daily & sophisticated shell built-in commands

1. compgen -b:display all built-in commands (completion of command)

2. type command: determine whether the command is a built-in command

3. pushd/popd: add/remove directories from stack

4. help command: display the help message of the command

R_LANG on MS Windows

1. Suits: R, RTools & Rstudio

2. environment variables:
Modify the variable  BINPREF in the file “/R-3.6.0/etc/x64/Makeconf” to “/path/to/Rtools/mingw_64/bin/” to solve a compiling error.

3. configure R environment:~/.Rprofile, R/R-x.x.x/etc/Rprofile.site