Headers File
The headers file can be used to send fixed-value headers. The most common use case is authentication headers.
Headers can be path-specific or applicable to all paths using the all element. Specific paths will take precedence over the all element.
Sample headers file:
all:
    Accept: application/json
/path/0.1/auth:
    jwt: XXXXXXXXXXXXX
/path/0.2/cancel:
    jwt: YYYYYYYYYYYYY
This will add the Accept header to all calls and the jwt header to the specified paths. 
tip
Alternatively, you can supply headers using the -H name=value argument. Those headers will apply to all paths.
You can use environment (system) variables in a headers file using: $$VARIABLE_NAME.
caution
Notice double $$ when supplying environment variables.