Summary of L7.3 Loop statements

Summary of circular statements

This article describes in detail the loop statement of the flow control statement, in turn:

1,for Loop Statement general use and special use

2,while loop statements are generally used with special customs

The 3,until statement uses

4, loop control: Use of continue,break in circular statements

5, use of functions

1,for Loop Statements

the general format for A for loop is : The list matches the variable sequentially, and do is followed by the execution body. The list defaults to one or more spaces or tab intervals

For variable in list

Do

Command1

Command2

...

CommandN

Done

Example: List 1 to 5 numbers

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Example: command referencing using commands

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special usage for the FOR loop: for (assignment; condition; Operation statement)

For ((EXPR1;EXPR2;EXPR3)); Do

Loop body

Done

EXPR1: Defines the control variable and assigns the initial value;

EXPR2: cyclic control conditions;

Entry condition: Control condition is "true"

Exit Condition: Control condition is "false"

EXPR3: Correcting control variables

Example: The sum of all positive integers within 100 is obtained;

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2,while Loop Statements

While loop General format syntax:

While Condtion; Do

Loop body

Done

Entry condition: When condition is "true";

Exit Condition: When condition is "false";

While CONDITION; Do

Loop body

Correction expressions for control variables

Done

Example: The sum of all positive integers within 100 is obtained;

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Special usage of the while loop:

Traverse each line of the file:

while read VARIABLE; Do

Loop body

Done </path/from/some_file

Example: Find all users whose UID is even, display their user name and ID number;

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3,UNITL Cycle

UNITL Loop Statement Format:

Until CONDITION; Do

Loop body

Modified expressions for cyclic control variables

Done

Entry condition: When condition is "false"

Exit Condition: When condition is "true"

Example: Sum of all positive integers within 100

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4, loop control (break,continue)

Circular control Meaning:

The # Break command does not perform the current loop within the body break, and the following statement exits from the current loop.

# Continue command is a program that ignores the following statement in this body, starting from the loop header

Break [n]: End loop prematurely; [n] indicates a few layers of loop to jump (end)

Continue [n]: End the cycle in advance and go directly to the next round; [n] continue to cycle into the first round

Use of loop control:

While loop:

While CONDITION; Do

.......

if CONDITION2; Then

Break [n]

Fi

Done

While CONDITION; Do

......

if CONDITION2; Then

Continue [n]

Fi

......

Done

Example: The sum of all even numbers within 100; continue use

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Dead Loop:

While true; Do

Loop body

if condtion; Then

Break

Fi

Done

until false; Do

Loop body

if CONDITION; Then

Break

Fi

Done

Example: every 3 seconds to see if the current system is known as "Gentoo" user login;

If a visit to Gentoo is logged in, it shows that Gentoo is logged in;

If not logged in, the display remains in the future and shows how many times it is already viewed;

Note Break uses

While using break

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Until using break

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5, use of functions

function use:

Function: Functions

Encapsulate a piece of code with a separate function and give it a name, and then use the given function name directly to

Call the whole code;

Function: Code reuse, modular programming;

How to use the function:

Define first: Write function code

Call after: give the function name and pass the parameters on demand

Define the method:

(1) Function F_name {

function body

}

(2) F_name () {

function body

}

Call Function:

F_name [Argu1, ARGU2, ...]

Custom function state return value:

return [#]

0: Success

1-255: Failure

Note: When the function code executes, once the return is encountered, the function code terminates and the function returns;

Example: Execute start,stop,restart to implement the file state judgment separately:

The Start,stop,restart is used as the function body, and the case uses the module call function.

#!/bin/bash#prog=$ (basename $0) lockfile= "/var/lock/subsys/$prog" #echo   $lockfileif  [ $#  -lt 1 ]; then   echo  "usage:  $prog  start|stop|restart| Status "   exit 1fistart ()  {   if [ -f  $lockfile  ];  thenecho  "$prog  is started yet."    elsetouch  $lockfile  && echo  "starting  $prog  ok ..."  | |  echo  "starting  $prog  failed ..."    fi}stop ()  {   if  [ -f  $lockfile  ]; thenrm -f  $lockfile  && echo  "Stop   $prog  ok ....  | |  echo  "stop  $prog  failed ..."    elseecho  "$prog  is stopped  yet. "    fi}restart ()  {   if [ -f  $lockfile  ]; THENRM  -f  $lockfile  && touch  $lockfile  && echo  "restarting  $porg  ok ..."     elsetouch  $lockfile  && echo  $prog  is stopped, Starting  $prog  ok. "   fi}status ()  {   if [ -f  $lockfile   ]; thenecho  "Running ..."    elseecho  "Stopped ..."    fi}case $ 1 instart)    start    ;; Stop)    stop   ;; Restart)    restart   ;; Status)    status   ;; *)    echo  "usage:  $prog  start|stop|restart|sttus"    exit  1esac[[email protected] /]# ./test.sh startstarting test.sh ok ... [[email protected] /]# cd /var/lock/subsys/[[email protected] subsys]#  Lsnetwork  test.sh                          #start创建了test. sh file [[email protected] subsys]# cd -[[email  protected] /]# ./test.sh stop[[email protected] /]# cd -[[email  protected] subsys]# lsnetwork                                     #stop删除了test. sh file [[email protected] subsys]# cd -[[email  protected] /]# ./test.sh restart[[email protected] /]# cd -[[email  protected] subsys]# lsnetwork  test.sh                              #restart执行了先删除再创建test. sh file

Function Modular Invocation

Modular programming

Function: Separate the code in the script file into multiple segments and put it in a different file

Suppose the/root/bin/srv directory has two files:

(1) Function file

(2) Script file

Using configuration files for scripts

Only variables are defined in a file

Script file Source This variable defines the file

Scope of the variable:

Local variables:

Local Variable=value

Survival time:

function execution begins, until function returns to the end;

Example: test01.sh function definition variable, test02.sh script reference test01.sh

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Summary of L7.3 Loop statements