Shell Script Basics (i)

First, Shell script introduction

The shell is a program written in C that is a bridge for users to use Linux. The shell is both a command language and a programming language.

• The shell is a scripting language;
• Can use logic judgment, loop and other syntax;
• can be customized functions;
• The shell is a collection of system commands;
• Shell script can realize the automation operation and maintenance, can greatly increase our operation and maintenance efficiency;

Ii. shell script structure and execution 1, structure

• The beginning requires "#!/bin/bash";
• The line in the script content that begins with # is interpreted as a description;
• Writing script notes: Author, time, function and other information, convenient to view later;
• The script's name ends with ". Sh", which distinguishes it from a shell script;

2, the way of implementation

1, as an executable program: Add execution permissions to the script chmod a+x test.sh , and then execute the script directly ./test.sh ;
2, as the Interpreter parameter: using SH execution # sh test.sh ;

3. Parameters

-X: sh -x test.sh View the script execution process
-N: sh -n test.sh detecting syntax errors

Iii. use of the date command

The date command is used to display or set the system time and date.
Command options:

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

Command instance:

[[email protected] ~]# Date//Display current DateTime April 17, 2018 Tuesday 19:54:50 cst[[email protected] ~]# cal  Current calendar April 2018 Day 1,234,561 2 3 4 5 6 7 8 9 10 11 12 13 1415 16 17 18 19 20 2122 23 24 25 26 2829 30[[email protected] ~]# date +%y//Display year 2018[[email protected] ~]# date +%y//year abbreviated 18 [[email protected] ~]# date "+%y-%m-%d%h:%m:%s%w"//month day, time division seconds, week 2018-04-17 19:58:30 2[[email prote CTED] ~]# Date +%f//Full date of another display 2018-04-17[[email protected] ~]# date +%w//week of the year [[Email pro Tected] ~]# Date +%t//Current time 20:01:19[[email protected] ~]# date "+%f%T"//datetime 2018-04-17 20:01: 45[[email protected] ~]# Date +%s//timestamp (shows the number of seconds from January 1, 1970 00:00:00 to the current experience) 1523966522[[email protected] ~ ]# date-d @1523966522//timestamp converted to specific date April 17, 2018 Tuesday 20:02:02 cst[[email protected] ~]# date +%s-d "201       8-04-17 20:04:59 "  Date converted to timestamp 1523966699[[email protected] ~]# date-d "-1 Year" +%y//previous 2017[[email protected] ~]# date-d "- 1 month "+%m//Previous month 03[[email protected] ~]# date-d"-1 day "+%d//The day before 16[[email protected] ~]# date-d" 2 Year 2month 2 Day "+%y-%m-%d//Current date next 2 years next February next 2 days 2020-06-19

Iv. variables in the shell

• When a string is used in a script, multiple times and the length of the string is long, you should use a variable instead
• When using conditional statements, use the 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 "Lnput a number:" N; echo $n If you do not write this n, you can use $reply directly
• Built-in variable $0,$1,$2 ... $ A represents the script itself, the first parameter, the second ... $#: Indicates the number of arguments
• Mathematical operations a=1;b=2;c=$ ($a + $b) or $[$a + $b]

Shell Script Basics (i)