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A general description is here.
‘R CMD Rprof|build|check’ are now based on R rather than Perl scripts. The only remaining Perl scripts are the deprecated ‘R CMD Sd2Rd’ and ‘install-info.pl’ (used only if ‘install-info’ is not found) as well as some maintainer-mode-only scripts.
*NB:* because these have been completely rewritten, users should not expect undocumented details of previous implementations to have been duplicated.
‘R CMD’ no longer manipulates the environment variables ‘PERL5LIB’ and ‘PERLLIB’.
‘R CMD build’ no longer creates an ‘INDEX’ file (‘R CMD INSTALL’ does so), and ‘--force’ removes (rather than overwrites) an existing ‘INDEX’ file.
It supports file ‘~/.R/build.Renviron’ analogously to ‘check’.
It now runs build-time ‘\Sexpr’ expressions in help files.
The 256-byte limit on names is now documented.
Subassignment by ‘[’, ‘[[’ or ‘$’ on an expression object with value ‘NULL’ coerced the object to a list.