Linux system components and command help get 4

    Linux file system:
        1, the name of the file name strictly case sensitive characters
        2, files can be used in addition to / any character other than
        3, the file name length can not exceed 255 characters
        4, to the beginning of the file as a hidden file.
            : Current directory
            ..: a current directory on the

        common commands:
            pwd: Printing working directory
                display the current directory
            cd: change directory change the working directory
                cd [/ the PATH / the tO / somedir]
                    cd: switching home directory
                        Note: bash, the home directory represents ~
                    cd ~: switch back to their home directory
                    cd ~ USERNAME: switch to specify the user's home directory
                    cd -: one located on the switch back and forth between the directory and the current directory
                    cd ../: switch to previous level directory
                    $ PWD: current work Catalog
                    $ OLDPWD: on a working directory

            ls: list list the contents of the specified directory
                LS [the OPTION] ... [the FILE] ...
                    -a: show all files, including hidden files
                    -A: and show all files except the ...
                    -l: --long, long list format, i.e., display detailed attribute information of the file
                        drwxr XR-2 X-12 is the root Jan 12:23 Anaconda 176 the root.
                        D: indicates the file type, there -, d, b, c, l, s, P
                        rwxr-XR-X
                            rwx: file owner rights owners
                            RX: file permissions genus groups
                            for other users (non-owner, is a group): RX
                        :. hidden permissions
                        2: the number of digital representation file is hard-linked
                        root : main document
                        root: file is a group
                        176: file size in bytes
                        Jan 12 12:23: file was last modified time
                        anaconda: file name
                    -h: - human-readable, file size unit conversion after conversion results may be inexact value
                    -d: Properties View directory itself, rather than file attributes inside
                    -r: reverse, reverse display
                    -R: recursive, recursive display

            cat: concatenate
                file text viewer
                    CAT [the OPTION] ... [the fILE] ...
                        -n, --number, Number All Output Lines, to the text line number displayed
                        -E, --show-ends, display linefeed $

            tac: CONCATENATE and Print files in reverse
                reverse view text files
                    tac [OPTION] ... [fILE] ...

            file: the Determine file of the type view the contents of the file type
                File [FILE] ...

            echo: A Line Run the display of text, echo
                echo [SHORT-OPTION] ... [STRING] ...
                    -n: not wrap
                    -e: let escape into force
                        \ n: line feed
                        \ T: tab

                    STRING quotes may be used single or double quotes may be used
                        in single quotes: strong references, variable reference replacement is not carried
                            ~] # echo '$ SHELL'
                        double quotes: weak reference variable reference will be replaced
                            ~] # echo "$ SHELL"
                Note: regular variable reference symbol
                    $ {name}

            shutdown or restart command: the shutdown
                the shutdown [the OPTIONS ...] [the TIME] [WALL ...]
                    the OPTIONS:
                        -H: HALT
                        -R & lt: reboot
                        -C: Cancel

                    the TIME:
                        now
                        HH: mm
                        + m

                    WALL: A Send Message to Everybody apos Terminal
            
            Date Related command:
                the Linux: read the date and time information from the hardware system startup, after completion of reading , it is no longer associated with the hardware
                    system clock
                    hardware clock
                date
                    display system date time: date [the OPTION] ... [+ the FORMAT]
                        ~] + #% F. date display date
                            2019-01-15
                        ~] # date + % T displays the current time
                            11:07:27
                        Predetermined ~] # date + "% F % T" format check the time, + can appear only once, if the intermediate spaces, can be used "" enclosed, so that the space to be construed as meaning other
                            2019-01-1511: 08:05
                        ~] # dATE S +% from at 0:00 on 1970 assigned time now
                            1547522443
                    set the system date and time: dATE [MMDDhhmm [[the CC] YY]] [SS.]
                
                Clock, hwclock: display hardware settings or clock
                        -s, --hctosys, the clock of the hardware, the system is adjusted with the same hardware clock
                        -w, --systohc, subject to the system clock, the hardware is adjusted to the same system clock
                
                cal: calendar
                    cal [[month ] year]