The "test-var-set" command:

Usage: fossil test-var-set VAR ?VALUE? ?--file FILE?

Store VALUE or the content of FILE (exactly one of which must be supplied) into variable VAR. Use a FILE of "-" to read from standard input.

WARNING: changing the value of a variable can interfere with the operation of Fossil. Be sure you know what you are doing.

Use "--blob FILE" instead of "--file FILE" to load a binary blob such as a GIF.