As all components of a pipe is ran in parallel, a shell typically forks a subprocess (a subshell) to handle its contents, making it impossible to propagate variable changes to the outside shell environment. To remedy this issue, the "pipemill" can instead be fed from a here document containing a command substitution, which waits for the pipeline to finish running before milling through the contents. Alternatively, a named pipe or a process substitution can be used for parallel execution. GNU bash also has a option to disable forking for the last pipe component.