Shell Learning notes-branching statements

One, single branch if statement 1. Syntax format

if  Then     program fi or if  [conditional judgment]    then     program fi

Note: There must be a space between the brackets and the conditional judgments

2. Example 1: Determine if the logged on user is root

#!/bin/Bashif["$USER"= = Root]; Then    Echo "Login User is root"fi######## #或者 ##########!/bin/Bashif["$USER"==Root] Then    Echo "Login User is root"fi

3. Example 2: Determine partition usage

#!/bin/bashtest=$ (dfgrepawk'{print $}'cut  '%'1) ;  then    Echo " warning!/dev/sda1 is full! " fi

Two, two-branch if statement 1. Syntax format

if  Then When the     condition is established    , the execution of the program if the else     condition is not established, the execution of the program fi or if  [ Conditional judgment]    then     When the condition is established, the program executed when the else     condition is not established  fi

2. Example 1: Enter a file to determine if there is a

#!/bin/"pleaseinput a file:"fileif [-F $file  then    echo"File: $file exists! " Else    Echo " File: $file not exists! " fi

3. Example 2: Determine if the Apache service is started, and if it is not, start the code

#!/bin/bashtest=$(PSAux |grephttpd |grep-V'grep'|WC-l)if[$test-GT0]; Then    Echo "$ (date) httpd is running!"Else    Echo "$ (date) httpd isn ' t running, 'll be started!"/etc/init.d/httpd Startfi

Three, multi-branch if statement 1. Syntax format

if  Then When     the conditional judgment 1 is established, the execution Program 1elif then     when the conditional Judgment 2 is established, executes the program 2 .... Omit more conditions ..... Else     When all conditions are not true, this program is finally executed fi

2. Example: Implementing a Calculator

#!/bin/bash# Input number A, number B and operator read-P"Please input number A:"Aread-P"Please input number B:"Bread-P"Please input operator[+|-|*|/]:"opt# Judging the correctness of the input content Testa=$(Echo$a |sed 's/[0-9]//g') Testb=$(Echo$a |sed 's/[0-9]//g') testopt=$(Echo$opt |sed 's/[+|\-|*|\/]//g')if[-N"$testa"-o-n"$testb"-o-n"$testopt"]; Then    Echo "input content is error!"Exit1elif["$opt"=="+"]; Thenresult=$ (($a +$b))elif["$opt"=="-"]; Thenresult=$ ($a-$b))elif["$opt"=="*"]; Thenresult=$ (($a *$b))Elseresult=$ (($a/$b))fiEcho "a $opt b = $result"

Iv. Case Statements

Case statements and IF...ELIF...ELSE statements are multi-branch conditional statements, but unlike if multi-branch conditional statements, case statements can only judge a conditional relationship, and an if statement may judge a variety of conditional relationships.

1. Syntax format

 Case inch " value 1 "     If the value of the variable equals the value 1, execute program 1    ;; " Value 2 "     If the value of the variable equals the value 2, execute program 2    ;; ..... Omit other branches ..... *)    If the value of the variable is not the above value, execute the program    ;; Esac

2. Example: Judging user input

#!/bin/Bashread-P"Please choose yes/no:"cmd Case$cmdinch "Yes")    Echo "Your Choose is yes!"    ;;"No")    Echo "Your Choose is no!"    ;;*)    Echo "Your Choose is error!"    ;;Esac

Shell Learning notes-branching statements