TYPED IN TERMINAL
echo $fact
num2=$(bc -l <<< "10^-15") #'here string' to bc - WORKS
echo $num2

The result of Script:

Code:

# ./demo.sh 
 
 
.00000000000000100000

-> Only the last variable has the wanted value!

When I put the single commands in the terminal:

Code:

# num1="10^-15" | bc -l #power function piped to bc - DOES NOT WORK
# echo $num1
 
# fact="10^-15" | bc -l #power function piped to bc - DOES ONLY WORK WHEN TYPED IN TERMINAL
# echo $fact
10^-15
# num2=$(bc -l <<< "10^-15") #'here string' to bc - WORKS
# echo $num2
.00000000000000100000

-> Here the both last variables have the wanted value - in different formats, but I can do further calculations with them.

Why does the pipe fail? Why does it work in the terminal with the "fact" variable?

I am using:

Code:

# bash -help
GNU bash, version 4.1.2(1)-release-(x86_64-redhat-linux-gnu)

Thanks!
Martin

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What, exactly, does num1="10^-15" print to the screen when you type it in and hit enter?

Absolutely nothing at all.

What, exactly, does it feed into bc when you type it in and feed it into a pipe?

Absolutely nothing at all.

Pipes pick up the text a program prints. If you want to pipe text into bc, that text has to print to the screen.

Code:

echo "text" | bc

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The pipe doesn't fail - it just doesn't have anything to transmit. Try echoing $num1 and pipe that to bc.

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You havent tried:

Code:

num1=$(echo "10^-15" | bc -l)

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The reason that num1 remains unset in your first example is because the assignment occurs in a subshell.

Regards,
Alister

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