R语言命令的缩写
ls [L]i[S]t objects common command in Unix-like operating systems
rm [R]e[M]ove objects common command in Unix-like operating systems
str [STR]ucture of an object
unz [UNZ]ip
getwd [GET] [W]orking [D]irectory
dir [DIR]ectory
sprintf [S]tring [PRINT] [F]ormatted
c [C]ombine values
regexpr [REG]ular [EXPR]ession