What is Shell? Get started quickly with Shell!

Shell is a command line interpreter that accepts application/user commands and then calls the operating system kernel. It is a powerful programming language, easy to write, easy to debug, and highly flexible.
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There are six shell parsers provided by Linux:cat /etc/shells

/bin/sh
/bin/bash, The default parser in CentOS is bash
/sbin/nologin
/bin/dash
/bin/tcsh
/bin/csh

Script format

Script #!/bin/bashstarts with
```
#!/bin/bash

echo "hello world"
```
Run the script file sh xx.shby: bash xx.sh, ,./xx.sh（注意需要.sh具备执行权限[x]，chmod 777）
By touch xx.shcreating script files, by vim xx.shediting script files.

System variable

$HOME, The home directory of the currently logged in user
$PWD, The current directory path
$SHELL, The default shell parser
$USER, The currently logged-in user

Custom variable

Basic grammar
- Define variable: variable = value
- Undo variable:unset 变量名
- Declare static variables:, readonly 变量名note that static variables cannot be undone, and restart is invalid.
- Variable definition rules: composed of letters, numbers, and underscores, and cannot start with a number. It is recommended that environment variables be capitalized.
- No spaces around the equal sign, strict requirements
- Variables are of string type by default , and numerical operations cannot be performed directly.
- If the value of the space variable, need quotes quotes
- Promote the local variables in the script to global variables:export 变量名

Special variable

$n
- Function description: n is a number. $0, On behalf of the script name;, $1-$9on behalf of the first to ninth parameters; parameters above the tenth need to be ${10}expressed.
  
  touch partmeter.sh
  
  vim partmeter.sh
```
#!/bin/bash

echo "$0 $1 $2"
```
  bash partmeter.sh p1 p2
  
  partmeter.sh p1 p2
$#
- Function description: Get the number of all input parameters, often used in loops
```
#!/bin/bash

echo "$#"
```
  bash partmeter.sh p1 p2
  
  2
$*
- Function description: Get all the parameters in the command line, and treat all the inputs as a whole
```
#!/bin/bash
    
echo "$*"
```
  bash partmeter.sh p1 p2
  p1 p2
$@
- Function description: Get all the parameters in the command line and treat all input parameters differently
```
#!/bin/bash

echo "$@"
```
  bash partmeter.sh p1 p2
  p1 p2
$?
- Function description: The return status of the last executed command. If the value of the variable is 0, it proves that the last command was executed correctly; if the value of the variable is non-zero, it proves that there was an error in the execution of the previous command.

Operator

Basic grammar
- $((运算式))or$[运算式]
- exprOperation ( There must be spaces between operators )
  - +,plus
  - -,Less
  - \*, Multiply
  - /,except
  - %, Take the remainder
  - Operation execution symbol: `expr 2 + 3

Conditional judgment

Basic grammar
- [ condition ], Note that there must be spaces before and after the expression
- [ test ], If the condition is not empty, it is true; [], if the condition is empty, it is false;
Commonly used judgment conditions
- = String comparison
- -lt, Less than
- -le, Less than or equal to
- -eq,equal
- gt,more than the
- ge,greater or equal to
- ne,not equal to
- -r, Has read permission
- -w, Have write permission
- -x, Have permission to execute
- -f, The file exists and is a regular file file
- -e, The file exists
- -d, The file exists and is a directory
Multi-condition judgment
- &&, The next command will be executed only when the previous command is executed successfully
- ||, The next command will be executed when the previous command fails

Process control

if

Basic syntax: ifthere []must be a space after it, and there must be a space before and after the conditional expression elif;, stands for else if; at the fiend.
```
#!/bin/bash

if [ condition1 ];then
    content1
elif [ condition2 ]
then
    content2
fi
```

case

Basic syntax: case $变量名 in value)format, ;;end of case, similar to break; *)represents the default branch; esacend;

#!/bin/bash

case $1 in
1)
	content1
	;;
2)
	content2
	;;
3)
	content3
	;;
*)
	content4
	;;
esac

for

Basic grammar 1: for((初始值;循环控制条件;变量变化))format; domethod body; doneend; specific accumulation of 1-100 sum:
```
#!/bin/bash

s=0
for((i=1;i<=100;i++))
do
	s=$[$s+$i]
done

echo $s
```
Basic syntax 2: for 变量 in value1 value2 value3Format; $@each input parameter is processed separately, and $*the input parameter is processed as a whole
```
#!/bin.bash

for i in "$*"
do
	echo "ceshi * $*"
done

for j in "$@"
do 
	echo "ceshi @ $@"
done
```
bash ceshi.sh zs ls

The output result is:

ceshi * zs ls

ceshi @ zs

ceshi @ ls

while

Basic grammar: while [ condition ]format; domethod body; doneend; the sum of concrete accumulation 1-100:
```
#!/bin/bash

s=0
i=0
while [ $i -le 100]
do
	s=$[$s+$i]
	i=$[$i+1]
done
```

Read to read console input

Basic syntax:read(选项)(参数)
- Options
  - -p, Specify the prompt when reading the value
  - -t, Specify the time to wait when reading the value (seconds)
- parameter
  - Variable: Specify the name of the variable to read the value

Example: Enter name on the console within seven seconds

#!/bin/bash

read -t 7 -p "input your name in 7 seconds" name

echo $name

function

System function

basename [string/pathname][suffix], Intercept the file name under the specified path
dirname 文件绝对路径, Extract the path of the file from the given file name containing the absolute path

Custom function

Basic grammar

// 声明函数
function funname[()]{
      
      
	action;
	[return int;]
}
funname // 执行函数

Shell tool

cut command

Cut bytes, characters, and fields from each line of the file, and output these bytes, characters, and fields.
Basic usage
- cut [选项参数] filename
- Option parameter
  - -f, Column number, which column to extract
  - -d, Separator, split the column according to the specified separator, the default separator is a tab\t
- Example: Use space as the separator to cut the second column of the file content and output the following content
  
  cut -d " " -f 2- cut.txt

and

Stream editor, processing one line of content at a time. When processing line content, store the current line content in a temporary buffer area called "mode space", then use the sed command to process the content in the buffer, and output the content after the processing is completed. Then process the next line until the end of the text.
Basic usage
- sed [选项参数] "命令功能" filename
- Option parameter
  - -e, Directly edit the action of sed on the specified column mode
- Command function (commonly used)
  - a, Add
  - d,delete
  - s, Find and replace
- For example:
  - Add content in the second line of the file: add content
    
    sed "2a add contend" filename
  - Delete the line containing wo in the file
    
    sed "/wo/d" filename
  - Replace wo in the file with ni
    
    sed "s/wo/ni/g" filename, gWhich stands for global replacement
  - Delete the second line of the file and replace wo in the file with ni
    
    sed -e "2d" -e "s/wo/ni/g" filename

awk

The text analysis tool reads the file line by line. By default, each line is sliced with a space as a separator, and the cut part can be analyzed and processed.
Basic usage: awk [选项参数] 'pattern1{action1} pattern2{action2} ...' filename; pattern; match the regular expression actioncommand matching contents in line with the execution.
- Option parameter
  - -F, Specify the input file separator
  - -v, Copy a user-defined variable
- Built-in variables
  - FILENAME,file name
  - NR, The number of records read
  - NF, The number of domains in the browse record (after cutting, the number of columns)
- Example: Search for all lines in the file starting with root, and output the seventh column of the line
  
  awk -F : '/^root/{print $7}' filename

sort

Files are sorted and output according to the standard output of the sorted results
Basic usage
- sort(选项)(参数)
  - Options
  - -n, Sorted by numerical value
  - -r, Sort in reverse order
  - -t, Set the separator character used for sorting
  - -k, Specify the column to be sorted
  - parameter
    - Specify the list of files to be sorted
  - Example: The files are divided by colons, and the third column is sorted in reverse order
    
    sort -t : -nrk 3 filename