does R read commands from scripts instantanously or seuqently during processing

Dear R community,


when I source a script into R via:


R --slave < scriptname.R


is the whole script file read at once during startup or is each 
indivdual line of code read seqnetially during the execution (i.e. 
directly before r processes the respective command)? In other words, can 
I savely edit the  scriptname.R file even when an active R process still 
runs the command above?


Thanks for your help
Jannis