1 $? $# $* $n $0

$?Once on output, it said if it is 0: that success; instead 0, indicates failure
$#It represents the script parameters个数
$*It represents the value of the corresponding parameter
$nExpressed as ( n>=1) parameter
$0Indicates that the script name

###这是测试脚本
#!/bin/sh
echo 这是脚本名字：$0
echo 总共有$#个人，分别是$*
echo 第一个人是$1，第二个人是$2
### 这是运行结果
这是脚本名字：sympol.sh
总共有2个人，分别是李四 张三
第一个人是李四，第二个人是张三

$?Test:
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2 semicolon

分号It is among more than one statement 分隔符号, when only a statement 末尾无需分号, after the last statement is also 无需分号, otherwise an error

3 quotes

单引号Is the original string
双引号can be extended for special characters
generally do not write is that when using double quotes

a=bcdef
echo $a # 输出bcdef
echo "$a" #双引号将进行变量扩展 ，输出bcdef
echo '$a' #单引号直接输出$a

4 brackets role

4.1 Command substitution

$( )And `backtick ( tabkey top) the same function: Replace command
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4.2 numerical operations

$(( ))Is an integer of numerical operations, can also be used (( ))in place of
Another $[ ]is performing mathematical operations;

4.3 test operation

[ ]In place of testoperators, it can also be used [[ ]]instead of using the double brackets, more rigorous logic

4.4 ${}

4.4.1 delimiters

$｛｝Delimiters, such as $ab, the equivalent of ${ab}, and ${a}bis just take the avalue of
using linux symbolic example, you can refer to my game to become familiar with linux

4.4.2 take the path, file name, suffix

#假设一个变量名为file
file=/dir1/dir2/dir3/my.file.txt
${file#*/}：删掉第一个 / 及其左边的字符串：dir1/dir2/dir3/my.file.txt
${file##*/}：删掉最后一个 / 及其左边的字符串：my.file.txt
${file#*.}：删掉第一个 . 及其左边的字符串：file.txt
${file##*.}：删掉最后一个 . 及其左边的字符串：txt
${file%/*}：删掉最后一个 / 及其右边的字符串：/dir1/dir2/dir3
${file%%/*}：删掉第一个 / 及其右边的字符串：(空值)
${file%.*}：删掉最后一个 . 及其右边的字符串：/dir1/dir2/dir3/my.file
${file%%.*}：删掉第一个 . 及其右边的字符串：/dir1/dir2/dir3/my

Way to remember are:

#Counting from the left represents the first (on the keyboard #at $the left, pay attention to the use of #time, remove the symbols *on the flag symbol ( /或.) to the left)
##: Indicates the last date from the left
%It represents the first counting from the right (on the keyboard %at $the right side, pay attention to the use of %time, remove the symbols *on the flag symbol ( /或.) to the right)
%%: Indicates the last date from the right
单一It is the symbol 最小匹配; 两个the symbol is最大匹配
*: Indicates to delete the contents of, for #and ##circumstances, it 位于specified character (in the example '/'和'.') of 左边, and its contents, remove all the characters on the left; for %and %%the circumstances, it 位于specified character (in the example '/'和'.') is 右边expressed delete the specified character and its contents on the right. Here the *position not interchangeable, i.e., not the *number on #or ##right, and vice versa.

5 (&), or (|)

5.1 and &

&: Indicates the task in the background, running in the background,redis-server &
&&: That the previous command executes successfully, only after the implementation of a command, such as:echo '1' && echo '2'

5.2 or |

|: A piping character, 上一条命令的输出as下一条命令参数
||: This represents a command execution 失败only after the next command, such as:cat nofile || echo '1'