df

Directive: df (view disk space)

Grammar: df-h (Readable Display)

[root@JueChen linux]# df -h
Filesystem      Size  Used Avail Use% Mounted on
rootfs          100G   87G   13G  88% /
none            100G   87G   13G  88% /dev
none            100G   87G   13G  88% /run
none            100G   87G   13G  88% /run/lock
none            100G   87G   13G  88% /run/shm
none            100G   87G   13G  88% /run/user
C:              100G   87G   13G  88% /mnt/c
D:               61G   27G   34G  45% /mnt/d
E:              121G   11G  110G   9% /mnt/e
G:              121G   24G   97G  20% /mnt/g
H:              106G   55G   52G  52% /mnt/h
K:               73G   39G   34G  54% /mnt/k
L:              932G  330G  602G  36% /mnt/l

free

Function: View memory usage

Grammar: free -m(-m denotes unit mb)

[root@JueChen linux]# free -m
              total        used        free      shared  buff/cache   available
Mem:          12203        5076        6895          17         230        6988
Swap:         12691          68       12622

Swap: For temporary memory, temporary disk space can be used as memory when the real memory of the system is not enough.

head

Function: View the first n lines of a file. If n is not specified, the default is to display the first 10 lines.

Syntax: head-n file path, n for numbers

tail

Function: View the content of the last n lines of a file, if not specified, the last 10 lines will be displayed by default.

Syntax: tail -n file path

[root@JueChen linux]# tail head.txt
       The full documentation for head is maintained as a Texinfo manual.  If the info and head programs are properly
       installed at your site, the command

              info coreutils 'head invocation'

       should give you access to the complete manual.



GNU coreutils 8.22                                   October 2018

Function: You can view the dynamic changes of a file by tail instructions

Grammar: tail-f

less

Function: View files, output with less content, press auxiliary function keys (number + return, space + up and down direction keys) to see more.

Syntax: less view file path

wc

Function: Used to statistics file content information (line number, word number, byte number)

Syntax: wc-lwc file paths that need to be counted

• - l Represents the number of rows
• - w denotes the number of words. The number of words is determined by the space.
• - c denotes bytes

[root@JueChen linux]# wc  head.txt
  60  277 2196 head.txt
[root@JueChen linux]# wc  -l head.txt
60 head.txt

date

Function: Represents the operation time and date (read, set)

Grammar 1:date

[root@JueChen linux]# date
Wed Aug 28 20:05:16 DST 2019

Grammar 2: date +%F

[root@JueChen linux]# date +%F
2019-08-28

Equivalent to date "+%Y-%m-%d"

[root@JueChen linux]# date "+%Y-%m-%d"
2019-08-28

Grammar 3: date "+% F% T"

[root@JueChen linux]# date "+%F %T"
2019-08-28 20:11:49

Grammar 4: Some time before or after acquisition

date -d "-1 day" "+%Y-%m-%d %H:%M:%S"

[root@JueChen linux]# date -d "-1 day" "+%Y-%m-%d %H:%M:%S"
2019-08-27 20:16:28

Optional values for symbols: + (after) before the latter (-)

Unit optional values: day, mouth, year

• % F denotes the full year, month and day.
• % T represents the complete time and seconds
• % Y means four months
• % m Bimonthly
• % d denotes date with leading 0
• % H denotes hours
• % M stands for minutes
• % S denotes the number of seconds

cal instruction

Function: Used to operate the calendar

[root@JueChen linux]# cal
     August 2019
Su Mo Tu We Th Fr Sa
             1  2  3
 4  5  6  7  8  9 10
11 12 13 14 15 16 17
18 19 20 21 22 23 24
25 26 27 28 29 30 31

- 3 means last month, this month and next month

[root@JueChen linux]# cal -3
      July 2019            August 2019         September 2019
Su Mo Tu We Th Fr Sa  Su Mo Tu We Th Fr Sa  Su Mo Tu We Th Fr Sa
    1  2  3  4  5  6               1  2  3   1  2  3  4  5  6  7
 7  8  9 10 11 12 13   4  5  6  7  8  9 10   8  9 10 11 12 13 14
14 15 16 17 18 19 20  11 12 13 14 15 16 17  15 16 17 18 19 20 21
21 22 23 24 25 26 27  18 19 20 21 22 23 24  22 23 24 25 26 27 28
28 29 30 31           25 26 27 28 29 30 31  29 30

Grammar: cal-y

A calendar that represents the output of a specified year.

clear /ctrl+L

Function: Clear the terminal already has commands and results (information)

Note: This command does not really clear the previous information, but hides the previous information to the top.

The Conduit

Pipeline:|

Function: Pipes are generally used for "filtration", "special" and "expansion treatment".

Grammar: Pipeline can not be used alone. It must be used with other commands. It plays a main auxiliary role.

The name of the document containing the letter "y" in the root directory needs to be queried through the pipeline

[root@JueChen //]# ls | grep y
sys

For the above commands

• The pipe acts as a demarcation line. The command in front has an output. The output in front is the input in the back, then filters and then outputs.
• grep instructions, mainly for filtering

[root@JueChen //]# ls
bin  boot  dev  etc  home  init  lib  lib64  media  mnt  opt  proc  root  run  sbin  srv  sys  tmp  usr  var
[root@JueChen //]# ls | wc -l
20