Below are the current and their defaults.

bucket:order string ["horizontal"] Bucket order: "horizontal", "vertical", "zigzag-x", "zigzag-y", "spacefill", "random", "spiral", or "circle".
bucket:orderorigin integer[2] [-1 -1] First bucket for "spiral" and "circle".
checkpoint:asfinal integer [0] Write checkpoint as final image or remove display channels.
checkpoint:command string [] Command to run at checkpoint.
checkpoint:exitat string [] Time to checkpoint and abort render.
checkpoint:interval string [] Time between checkpoints, e.g. "1h30m" for 1 hour 30 min.
checkpoint:keepfiles integer [0] Individually name and preserve checkpoint renders.
curve:maxdepth integer [0] Description
curve:minwidth float [0] Description
curve:stochasticshadows integer [1] Use stochastic transparency on transmission rays on curves.
deep:flagvolumes integer [1] Off [0], auto (off for dtex, on otherwise) [1], or on [2]
dice:offscreenmultiplier float [1] Dicing rate multiplier for objects outside the viewing frustum.
dice:triangle string ["trim"] The method used for dicing "triangles". The default method "trim" renders triangles by trimming a quad. The "blp" method renders a quad with two points being coincident.
hider:adaptall integer [0] Description
hider:aperture float[4] [0 0 1 0] Aperture [nsides rotation roundness rotation].
hider:cameraraydiffmult float [1] Description
hider:darkfalloff float [0.025] Description
hider:dof string ["default"] "default" is the same as "legacy"
hider:extrememotiondof integer [0] Description
hider:incremental integer [0] Description
hider:maxsamples integer [0] Description
hider:minextrasamples integer [-1] Description
hider:minsamples integer [-1] Description
hider:pixelfiltermode string ["weighted"] Description
hider:samplemotion integer [1] Description
hider:type string ["raytrace"] Render mode "raytrace" or "bake".
limits:brickmemory integer [2097152] Brickmap cache size in kB (2048 MB).
limits:bucketsize integer[2] [16 16] Description
limits:deepshadowerror float [0.01] Description
limits:geocachememory integer [2097152] Geometry cache size in kB (2048 MB).
limits:gridsize integer [289] Description (17 x 17 = 289)
limits:matrixcachememory integer [0] Matrix cache in kB.
limits:nurbcurvaturedicefactor integer [3] NURBs max curvature multiplier.
limits:octreememory integer [20480] None cache size in kB (20 MB).
limits:opacitycachememory integer [1048576] Opacity cache in kB (1024 MB).
limits:othreshold float[3] [0.99609375 0.99609375 0.99609375] Opacity threshold (255/256).
limits:pointmemory integer [20480] Point cache size in kB (20 MB).
limits:proceduralbakingclumpsize integer [0] Description. Only used for `prman -capture`
limits:ptexturemaxfiles integer [128] Description
limits:ptexturememory integer [32768] PTex cache size in kB (32 MB).
limits:rendermemory float [0] Memory limit as percentage of machine memory.
limits:rendertime integer [0] Maximum render time in minutes.
limits:shadesize integer [289] The maximum number of vertices, not grid (17 x 17 = 289).
limits:texturememory integer [2097152] Texture cache size in kB (2048 MB).
limits:textureperthreadmemoryratio float [0.5] Description
limits:threads integer [0] Number of render threads.
lpe:customLPE:<name> string <lpe> Name custom LPE
lpe:diffuse1 string [] D1 diffuse lobe name for light path expressions.
lpe:specular1 string [] S1 specular lobe name for light path expressions.
lpe:user1 string [] U1 user lobe name for light path expressions.
procedural:splitmode integer [0] Split on demand [0], up front [1], or hybrid [2].
Ri:ClippingPlane float[6] [0 0 0 0 0 0] Creating a clipping plane at [Nx Ny Nz Px Py Pz] in world space.
Ri:CropWindow float[4] [0 1 0 1] Crop window [left right top bottom] in NDC space.
Ri:FormatPixelAspectRatio float [1] Device pixel aspect ratio.
Ri:FormatResolution integer[2] [640 480] Device resolution.
Ri:Frame integer [0] Frame number.
Ri:FrameAspectRatio float [-1] Description
Ri:PixelVariance float [0.001] Description
Ri:ScreenWindow float[4] [0 0 0 0] Screen window [left right top bottom] in screen space.
Ri:Shutter float[2] [0 0] Scene shutter.
ribparse:varsubst string [] RIB parse only.
scenedump:filename string [] Path to dump RIB scene.
searchpath:archive string [] Description
searchpath:dirmap string [] Description
searchpath:display string [] Description
searchpath:procedural string [] Description
searchpath:rifilter string [] Description
searchpath:rixplugin string [] Description
searchpath:shader string [] Description
searchpath:texture string [] Description
shade:debug integer [0] Shading debug level.
shade:derivsfollowdicing integer [1] Description
shade:frequencyframetime float [-inf] Shading time for frequency "frame".
shade:mergedifferingobjectids integer [1] Allow ray batches to merge geometry when object IDs differ.
Shutter:offset float [0] Offset shutter open/close.
statistics:displace_ratios float[2] [0.1 1] Description
statistics:filename string [] Description
statistics:iotracefilename string [] Description
statistics:level integer [0] Description
statistics:maxdispwarnings integer [100] Description
statistics:shaderprofile string [] Description
statistics:stylesheet string [] Description
statistics:texturestatslevel integer [0] Description
statistics:trackassetobjects integer [0] Description
statistics:xmlfilename string [] Description
stitch:refwarning integer [1] Enable stitch warnings.
trace:depthmode string ["separate"] Integrators track diffuse/specular depth "separate" or "combined".
trace:maxdepth integer [25] Maximum ray depth.
user:<name> <type> <value> Arbitrary global user data.
volume:aggregateprecision float [1] Aggregate volume precision, effecting memory usage.