How do I calculate the log of a number using bc?

Question

This is the first time I am using bc. I want to calculate the log (base 10) of a number. How do I this?

Answer 1

Invoke bc with the -l option (to enable the math library) like so

$ echo 'l(100)/l(10)' | bc -l
2.00000000000000000000

Use the l function which is the natural log. Take the log of the number you are interested in then divide by the natural log of 10.

Answer 2

the logarithm of x in respect to base b can be computed given any logarithm function to an arbitrary base k -- that's actually pretty cool!

log_b(x) = log_k(x) / log_k(b)

e.g.

log_b(x) = ln(x) / ln(b)

if b=10:

log_10(x) = ln(x) / ln(10)

and -l in bc enables the math library

so that's why this works:

# bc -l
l(100) / l(10)
2.00000000000000000000

Answer 3

If you start bc with the -l switch, then there's a function l() that calculates the natural log of its argument.