# terrain attributes computed with SAGA-GIS
library(RSAGA)
# set up rsaga environment
work_env <- rsaga.env(path = "path/saga-6.3.0_x64/saga-6.3.0_x64", modules = "path/saga-6.3.0_x64/saga-6.3.0_x64/tools")

rsaga.get.libraries(path = work_env$modules)
rsaga.get.modules(env = work_env,'grid_filter')
rsaga.get.modules(libs = "io_gdal", env = work_env)
rsaga.get.usage(lib = "io_gdal", module = "Export Raster", env = work_env)
#rsaga.geoprocessor

rsaga.slope(in.dem = "path\dem.tif",
        out.slope = "path\slope",
        unit.slope = "degrees",
        out.aspect = "path\aspect",
        unit.aspect = "degrees",
        out.cgene = "path\general_curvature",
        out.clong = "path\longitudinal_curvature",
        out.ccros = "path\crosssectional_curvature",
        env = work_env)

Warning message:
running command '"path/saga-6.3.0_x64/saga-6.3.0_x64/saga_cmd.exe"
-f=q ta_morphometry "Slope, Aspect, Curvature" -ELEVATION
"pathdem.tif" -SLOPE "path/slope.sgrd" -ASPECT
"path/aspect.sgrd" -C_GENE "path/general_curvature.sgrd" -C_LONG
"path/longitudinal_curvature.sgrd" -C_CROS
"path/crosssectional_curvature.sgrd" -METHOD "6" -UNIT_SLOPE "1"
-UNIT_ASPECT "1"' had status 5