Question posted at VFX Overflow Facebook group by Matthew Shaw:
Anyone out there know if there is a Nuke equivalent to Shake's 1-sys.runLevel command that enables a node (or diffferent branch) to switch on upon render? For example can be used to switch on motionblur, or a process hungry branch when rendering (through a select node).
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Hugh Macdonald
[ Editor ]
You need to use the '$gui' variable.
Very useful to add it as an expression to the 'disable' knob of a node ("Node" tab, R-Click on 'disable' and select "Add Expression". Just put in '$gui', and the node will get a single line through it rather than an X. This means that the disable is an expression rather than fixed. With '$gui', it'll be enabled when rendering, but disabled in the GUI.)
Elsewhere, just use expressions like:
$gui?<valueInGui>:<valueWhenRendering>
Why is this $gui not gui or similar? Was looking for this today, and ended up using [python nuke.env\['gui'\]] after trying various tcl expressions like root.gui etc..