1. cp : copy files and directories
1.1 Basic use of cp : cp SRC DEST
SRC is a single file：
If DEST does not exist ：create DEST and copy the content of SRC to DEST；
If DEST exists：
If DEST is a file ：overwrite DEST , and -i option is recommanded；
If DEST is a directory ：Create a file named SRC in the directory of DEST,and fill the new file with the content of source SRC；
SRC are multiple files：
DEST must exist and is a directory
If SRC is a directory ：-r option is recommanded.
If DEST does not exist ：Create a directory named DEST and copy all of the files in SRC to DEST.
If DEST exists ：
If DEST is a file ：An error is reported.
If DEST is a directory：copy all of the files in SRC to DEST.
1.2 Commondly used options
-i：interactive
-r, -R: recursion
-a: 归档，相当于-dR --preserv=all
The original state and attributes of copied files are not always saved,for example ,if you copy a symlink file,it actually copies not the symlink file itself,but create a file with the same name as the symlink file and then fill the new file with the content of the file linked by the symlink file. -a is used to resolve this problem.
-p: --preserv=mode,ownership,timestamp
-v: --verbose
-f: --force
2. mv : move (rename) files
2.1 The same basic use as cp
2.2 Commondly used options
-i: interactive
-f: forced
3. rm : remove files or directories
3.1 Basic use of rm：rm [OPTION]... FILE...
3.2 Commondly used options
-i: interactive
-f: forced
-r: recursion
rm -rf