shell编程if条件判断.docx

shell编程——if条件判断if 语句格式if??条件then?Commandelse?Commandfi??????????????????????????????别忘了这个结尾If语句忘了结尾fitest.sh: line 14: syntax error:?unexpected end of fi????if 的三种条件表达式ifcommandthenif?函数then?命令执行成功，等于返回0 （比如grep ,找到匹配）执行失败，返回非0 （grep,没找到匹配）if [ expression_r_r_r??]then??表达式结果为真，则返回0，if把0值引向thenif test expression_r_r_rthen?表达式结果为假，则返回非0，if把非0值引向then?????????[ ] ??——快捷if[ -f /etc/shadow ] echo This computer uses shadow passwors??? 可以理解为then????如果左边的表达式为真则执行右边的语句?????shell的if与c语言if的功能上的区别?shell if????c语言if0为真，走then?正好相反，非0走then?不支持整数变量直接if必须:if [ i –ne 0 ]但支持字符串变量直接ifif [ str ] 如果字符串非0?支持变量直接ifif (i )?=================================以command作为if 条件===================================??????以多条command或者函数作为if 条件echo –n “input:”read userif多条指令,这些命令之间相当于“and”（与）grep $user /etc/passwd /tmp/null??????who -u | grep $userthen?????????????上边的指令都执行成功,返回值$?为0，0为真，运行then?echo $user has loggedelse?????指令执行失败，$?为1，运行else ????????????????????????????echo $user has not loggedfi???# sh test.shinput : macgmacg?????pts/0????????May 15 15:55???.??????????2075 (192.168.1.100)macg has logged???# sh test.shinput : dddddd has not logged??????以函数作为if条件??(函数就相当于command,函数的优点是其return值可以自定义)if以函数作为if条件，getynthen???函数reture值0为真，走thenecho your answer is yeselse??函数return值非0为假，走elseecho your anser is nofi?????if command??等价于 command+if $?$ vi testsh.sh#!/bin/shifcat 111-tmp.txt | grep ting1thenecho foundelseecho no foundfi?$ vi testsh.sh#!/bin/shcat 111-tmp.txt | grep ting1if [ $? -eq 0 ]thenecho $?echo foundelseecho $?echo no foundfi$ sh testsh.shno found??$ sh testsh.sh1no found$ vi 111-tmp.txtthat is 222filethisting1 is 111file$ sh testsh.shthisting1 is 111filefound$ vi 111-tmp.txtthat is 222filethisting1 is 111file$ sh testsh.shthisting1 is 111file0found???========================================以条件表达式作为 if条件=============================????传统if 从句子——以条件表达式作为 if条件if [ 条件表达式 ]then?command?command?commandelse?command?commandfi??????条件表达式文件表达式if [ -f??file ]????如果文件存在if [ -d ...???]????如果目录存在if [