Shell Programming flow control (conditional statements and loop statements)

1 If ... else ...

Basic Format:

If condition

Then

Commend

Else

Commend

Fi

    Of course you can write to a line, use [] to indicate the boundary, or to indicate a branch. For example:  if [ $ (ps -ef | grep -c "SSH" ) -gt 1 ];then echo "true" ; fi    judging conditions in square brackets to denote demarcation, meaning to count the current process

< Span class= "pun" >< span class= "str" >< Span class= "PLN" >< Span class= "pun" > Number of strings with ' ssh ', if this number is greater than 1, then output true, end.

< Span class= "pun" >< span class= "str" >< Span class= "PLN" >< Span class= "pun" >    also has a format with Elif, as follows:

< Span class= "pun" >< span class= "str" >< Span class= "PLN" >< Span class= "pun" >        If condition

< Span class= "pun" >< span class= "str" >< Span class= "PLN" >< Span class= "pun" >        then

< Span class= "pun" >< span class= "str" >< Span class= "PLN" >< Span class= "pun" >            commend /span>

< Span class= "pun" >< span class= "str" >< Span class= "PLN" >< Span class= "pun" >        elif condition1

< Span class= "pun" >< span class= "str" >< Span class= "PLN" >< Span class= "pun" >        then

< Span class= "pun" >< span class= "str" >< Span class= "PLN" >< Span class= "pun" >            &NBSP;COMMEND1 /span>

< Span class= "pun" >< span class= "str" >< Span class= "PLN" >< Span class= "pun" >        Else

< Span class= "pun" >< span class= "str" >< Span class= "PLN" >< Span class= "pun" >            Commend2

< Span class= "pun" >< span class= "str" >< Span class= "PLN" >< Span class= "pun" >        fi

< Span class= "pun" >< span class= "str" >< Span class= "PLN" >< Span class= "pun" >    instance:

a=10b=20if [$a = = $b] #此处方括号和内部的内容之间必须有空格then echo "a equals B" elif [$a-gt $b] #这种写法也可以改写成test $a-gt $bthen echo "A greater than B" elif [$a-lt $b]then echo "A less than B" else echo "does not meet the conditions" fi# note, c=[$a-l T $b];echo $c can not get any output value, it is not like Python, true and false can not be printed out, it is really want to try to use [$a-lt $b] && echo hehe (| | ) also (get hehe on the front is ture) a=1b=2[$a-lt $b] && echo hehe1 #且, the first one is true, the second one still needs to be judged, so as to know whether the overall is true or false[$a-gt $b] || Echo Hehe2 #或, the first one is true, the second one does not have to be executed to get the overall return value is true 

2 for Loop

basic format:for I in Item1 item2

Do

Commend

Done

It is still separated by semicolons when written in a line: For I in Item1 item2; Do commend; Done

Example:

For loop in 1 2 3 4 5do    echo ' The value is: $loop ' done    #结果是 #the value is:1#the value is:2 ... Until 5.

One thing to note, for I in "a string", the output of I will be: a string, here and Python is not the same, here will not take the letter one by one to disassemble the iteration.

3 While loop

Executes as long as the condition is met. The General format is:

While condition #condition not write, or write true, it can cause an infinite loop.

Do

Commend

done$

example; #得到的输出是1, 2,3,4,5,let is used to calculate multiple expressions, do not need to add variable indicator $, self-add operation: Letno++, self- subtraction: Letno--。 abbreviated form let no+=10,let no-=20, respectively, is equivalent to let no=no+10,let no=no-20.

Int=1while (($int <=5)    #写成: [$int-le 5] line do    echo $int let    ' int++ ' done

4 until cycle

It has been circulating until there is a ture. Just the opposite of Wihile.

Untill condition

Do

Commend

Done

Example:      

A=0until [! $a-lt]    #除非a大于等于10否则一直运行do   echo $a   a= ' expr $a + 1 ' done

5 Multiple SELECT statements

is to select one of the multiple statements, note the quotation marks, and wrap.

Instance:

Anum=3case $aNum    in #变量aNum可以是数字或者字符串abc等    1) echo ' You selected 1 '    ;;    #俩分号    2)  echo ' You have chosen 2 '    ;;    3)  echo ' You have chosen 3 '    ;;    4)  echo ' You have chosen 4 '    ;;    *)  echo ' You did not enter a number between 1 and 4 '    ;; Esac    #esca是反过来的

6 jumping out of the loop

The break command jumps out of all loops.

Instance:

While:d o    echo-n "Enter a number from 1 to 5:"    read Anum case    $aNum in        1|2|3|4|5) echo "The number you entered is $aNum!"        ;        *) echo "The number you entered is not between 1 and 5!" Game Over "    #* represents a break #直接跳出并且终止这个程序 other than 12345        ;;    Esacdone

　Continue jump out of this layer of cycle

Instance:

While:d o    echo-n "Enter a number from 1 to 5:"    read Anum case    $aNum in        1|2|3|4|5) echo "The number you entered is $aNum!"        ;        *) echo "The number you entered is not between 1 and 5!"            Continue the        number between #如果输入的值不是1-5, the Contiuune will be transferred directly to the "echo-n" input number between 1 and 5 "            echo" Game Over ";        ;    Esacdone

　　

Shell Programming flow control (conditional statements and loop statements)