shell if 语句

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 "$user has logged"else 指令执行失败，$?为1，运行else echo "$user has not logged"fi # 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 yes"else 函数return值非0为假，走elseecho "your anser is no"fi if command 等价于 command+if $?$ vi testsh.sh#!/bin/shifcat 111-tmp.txt | grep ting1thenecho foundelseecho "no found"fi $ vi testsh.sh#!/bin/shcat 111-tmp.txt | grep ting1if [ $? -eq 0 ]thenecho $?echo foundelseecho $?echo "no found"fi$ 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 [ -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int2 if [ int1 -ne int2 ] 如果不等于 if [ int1 -ge int2 ] 如果>=if [ int1 -gt int2 ] 如果>if [ int1 -le int2 ] 如果<=if [ int1 -lt int2 ] 如果< 字符串变量表达式If [ $a = $b ] 如果string1等于string2 字符串允许使用赋值号做等号if [ $string1 != $string2 ] 如果string1不等于string2 if [ -n $string ] 如果string 非空(非0），返回0(true) if [ -z $string ] 如果string 为空if [ $sting ] 如果string 非空，返回0 (和-n类似) 条件表达式引用变量要带$if [ a = b ] ;then echo equalelseecho no equalfi[macg@machome ~]$ sh test.shinput a:5input b:5no equal （等于表达式没比较$a和$b,而是比较和a和b,自然a!=b)改正：if [ $a = $b ] ;then echo equalelseecho no equalfi[macg@machome ~]$ sh test.shinput a:5input b:5equal -eq -ne -lt -nt只能用于整数，不适用于字符串，字符串等于用赋值号=[macg@machome ~]$ vi test.shecho -n "input your choice:"read varif [ $var -eq "yes" ]thenecho $varfi[macg@machome ~]$ sh -x test.shinput your choice:ytest.sh: line 3: test: y: integer expression_r_r_r expected 期望整数形式，即-eq不支持字符串 =放在别的地方是赋值,放在if [ ] 里就是字符串等于,shell里面没有==的,那是c语言的等于 无空格的字符串，可以加" ",也可以不加[macg@machome ~]$ vi test.shecho "input a:"read aecho "input is $a"if [ $a = 123 ] ; thenecho equal123fi[macg@machome ~]$ sh test.shinput a:123input is 123equal123 = 作为等于时，其两边都必须加空格，否则失效等号也是操作符，必须和其他变量，关键字，用空格格开 (等号做赋值号时正好相反，两边不能有空格）[macg@machome ~]$ vi test.shecho "input your choice:"read varif [ $var="yes" ]thenecho $varecho "input is correct"elseecho $varecho "input error"fi[macg@machome ~]$ vi test.shecho "input your choice:"read varif [ $var = "yes" ] 在等号两边加空格thenecho $varecho "input is correct"elseecho $varecho "input error"fi[macg@machome ~]$ sh test.shinput your choice:yyinput is correct[macg@machome ~]$ sh test.shinput your choice:n ninput is correct 输错了也走then,都走then,为什么?因为if把$var="yes"连读成一个变量，而此变量为空，返回1，则走else [macg@machome ~]$ sh test.shinput your choice:yyinput error[macg@machome ~]$ sh test.shinput your choice:no noinput error一切正常 If [ $ANS ] 等价于 if [ -n $ANS ] 如果字符串变量非空（then） , 空(else)echo "input your choice:"read ANSif [ $ANS ]thenecho no emptyelseecho empthfi [macg@machome ~]$ sh test.shinput your choice: 回车 empth 说明“回车”就是空串[macg@machome ~]$ sh test.shinput your choice:34no empty 整数条件表达式，大于，小于，shell里没有> 和< ,会被当作尖括号，只有-ge,-gt,-le,lt[macg@machome ~]$ vi test.shecho "input a:"read aif [ $a -ge 100 ] ; thenecho 3bitelseecho 2bitfi[macg@machome ~]$ sh test.shinput a:1233bit[macg@machome ~]$ sh test.shinput a:202bit 整数操作符号-ge,-gt,-le,-lt, 别忘了加-if test $a ge 100 ; then[macg@machome ~]$ sh test.shtest.sh: line 4: test: ge: binary operator expectedif test $a -ge 100 ; then[macg@machome ~]$ 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1 –a 表达式2 ] 逻辑或 -o 条件表达式的或if [ 表达式1 –o 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 -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） [macg@mac-home ~]$ vi test.sh:echo "input the num:"read numecho "input is $num"if [ -z "$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 $JHHOME" -----------------------[macg@mac-home ~]$ sh test.shinput the num:pppinput is pppJHHOME is目录-d $HOME/$num 不存在，所以$JHHOME没被then赋值[macg@mac-home ~]$ mkdir ppp[macg@mac-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="y"elseANS="n"fiecho $ANS[macg@machome ~]$ sh test.shinput your choice:ny[macg@machome ~]$ 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$ANS="Yes"，而变量又为空，所以走else了[macg@machome ~]$ vi test.shecho "input your choice:"read ANS echo "input your choice:"read ANSif [ $ANS = "Yes" -o $ANS = "yes" -o $ANS = "y" -o $ANS = "Y" ]thenANS="y"elseANS="n"fiecho $ANS[macg@machome ~]$ sh test.shinput your choice:non[macg@machome ~]$ sh test.shinput your choice:yesy[macg@machome ~]$ 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 $num -eq 0 thenecho "try again"elseecho "good"fi man test[macg@machome ~]$ man test[(1) User Commands [(1)SYNOPSIS test EXPRESSION [ EXPRESSION ] [-n] STRING the length of STRING is nonzero -n和直接$str都是非0条件 -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 -nt FILE2 FILE1 is newer (modification date) 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 文件存在 -f FILE FILE exists and is a regular file 文件存在且是普通文件 -h FILE FILE exists and is a symbolic link (same as -L) -L FILE FILE exists and is a symbolic link (same as -h) -G FILE FILE exists and is owned by the effective group ID -O FILE FILE exists and is owned by the effective user ID -p FILE FILE exists and is a named pipe -s FILE FILE exists and has a size greater than zero -S FILE FILE exists and is a socket -w FILE FILE exists and is writable -x FILEFILE exists and is executable ======================if简化语句================================= 最常用的简化if语句 && 如果是“前面”，则“后面”[ -f /var/run/dhcpd.pid ] && rm /var/run/dhcpd.pid 检查 文件是否存在，如果存在就删掉 || 如果不是“前面”，则后面[ -f /usr/sbin/dhcpd ] || exit 0 检验文件是否存在，如果存在就退出 用简化 if 和$1,$2,$3来检测参数，不合理就调用help[ -z "$1" ] && help 如果第一个参数不存在（-z 字符串长度为0 ）[ "$1" = "-h" ] && help 如果第一个参数是-h,就显示help例子#!/bin/sh[ -f "/etc/sysconfig/network-scripts/ifcfg-eth0" ] && rm -f /etc/sysconfig/network-scripts/ifcfg-eth0cp ifcfg-eth0.bridge /etc/sysconfig/network-scripts/ifcfg-eth0[ -f "/etc/sysconfig/network-scripts/ifcfg-eth1" ] && rm -f /etc/sysconfig/network-scripts/ifcfg-eth1cp ifcfg-eth1.bridge /etc/sysconfig/network-scripts/ifcfg-eth1[ -f "/etc/sysconfig/network-scripts/ifcfg-eth0:1" ] && rm -f /etc/sysconfig/network-scripts/ifcfg-eth0:1