4.控制流结构
4.1.控制结构 4.2.if then else语句 格式:if 条件1 //如果条件1为真then 命令1 //那么,执行命令1elif 条件2 //如果条件1不成立,而条件成立then 命令2 //那么,执行命令2……更多的elif then组合……else 命令2 //如果条件1、2……都不成立,那么执行命令3fi //完成,if语句必须以单词fi终止
最简可以是形式:
if 条件then 命令fi
等价于
if 条件; then 命令
;可以在脚本的一行里写多条命令
例程1:#!/bin/bash#if testif [ "10" -lt "12" ]then echo "Yes, 10 is less than 12"else echo "No"fi
注意语法细节(在我的ubuntu 12.04 LTS中):
1.if或elif与[ 之间必须有一个空格,否则会出错!!! 2.条件的 [ 之后 和 ]之前也必须有一个空格 经历 因为没有注意这个细节,写的脚本会报出类似下面的错误 ./cpdir.sh: 行 10: if[ 2 -ne 2 ]: 未找到命令 ./cpdir.sh: 行 11: 未预期的符号 `then' 附近有语法错误 ./cpdir.sh: 行 11: `then' 例子 if [ "10" -lt "12" ]在if后有空格、[后有空格、]前有空格,这就是正确的 if ["10" -lt "12"]在[后没空格、]前没空格就是错误的 if和elif后的判断条件(用 man test 查看详细内容) [ EXPRESSION ] EXPRESSION is true !EXPRESSION EXPRESSION is false EXPRESSION1 -a EXPRESSION2 both EXPRESSION1 and EXPRESSION2 are true EXPRESSION1 -o EXPRESSION2 either EXPRESSION1 or EXPRESSION2 is true [ -n ] STRING the length of STRING is nonzero -z STRING the length of STRING is zero STRING1 = STRING2 the STRINGs are equal STRING1 != STRING2 the STRINGs are not equal INTEGER1 -eq INTEGER2 INTEGER1 is equal to INTEGER2 INTEGER1 -ge INTEGER2 INTEGER1 is greater than or equal to INTEGER2 INTEGER1 -gt INTEGER2 INTEGER1 is greater than INTEGER2 INTEGER1 -le INTEGER2 INTEGER1 is less than or equal to INTEGER2 INTEGER1 -lt INTEGER2 INTEGER1 is less than INTEGER2 INTEGER1 -ne INTEGER2 INTEGER1 is not equal to INTEGER2 FILE1 -ef FILE2 FILE1 and FILE2 have the same device and inode numbers FILE1 -nt FILE2 FILE1 is newer(modification data) than FILE2 FILE1 -ot FILE2 FILE1 is older than FILE2 -b FILE FILE exists and is block special -c FILE FILE exists and is character special -d FILE FILE exists and is a directory -e FILE FILE exists -f FILE FILE exists and is a regular file -g FILE FILE exists and is set-group-ID -G FILE FILE exists and is owned by the effective group ID -k FILE FILE exists and has its sticky bit set -L FILE FILE exists and is a symbolic link -O FILE FILE exists and is owned by the effective user ID -p FILE FILE exists and is a named pipe -r FILE FILE exists and is readable -s FILE FILE exists and has a size greater than zero -S FILE FILE exists and is a socket -f [ FD ] file directory FD(stdout by default) is opened on a terminal -u FILE FILE exists and its set-user-ID bit is set -w FILE FILE exists and is writable -x FILE FILE exists and is executable 例程2:#!/bin/bash#if test2echo -n "Enter your name:"read NAME#did the user just hit returnif[ "$NAME" = "" ]#注意字符串变量比较的具体格式then echo "You did not enter any information"else echo "Your name is ${NMAE}" #注意字符串变量输出使用的具体格式fi
例程3:
#!/bin/bash#if cp testif cp myfile.bak myfilethen echo "good copy"else echo "`basename $0`:error could not copy the files" > &2fi
例程4:
#!/bin/bash#if elifecho -n "Enter your name:"read NAMEif [ -z $NAME ] || [ "$NMAE"="" ]then echo "You did not enter a name"elif [ "$NMAE"="root" ]then echo "Hello root"elif[ "$NAME"="perfect" ]then echo "Hello perfect"else echo "You are not root or perfect, but hi $NAME"fi
4.3.case语句
格式:case 值 in模式1) 命令1 ;;模式2) 命令2 ;;esac
简介:
case取值后面必须为单词 in ,每一模式必须以右括号结束。取值可以是变量或常数。匹配发现取值符合某一模式后,其间所有命令开始执行直至 ;; 。模式匹配符 * 表示任意字符, ? 表示任意单个字符, [ .. ]表示类或范围中任意一个字符 例程1:#!/bin/bash#case selectecho -n "Enter a number from 1/2/3/y/Y:"read ANScase $ANS in1) echo "You select 1" ;;2) echo "You select 2" ;;3) echo "You select 3" ;;y|Y) #y或者Y echo "you select $ANS" ;;*) echo "`basename $0`:This is not between 1 and 3" > &2 exit; ;;esac
例程2:
4.4.for循环 格式:for 变量名 in 列表do 命令1 命令2 ……done
简介:
当变量值列表里,for循环即执行一次所有命令,使用变量名访问列表中取值。命令可为任何有效的shell命令和语句。变量名为任意单词。in列表用法是可选的,如果不用它,for循环使用命令行的位置参数。in列表可以包含替换、字符串和文件名 例程1:#!/bin/bash#fortest1for lop in 1 2 3 4 5do echo $loopdone
例程2:
#!/bin/bash#fortest2for loop in "orange red blue grey"do echo $loopdone
区别于
#!/bin/bash#fortest2for loop in orange red blue greydo echo $loopdone
例程3:
#!/bin/bash#fortest3for loop in `cat listfile`#``反斜杠里可以放shell命令#将listfile里面的字符串作为in的列表来用的具体格式(文件中的字符串以空格或回车作为分隔)do echo $loopdone
4.5.until循环
格式:until 条件do 命令1 命令2 ……done
简介:
条件可为任意测试条件,测试发生在循环末尾(先执行do和done中间的命令,再执行until里的判断,直到until里面的条件为真再停止循环),因此循环至少执行一次 例程1(可以用于服务器管理员监控分区,循环执行检查分区使用是不是还在安全范围内,并将结果mail给系统管理员):#!/bin/bash#监控分区#定义监控的分区Part="/backup"#得到磁盘使用的百分比LOOK_OUT=`df | grep $Part | awk '{print $5}' | sed 's/%//g'` #这条语句将``里面命令执行的结果赋值给LOOK_OUTecho $LOOK_OUTuntil[ "LOOK_OUT" -gt "90" ]do echo "Filesystem /backup is nearly full" | mail root LOOK_OUT=`df | grep $PART | awk '{print $5}' | sed 's/%//g'` sleep 3600 #程序休眠一小时done
4.6.while循环
格式:while 命令do 命令1 命令2 ……done
或
while 条件do 命令1 命令2 ……done
注意语法细节:
1.while的条件放在 [] 中 1.while与[ 之间必须有一个空格,否则会出错!!! 2.条件的 [ 之后 和 ]之前也必须有一个空格 简介: 在while和do之间虽然通常指使用一个命令,但可以放几个命令,命令通常用作测试条件 例程1:#!/bin/bash#whileread1echo "按住+D 退出输入"while echo -n "输入你最喜欢的电影:";read FILMdo echo "Yeah, ${FILM}是一部好电影"done
例程2:
#!/bin/bash#whileread2while read LINEdo echo $LINEdone < names.txt#这样是合法的格式,通过在done后面放上
类比格式:
#!/bin/bash#whileread2while read LINE < names.txtdo echo $LINEdone#这个格式将会无限循环只输出names.txt的第一行的内容
4.7.break控制
格式: break [ n ] break 其实就是 break 1 简介: 退出循环 如果是在一个嵌入循环中,可以指定n来跳出的循环的个数 例程1:#!/bin/bash#breakoutwhile :#while : 表示永远为真,注意while和: 之间一定要有一个空格,否则会有语法错误do echo -n "Enter any number[ 1...5 ]:" read ANS case $ANS in 1|2|3|4|5) echo "You enter a number between 1 and 5." ;; *) echo "Wrong bumber, Bye." break #break 用于跳出while循环,而不是用于跳出case,这里不同于C等语言 ;; esacdone
4.8.continue控制
简介: 跳出循环步 continue不同于break,continue不会跳出整个循环,而只是跳出这个循环中的某一步而已 例程1:#!/bin/bash#continuewhile:do echo -n "Enter any number [ 1...5 ]:" read ANS case $ANS in 1|2|3|4|5) echo "You enter a number between 1 and 5" ;; *) echo -n "Wrong number, continue(y/n)?:" read IS_CONTINUE case $IS_CONTINUE in y|yes|Y|YES continue ;; *) break ;; esac esacdone