03_directory-related commands

1. Category of directory
    The shell working directory.
    The current directory which refers to the shell working directory.
    HOME directory:
        root : /root
        Average user : /home/USERNAME
2. The cd command
    Change the shell working directory / Change the current directory to DIR.
    e.g :
        cd or cd~ : Switch to the current user's home directory.
        cd ~USERNAME :     Switch to the home directory of the specified user.
        cd - : Switch back and forth between the previous directory and the current directory.
        . : The current directory.
        .. : The parent directory of the current directory.
    Environment variables related to the cd command:
        PWD : Used to save the current directory.
            The pwd command is used to read the value of the PWD environment variable. 
        OLDPWD : Used to save the previous directory.
            That's why you can switch back and forth between the previous directory and the current directory.
            cd- is equivalent to cd $OLDPWD.

3. The ls command : List contents of the specified directory.
    Execution result of ls command with the option of -l : 
        -rw-r--r-- 1 root root 44800 Aug 14 14:32 install.log
            The leftmost one : refers to the file type.
            The next nine : refers to the file access authority.
            The next digital 1 :  The number of a file is hard linked.
            the first root : The owner of the file.
            the second root : The group of the file.
            44800 : The size of the file.
            Aug 14 14:32 : The last time the file was modified.
            install.log : the file name.

 

 

1. mkdir
    1.1 mkdir用来创建目录的：在这里目录和文件是一回是
    [root@bogon tmp]# mkdir test/abc  
        如果test存在：创建成功
        如果abc存在，则不会创建新的替换，且会报错。
        如果test不存在：创建失败
    -p, --parents : no error if existing, make parent directories as needed
        [root@bogon tmp]# mkdir test/abc
        如果test不存在,先创建test,后创建abc
            如果abc存在，则不会创建新的替换，且不会报错。
    -v, --verbose : print a message for each created directory
2. rmdir : remove empty directories
3. tree : list contents of directories in a tree-like format.