Why does this compound command report errors when copying directories?

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http://unix.stackexchange.com – if one executes the following two commands in one line, as follows, rm -rf dir ; cp -r dir2 dir it may complain that cp can not create directory dir/subdir: File exists but if these two commands are executed in two lines, no errors will be thrown. I am just wandering what is the difference? and more importantly, how to execute two commands in one line, with the effect exactly the same as by two lines... EDIT: I change it to cp -r dir2 dir. Besides, what is in dir or dir2 is huge, typically 4gb. (HowTos)