Shell script introduction, Structure and execution, date command usage, variables in shell scripts

Shell Script Introduction

• Shell is a scripting language

  • The shell has its own syntax, and can use logical judgments, loops, and so on.

• Functions can be customized to reduce duplicate code

• The shell is a collection of system commands

• Shell scripts enable automated operations, which can greatly increase our operational efficiency

Shell script structure and execution structure

• The first line of the script must be #!/bin/bash

• The behavior that begins with # in the contents of the script as an explanatory note

• Write a script note: Author, time, features and other information, easy to view later

• The script name ends with ". Sh" and is used to differentiate between this is a shell script

Execution method

1. Add Execute permission "chmod a+x test.sh" to the script, then execute the procedure directly

2, bash test.sh; SH test.sh

• SH parameter

  • -x:sh-x test.sh Viewing the script execution process
  • -n:sh-n test.sh to see if there is a syntax error in the script

  Create a shell script:
  [Email protected] shell]# vim 01.sh
  #! /bin/bash//fixed format
  echo "123"
  W
  Ls

  Add execute permissions to the script:
  [Email protected] shell]# chmod a+x 01.sh

  Execute script
  [Email protected] shell]#./01.sh
  Abc
  22:46:45 up 2:33, 1 user, load average:0.00, 0.01, 0.05
  USER TTY from [email protected] IDLE jcpu PCPU
  Root pts/0 192.168.159.1 20:17 5.00s 0.06s 0.00s/bin/bash./01.sh
  01.sh

• In the current terminal, put the #! in 01.sh /bin/bash after the execution of the script, you will see that the results are the same, no problem, this means that the machine is able to recognize the inside of a single command, to run the command inside, but if you change a machine, it will not be able to carry out

• In the first line, the file header specifies #!/bin/bash, and the next command to run is the interpreter that is used to operate it, usually performed by the/bin/bash interpreter.

Date command usage

• The date command is used to display or set the system time and date

• Syntax: Date option parameter

• Options

  • -D: Displays the date and time specified by the string (double quotation marks must precede and before the string)
  • -S: Sets the time and date based on the string (double quotation marks must be added before and after the string)

• Parameters

  Show Current time:
  [[Email protected] ~]# Date
  Wednesday, February 07, 2018 09:56:28 CST

  To view the system calendar:
  [[email protected] ~]# cal
  February on 2018
  Day 123456
  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

  Cal-y can view a calendar of one year

  + Time Date format: Specify the format of the date and time display

  Date +%y (%Y): Displays the year in four-bit (two-bit) number format
  [Email protected] ~]# date +%y
  2018
  [Email protected] ~]# date +%y
  18

  Date "+%y-%m-%d%h:%m:%s%w"
  means: Year, month, day, time, minute, second, week
  [Email protected] ~]# Date "+%y-%m-%d%h:%m:%s%w"
  2018-02-07 10:02:43 3
  When the above parameter combination, there is a special symbol in the middle to add double quotation marks

  Date +%f: Displays the full month day
  [Email protected] ~]# date +%f
  2018-02-07

  Date +%w: Shows the current time is the week ordinal of a year
  [Email protected] ~]# date +%w
  06

  Date +%t: Shows the current time
  [Email protected] ~]# date +%t
  10:05:50

  Date +%s: Timestamp
  Displays the number of seconds from January 1, 1970 00:00:00 to the current experience
  [Email protected] ~]# date +%s
  1517969292

  Time Stamp conversion:
  [[Email protected] ~]# date +%s-d "20180207 10:10:00"
  1517969400
  [Email protected] ~]# date-d @1517969400
  Wednesday, February 07, 2018 10:10:00 CST

Print specified date & time

• Sometimes you need to use the date or time before n days (hours, minutes, seconds)

  Two days ago:
  Date-d "-2 Day"
  [[Email protected] ~]# Date
  Wednesday, February 07, 2018 10:14:10 CST
  [[email protected] ~]# date-d "-2 Day"
  Monday, February 05, 2018 10:14:13 CST

  Two days later:
  Date-d "+2 Day"
  [[Email protected] ~]# Date
  Wednesday, February 07, 2018 10:14:58 CST
  [[email protected] ~]# date-d "+2 Day"
  Friday, February 09, 2018 10:15:02 CST

  Two months a year before:
  Date-d "-1 year-2 month-1 Day"
  [[Email protected] ~]# Date
  Wednesday, February 07, 2018 10:16:32 CST
  [[email protected] ~]# date-d "-1 year-2 month-1 Day"
  December 06, 2016 Tuesday 10:16:34 CST

Time setting

• Manual setup Time: Date-s "year-month-day time: minutes: Seconds"

  [[email protected] ~]# date-s "2017-01-01 12:00:00"
  Sunday, January 01, 2017 12:00:00 CST
  [[Email protected] ~]# Date
  Sunday, January 01, 2017 12:00:02 CST

• Synchronize Network time: Ntpdate command

  [[email protected] ~]# Yum install-y NTP
  Install the Ntpdate command

  [Email protected] ~]# ntpdate ntp1.aliyun.com
  7 Feb 10:24:16 ntpdate[2681]: Step time server 182.92.12.11 offset 34726773.904725 sec
  [[Email protected] ~]# Date
  Wednesday, February 07, 2018 10:24:24 CST
  Ntpdate back with NTP time server address

Variables in shell scripts

• When a string is used more frequently in a script and the string length is long, you should use a variable instead of the string.

• When using conditional statements, you often use variable if[$a-gt 1];then ...; Fi

• Replace n= ' Wc-l 1.txt ' with a variable when referencing the result of a command

• When writing and user interaction scripts, variables are also essential for read-p "input a number:" N; Echo $n If you don't write this n, you can use ￥reply directly.

• Built-in variable $0,$1,$2 ... $ A represents the script itself, the first argument, the second argument of $ $n represents the nth parameter

• Mathematical Operations a=1;b=2;c=$ (($a + $b)) or $[$a + $b]

Shell scripting introduction, Structure and execution, date command usage, variables in shell scripts