Shell script-basic editing specifications and variables

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Shell script overview

Used to interpret commands between the system kernel and users. "翻译官"
Save the commands to be executed to a text file in order.
Give the file executable permissions [默认权限为644所以给予权限"+x"]
. Various Shell control statements can be combined to complete more complex operations.

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Shell script application scenarios

►Repetitive operation

►Interactive tasks

►Batch transaction processing

►Service operation status monitoring

Regular inspections | Perform optimization | Save logs

►Timed task execution

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Shell operating environment

Logged-in user Shellto automatically load when a Shellprogram
bashis the Linuxsystem used by default Shellprogram
bashfiles are located/bin/bash

cat /etc/shells

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The composition of a shell script

First you need to declare an interpreter on the first line

Shell：#!/bin/bash

Indicates that the following code statements are executed through the /bin/bash program to interpret and execute
Python: #!/usr/bin/python
Expect:#!/usr/bin/expect

Next can be a comment# or a command statement

!#/bin/bash //解释器
#There is nothing here //注释
echo ”阿巴阿巴“ &> /dev/null //丢入无尽黑洞

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Shell script execution

①Pass ./Execute script

echo "echo awsl" > start.sh

We can’t execute it at this time, because we don’t have enough permissions

chmod +x start.sh

By chmodlifting its executable permissions
At this point it shined and that can be executed at any time

./start.sh

Absolute/relative path: bash environment execution will not change the operating environment in the file

②Execute the script through sh [no permission required]

At this point we subtract its permissions

chmod -x start.sh

sh start.sh

③Execute script through source [no permission required]

source start.sh

shAnd sourcealthough no need to execute permissions can be performed, but it will inherit the current Shellenvironment as an execution environment

Redirection and pipeline operations

①Switching hardware equipment

Types of	Default device	Device file	Device description number
Standard input	keyboard	/dev/stdin	0
Standard output	monitor	/dev/stdout	1
Guarantee error	monitor	/dev/stderr	2

②Redirection operation

Types of	Operator	use
Redirect input	<	Read data from the specified file, non-keyboard input
Redirect output	>	The output result will overwrite the original content
Redirect output	>>	The output is appended to the end of the file
Standard error output	2>	Save the error message and overwrite the original content
Standard error output	2>>	Append error information to the specified file
Mixed output	&>	Save the standard output and error content to the same file
Mixed output	2>&1	Redirect standard error output to standard output

For example:

echo 123456 > passwd;passwd fox --stdin < passwd;

When SeLinux is turned on, this operation will be rejected

③Pipeline operator

Use the output result of the command on the left as the input object of the command on the right. Multiple pipe characters can be used in the same line of command

命令 | 命令

④Control of awk column

Awk -FControl bar, the control paragraph F：indicates segmentation symbols, colon, if not written, and a space default identification tab as single quote as the content segmentation, printprinting

• NR indicates which row is the row currently being processed
• FNR indicates that the currently processed line is the first few lines of the currently processed file
• NF indicates how many columns of data the current row has

The role of shell variables

Used to store specific parameter values ​​that the system and users need to use

►Custom variables: defined, modified and used by the user

►Environmental variables: use fixed names. Preset by the system or defined by the user

►Position variable: changes according to the changes of the user setting system

►Predefined variables: BashA type of variables built in. Can not be modified

①Custom variables

Define a new variable
usually start with a letter or underscore, case sensitive, all uppercase is recommended

变量名=变量值

View the value of a variable echo $变量名

• Double quotes: Allow to quote other variable values ​​through the $ symbol
• Single quote: It is forbidden to quote other variable values, and $ is regarded as a normal character
• Backtick: command replacement, extract the output result after the command is executed
  
  

②Enter the content from the keyboard through read to assign a value to the variable

Use -pto add a hint

read -p "是否打响指" input
echo ${input}

Variable scope

By default, the new variables defined only in the current Shellactive context, referred to as local variables
by internal command exportvariable specified export global variable, so that bashwhen the switch is still effective environment variables

export 变量名
export 变量名=变量值

Operation of integer variables

Operators: + addition,-subtraction, * multiplication, / division,% remainder
common algorithm expressions

expr 变量1 运算符 变量2
+ - \*[乘法运算] /除法运算 %求模(取余)
num=$(expr 2 \* 2)
num=`expr 2\*2`
num=$((2*2))
num=$[2*2]
let num=2*2

i++ 相当于 i=$[i+1]
i-- 相当于 i=$[i-1]
i+=2 相当于 i=$[i+2]

Special shell variables

①Environmental variables

Created by the system in advance, to set the user's working environment
profiles: /etc/profile, ~/.bash+pro
use the envcommand to view the current environment variables in the environment
variable USERindicates the user names, HOMErepresenting the user's home directory, LANGrepresentation languages and character sets

②Read-only variables

Used for variable values ​​not to be modified

product=benet
readonly product

③Position variable

When executing a command line operation, the first field represents the name of the command or script program, and the remaining string parameters are assigned to the positional variables in order from left to right.

vim start.sh
#!/bin/bash
echo $1
echo $2
echo $1 + $2

④Predetermined variables

$* $@ 表示命令或脚本要处理的参数
$* 把所有参数看成以空格分隔的一个字符串整体返回
11 22 33 44 为一个整体
$@ 加上双引号后会分割都是整体
11 22 33 44 他们每个都是整体
$# 表示命令或脚本处理参数的个数
$? 表示前一条命令或脚本执行后的返回状态码 [0为true非0为false]

time=backup-`date +%Y%m%d`.tgz
tar zcf $time $* &> /dev/null
echo "已执行 $0 个脚本,"
echo "共完成 $# 个对象的备份"
echo "具体内容包括: $*"

Calculation tool

①Use bc operation

bash itself does not support floating point operations

echo "1+2" | bc

scale=整数 #保留小数个数
echo "scale=5;1.11111+2.22222" | bc

②Use awk operation

Awk displays five decimal places and six significant digits by default

num=$(awk 'BEGIN {print 5/3}')
echo $num