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bashpipebc

Pipe to bc gives inconsistent syntax errors

I have to pipe integers comparisons (and more complex arithmetic and boolean operations) to bc, and I have to keep track of syntax errors when some data is missing.

It seems I found some inconsistent behaviour with bc.

Of course :

echo "1==7" | bc gives boolean 0

echo "==7" | bc gives (standard_in) 1: syntax error

But :

echo -e "==1\n==7" | bc
(standard_in) 1: syntax error`
7

instead of 7, one would expect (standard_in) 2: syntax error on the second line.

For the sake of these explanations, I simplified the input of bc but I have to resolve this strange behaviour with much more complex arithmetic and boolean operations.

WORKAROUND As @KamilCok noticed below, a workaround consists in inserting newlines

echo -e "==1\n==7" | awk 'ORS="\n\n"' | bc

Solution

  • I believe this is a bug in yacc parser within GNU bc. I see that inserting a valid statement between comparisons seems to reset the state.

    echo -e "==1\n1\n==7" | bc
(standard_in) 1: syntax error
1
(standard_in) 3: syntax error

    So a workaround would be to input an empty valid statement (like just 1) between comparisons. A real solution would be to write a patch to GNU bc and notify the developers.