Shell Programming Basics

In this paper, the basic syntax of the shell is briefly recorded for future use!

1. Shell variables

When you define a variable name without the dollar sign $, you only need to add the dollar sign before the variable name when using the variable. As

1 name="LW"2echo ${name}     #或者 $name, but it is recommended to add braces

Note: When assigning a value to a variable, the equals sign "=" cannot have spaces on either side

The name of the variable follows the rules:

1) The first character must be a letter (a-z, a-z).

2) cannot have spaces in the middle, you can use the underscore (_).

3) punctuation cannot be used.

4) You cannot use the keywords in bash (you can view the reserved keywords using the help command).

2. Shell string

a shell string can use either single or double quotation marks. However, there are some differences between these two types of quotes in the process of use.

1) Single quote: The string in single quotation marks has two limitations: the variable in single quotation marks is not valid; single quotation marks cannot appear in single quotation marks (not after using the escape character for single quotes). In short, any character in a single quote will be output as it is.

2) Double quotes: double quotes with no single quotation marks, you can output variables, or you can use escape characters.

Such as:

1 course="python"2echo "The course is: ${course}"

3) Other operations of the string

 1  first= "hello"  2  " python " 3   #字符串的拼接  4  newstr= "${first}, ${ last   " 5  Newstr=${first}", "${last   6   #获取字符串的长度  7  len=${#first}  8   #提取子字符串  9  substr=${first:0 : 5 } 

3. Shell Array

Array definitions:array= (value1, value2, Value3, Valuen)

Such as:

1 course= (' python ', ' shell ', 'perl')2 val1=${course[1]} #获取course [ 1] Data in 3echo ${course[@]}        #输出数组中的全部值

4. If,for,while Statement

Numeric comparison operations:

-eq: equals -ne: Not Equal to

-GT: Greater than -ge: greater than or equal to

-lt: Less than -le: Less than or equal to

if: num1=${2*6} num2=${12}

1)if statement

 1  if  Test ${num1}-eq ${num2}  2  then  3  " equal " 4  elif  Test ${num1}-gt ${num2}  Span style= "color: #008080;" >5  echo   "num1 bigger than num2"  Span style= "color: #008080;" >6  7  echo  num1 less   than num2 " 8  fi  

2) for statement

1 # for in 'seq'            #输出1 ~2for in  3456            4563  Do 4 Echo $i 5  Done 　　

3) While statement

1  while  : 2  Do 3 Echo "Hello" 4  Done 5 4 ) until statement 6 until condition 7  Do 8 Command 9  Done 　　

5. Shell function

such as the addition function:

1 Add () {2     Echo '$:'$1', $:'$23Return $ (($1+$2))4 }5Add3 46 Echo "The sum is: $?"

A few special character meanings:

$#: Number of arguments passed to script

$*: Displays all parameters passed to the script in a single string

$$: The current process ID number for the script to run

$!: ID number of the last process running in the background

[Email protected]: Same as $# , but use quotation marks and return each parameter in quotation marks.

$-: Displays the current options used by the Shell , same as the set command function.

$?: Displays the exit status of the last command. 0 indicates no error, and any other value indicates an error.

Reference Links:

Http://www.w3cschool.cc/linux/linux-shell-func.html

Shell Programming Basics