shell编程if条件判断.docx

1、shell编程if条件判断shell编程if条件判断if 语句格式if条件thenCommandelseCommandfi别忘了这个结尾If语句忘了结尾fitest.sh: line 14: syntax error:unexpected end of fiif 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_rthen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，i

2、f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 ifc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

3、“and”（与）grep $user /etc/passwd /tmp/nullwho -u | grep $userthen上边的指令都执行成功,返回值$?为0，0为真，运行thenecho $user has loggedelse指令执行失败，$?为1，运行else echo $user has not loggedfi# sh test.shinput : macgmacgpts/0May 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

4、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 nofiif 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 ting

5、1if $? -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

6、=传统if 从句子以条件表达式作为 if条件if 条件表达式 thencommandcommandcommandelsecommandcommandfi条件表达式 文件表达式if -ffile 如果文件存在if -d .如果目录存在if -s file如果文件存在且非空if -r file如果文件存在且可读if -w file 如果文件存在且可写if -x file 如果文件存在且可执行 整数变量表达式if int1 -eq int2 如果int1等于int2if int1 -ne int2 如果不等于 if int1 -ge int2 如果=if int1 -gt int2 如果if int

7、1 -le int2 如果=if int1 -lt int2 如果和 ,会被当作尖括号，只有-ge,-gt,-le,ltmacgmachome $ vi test.shecho input a:read aif $a -ge 100 ; thenecho 3bitelseecho 2bitfimacgmachome $ sh test.shinput a:1233bitmacgmachome $ sh test.shinput a:202bit整数操作符号-ge,-gt,-le,-lt, 别忘了加-iftest $age100 ; thenmacgmachome $ sh test.shtes

8、t.sh: line 4: test: ge: binary operator expectediftest $a -ge 100 ; thenmacgmachome $ sh test.shinput a:1233bit=逻辑表达式=逻辑非 !条件表达式的相反if ! 表达式 if ! -d $num 如果不存在目录$num逻辑与 a条件表达式的并列if 表达式1a表达式2 逻辑或 -o条件表达式的或if 表达式1o 表达式2 逻辑表达式 表达式与前面的=!= -d f x -ne -eq -lt等合用 逻辑符号就正常的接其他表达式，没有任何括号（），就是并列if -z $JHHOME -a

9、 -d $HOME/$num 注意逻辑与-a与逻辑或-o很容易和其他字符串或文件的运算符号搞混了最常见的赋值形式，赋值前对=两边的变量都进行评测左边测变量是否为空，右边测目录(值)是否存在（值是否有效）macgmac-home $ vi test.sh:echo input the num:read numecho input is $numif -z $JHHOME -a -d $HOME/$num 如果变量$JHHOME为空，且$HOME/$num目录存在thenJHHOME=$HOME/$num则赋值fiecho JHHOME is $JHHOME-macgmac-home $ sh t

10、est.shinput the num:pppinput is pppJHHOME is目录-d $HOME/$num不存在，所以$JHHOME没被then赋值macgmac-home $ mkdir pppmacgmac-home $ sh test.shinput the num:pppinput is pppJHHOME is /home/macg/ppp一个-o的例子，其中却揭示了”=”必须两边留空格的问题echo input your choice:read ANSif $ANS=Yes-o $ANS=yes -o $ANS=y -o $ANS=Y thenANS=yelseANS=

11、nfiecho $ANSmacgmachome $ sh test.shinput your choice:nymacgmachome $ sh test.shinput your choice:noy为什么输入不是yes,结果仍是y(走then）因为=被连读了，成了变量$ANS=Yes，而变量又为空，所以走else了macgmachome $ vi test.shecho input your choice:read ANSecho input your choice:read ANSif $ANS = Yes-o $ANS = yes -o $ANS = y -o $ANS = Y the

12、nANS=yelseANS=nfiecho $ANSmacgmachome $ sh test.shinput your choice:nonmacgmachome $ sh test.shinput your choice:yesymacgmachome $ sh test.shinput your choice:yy=以test 条件表达式作为if条件=if test $num -eq 0等价于if $num eq 0 test表达式,没有 if test $num -eq 0thenecho try againelseecho goodfiman testmacgmachome $ ma

13、n test(1)User Commands(1)SYNOPSIStest EXPRESSION EXPRESSION -n STRINGthe length of STRING is nonzero-n和直接$str都是非0条件-z STRINGthe length of STRING is zeroSTRING1 = STRING2the strings are equalSTRING1 != STRING2the strings are not equalINTEGER1 -eq INTEGER2INTEGER1 is equal to INTEGER2INTEGER1 -ge INTE

14、GER2INTEGER1 is greater than or equal to INTEGER2INTEGER1 -gt INTEGER2INTEGER1 is greater than INTEGER2INTEGER1 -le INTEGER2INTEGER1 is less than or equal to INTEGER2INTEGER1 -lt INTEGER2INTEGER1 is less than INTEGER2INTEGER1 -ne INTEGER2INTEGER1 is not equal to INTEGER2FILE1 -nt FILE2FILE1 is newer

15、 (modification date) than FILE2FILE1 -ot FILE2FILE1 is older than FILE2-b FILEFILE exists and is block special-c FILEFILE exists and is character special-d FILEFILE exists and is a directory-e FILEFILE exists文件存在-f FILEFILE exists and is a regular file文件存在且是普通文件-h FILEFILE exists and is a symbolic l

16、ink (same as -L)-L FILEFILE exists and is a symbolic link (same as -h)-G FILEFILE exists and is owned by the effective group ID-O FILEFILE exists and is owned by the effective user ID-p FILEFILE exists and is a named pipe-s FILEFILE exists and has a size greater than zero-S FILEFILE exists and is a socket-w FILEFILE exists and is writable-x