  In the seminal work by G. Eilenberger [Z. Phys. 214, 195 (1968)], the
quasiclassical expression for the free energy of spin-singlet superconductor
has been suggested. Starting from the Luttinger-Ward formulation we derive the
Eilenberger free energy and find its generalization for superconductor or
superfluid with spin-triplet correlations. Besides ordinary superconductors
with various scattering mechanisms, the obtained free energy functional can be
used for systems with spin-triplet pairing such as superfluid $^3$He and
superconducting systems with spatially-inhomogeneous exchange field or
spin-orbit coupling. Using this general result we derive the simplified
expression for the free energy in the diffusive limit in terms of the
momentum-averaged propagators.
