Shell Programming-Basic syntax

Tag: element represents article string output shard for form ber

The role of variables

Forbidden spaces on both sides of the equals sign

For loop

#语法一 for  variable in value 1 value 2 value 3     : Do       block done     # #语法二 the for  variable ' command ' do     block done    # #语法三 for  (initial value; loop control ; Variable) do      program block done      

e.g. Output 3 times Uptime

#!/bin/bash for    i in 1 2 3 4 5     does        echo "$i-->$ (uptime)"    done  

e.g. bulk unzip all. tar.gz files under the current folder

#!/bin/bash for    i in ' ls./*.tar.gz '      does         tar-zxvf $i >/dev/null     Done  

e.g. seeking 1~100 and

#!/bin/bash    #注意变量赋值的时候, = There is absolutely no space on either side  sum=0 for  ((i=1; i<=100; i++)) do       sum=$ (($sum + $i)    ) Done    echo "1+2+3+...+100= $sum"

Array

• Definition of an array

　The attribute of an array is a set of identical data types

Let's look at two types of data: One is a numeric type, the other is a string type, and although the shell itself is a weak type, it can be so distinguished.

　　array of numeric types: A pair of parentheses represents an array, and the elements in the array are separated by a "space".

As an example:

Arr_number= (1 2 3 4 5);

　　array of string types: Similarly, use a pair of parentheses to represent the array, where the elements in the array are enclosed in double or single quotation marks, and also with "spaces."

arr_string= ("abc" "EDF" "SSS"); or arr_string= (' abc ' EDF ' SSS ');

• Operation of the array

We use the numeric type array arr_number= (1 2 3 4 5) as the source array for the relevant explanation: Get the array length, read the value of a subscript, assign a value to a subscript, delete, assign and replace, and traverse.

The shell looks to get the value of a variable, starting with the $ character, such as: $a or ${a}.

　　Get array length

arr_length=${#arr_number [*]} or ${#arr_number [@]}, i.e. form: ${#数组名 [@/*]} to get the length of the array.

　　Reads a value of a subscript

ARR_INDEX2=${ARR_NUMBER[2]}, i.e. form: ${array name [subscript]}

　　Assign a value to a subscript

Here are two questions to ask:

　　　　The first question is , what happens if the subscript element already exists?

　　　　A : the underlying value is modified to be the new specified value.

For example: arr_number[2]=100, the array is modified to (1 2 100 4 5)

　　　　The second question is , if the specified subscript already exceeds the size of the current array, such as the size of the arr_number above is 5, specify an subscript of 10 or 11 or greater than 5 of any value?

　　　　A : the newly assigned value is appended to the end of the array.

For example: arr_number[13]=13, the array is modified to (1 2 100 4 5 13)

　　Delete operation

Clear an element: unset arr_number[1], this clears the array labeled 1;

Empties the entire array: unset arr_number;

　　Shard Access

The Shard Access form is: ${array name [@ or *]: start subscript: End subscript}, note that the value of the end subscript element is not included.

For example: ${arr_number[@]:1:4}, where Shard Access starts with subscript 1 and the number of elements is 4.

　　Mode substitution

Form: ${array name [@ or *]/mode/new Value}

For example: ${arr_number[@]/2/98}

　　Traversal of an array

Array traversal We use the FOR statement to demonstrate:

For V in ${arr_number[@]}; Do

Echo $v;

Done

Shell Programming-Basic syntax