integral(abs(cos(x)),(x,0,pi))

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It is my understanding that Maxima has a difficult time with absolute values in integrals. You can get the answer using a numerical approximation via:

numerical_integral(abs(cos(x)),0,pi)

Or via sympy you can use:

import sympy
print sympy.integrals.integrate(abs(cos(x)),(x,0,pi)).n()