$PreInitialization

is a symbol whose value, if any, is evaluated with ReleaseHold[$PreInitialization] before any other initializations in a Wolfram Language session.

Details

Persistent values of $PreInitialization are typically set with InitializationValue[$PreInitialization]=Hold[expr].
Values defined in different persistence locations on $PersistencePath are joined with Flatten.
Since $PreInitialization is used early in the startup of a Wolfram Language session, not all capabilities will already be initialized. For example, remote access such as of the cloud from the desktop will not yet be available.

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Set a value to be evaluated early every time the kernel starts:

View the current initialization value:

Initialization values are stored in persistent objects:

List the existing initialization values for $PreInitialization in all editable persistence locations:

Remove them:

Create an initialization value for $PreInitialization in a nondefault location:

Remove by default removes only the value in the location given by $PersistenceBase:

Remove all values that might exist anywhere in all editable locations given by $PersistencePath:

The default value is still present:

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