if条件 then Command else Command @H_502_14@fi别忘了这个结尾 |
If语句忘了结尾fi test.sh: line 14: Syntax error:@H_502_14@unexpected end of fi |
@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@if 的三种条件表达式
if command then if 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then |
if test expression_r_r_r then |
表达式结果为假,则返回非0,if把非0值引向then |
@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@
@H_502_14@@H_502_14@@H_502_14@@H_502_14@[ ] &&――快捷if
[ -f "/etc/shadow" ] && echo "This computer uses shadow passwors" |
@H_502_14@&& 可以理解为then 如果左边的表达式为真则执行右边的语句 |
@H_502_14@
@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@shell的if与c语言if的功能上的区别
shell if | c语言if |
@H_502_14@0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 �Cne 0 ] 但支持字符串变量直接if if [ str ] 如果字符串非0 |
支持变量直接if if (i ) |
@H_502_14@
@H_502_14@=================================以command作为if 条件===================================
@H_502_14@@H_502_14@
@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@以多条command或者函数作为if 条件
echo �Cn “input:” read user if 多条指令,这些命令之间相当于@H_502_14@“and”(与) grep $user /etc/passwd >/tmp/null who -u | grep $user then上边的指令@H_502_14@都执行成功,返回值$?为0,@H_502_14@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.sh input : macg macgpts/0May 15 15:55.2075 (192.168.1.100) macg has logged # sh test.sh input : ddd ddd has not logged |
@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@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, getyn then函数reture值@H_502_14@0为真,走then echo " your answer is yes" else函数return值非0为假,走else echo "your anser is no" fi |
@H_502_14@@H_502_14@@H_502_14@@H_502_14@if command等价于 command+if $?
$ vi testsh.sh #!/bin/sh @H_502_14@if @H_502_14@cat 111-tmp.txt | grep ting1 @H_502_14@then echo found else echo "no found" fi |
$ vi testsh.sh #!/bin/sh @H_502_14@cat 111-tmp.txt | grep ting1 @H_502_14@if [ $? -eq 0 ] @H_502_14@then @H_502_14@echo $? echo found else @H_502_14@echo $? echo "no found" fi |
$ sh testsh.sh no found |
$ sh testsh.sh 1 no found |
$ vi 111-tmp.txt that is 222file thisting1 is 111file $ sh testsh.sh thisting1 is 111file found |
$ vi 111-tmp.txt that is 222file thisting1 is 111file $ sh testsh.sh thisting1 is 111file 0 found |
@H_502_14@@H_502_14@@H_502_14@
@H_502_14@========================================以条件表达式作为 if条件=============================
@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@传统if 从句子――以条件表达式作为 if条件
@H_502_14@if [ 条件表达式 ]
@H_502_14@then
@H_502_14@@H_502_14@command
@H_502_14@@H_502_14@command
@H_502_14@@H_502_14@command
@H_502_14@else
@H_502_14@@H_502_14@command
@H_502_14@@H_502_14@command
@H_502_14@fi
@H_502_14@@H_502_14@@H_502_14@
@H_502_14@条件表达式
@H_502_14@if [ -ffile ]如果文件存在
@H_502_14@if [ -d ...]如果目录存在
@H_502_14@if [ -s file]如果文件存在且非空
@H_502_14@if [ -r file@H_502_14@@H_502_14@]@H_502_14@@H_502_14@@H_502_14@@H_502_14@如果文件存在且可读
@H_502_14@if [ -w file@H_502_14@@H_502_14@] @H_502_14@@H_502_14@@H_502_14@如果文件存在且可写
@H_502_14@if [ -x file@H_502_14@@H_502_14@] @H_502_14@@H_502_14@@H_502_14@如果文件存在且可执行 @H_502_14@
- @H_301_410@整数变量表达式
@H_502_14@if [ int1 -eq int2 ]如果int1等于int2
@H_502_14@if [ int1 -ne int2 ]如果不等于 @H_502_14@
@H_502_14@if [ int1 -ge int2 ]@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@如果>=
@H_502_14@if [ int1 -gt int2 ]@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@如果>
@H_502_14@if [ int1 -le int2 ]@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@如果<=
@H_502_14@if [ int1 -lt int2 ]@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@如果<
@H_502_14@@H_502_14@@H_502_14@
- @H_301_410@字符串变量表达式
@H_502_14@If@H_502_14@@H_502_14@[ $a = $b ]@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@如果string1等于string2
@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@字符串允许使用赋值号做等号
@H_502_14@if@H_502_14@@H_502_14@[ $string1 !=@H_502_14@@H_502_14@$string2 ]@H_502_14@@H_502_14@@H_502_14@如果string1不等于string2@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@
@H_502_14@if[ -n $string]如果string 非空(非0),返回0(true)@H_502_14@@H_502_14@
@H_502_14@if@H_502_14@@H_502_14@[ -z $string@H_502_14@@H_502_14@]@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@如果string 为空
@H_502_14@if[ $sting ]如果string 非空,返回0 (和-n类似) @H_502_14@@H_502_14@
@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@条件表达式引用变量要带$
if [ a = b ] ;then echo equal else echo no equal fi |
[macg@machome ~]$ sh test.sh input a: 5 input b: 5 @H_502_14@no equal(等于表达式没比较$a和$b,而是比较和a和b,自然a!=b) |
@H_502_14@改正:
if [ $a = $b ] ;then echo equal else echo no equal fi |
[macg@machome ~]$ sh test.sh input a: 5 input b: 5 equal |
@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@
@H_502_14@@H_502_14@@H_502_14@-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.sh echo -n "input your choice:" read var if[ $var -eq "yes" ] then echo $var fi [macg@machome ~]$ sh -x test.sh input your choice: y @H_502_14@test.sh: line 3: test: y: integer expression_r_r_r expected @H_502_14@期望整数形式,即-eq不支持字符串 |
@H_502_14@@H_502_14@@H_502_14@=放在别的地方是赋值,放在if [ ] 里就是字符串等于,@H_502_14@shell里面没有==的,那是c语言的等于
@H_502_14@@H_502_14@@H_502_14@@H_502_14@无空格的字符串,可以加" ",也可以不加
[macg@machome ~]$ vi test.sh echo "input a:" read a echo "input is $a" if [ $a = 123 ] ; then echo equal123 fi |
[macg@machome ~]$ sh test.sh input a: 123 input is 123 equal123 |
@H_502_14@@H_502_14@= 作为等于时,其两边都必须加空格,否则失效
@H_502_14@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.sh echo "input your choice:" read var if [@H_502_14@$var="yes"] then echo $var echo "input is correct" else echo $var echo "input error" fi |
[macg@machome ~]$ vi test.sh echo "input your choice:" read var if [@H_502_14@$var = "yes"]在等号两边加空格 then echo $var echo "input is correct" else echo $var echo "input error" fi |
[macg@machome ~]$ sh test.sh input your choice: y y input is correct [macg@machome ~]$ sh test.sh input your choice: n n input is correct 输错了也走then,都走then,为什么? @H_502_14@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.sh input your choice: y y input error [macg@machome ~]$ sh test.sh input your choice: no no input error 一切正常 |
@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@If[$ANS]等价于if [ -n $ANS ]
@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@如果字符串变量非空(then),空(else)
echo "input your choice:" read ANS if [ $ANS ] then echo no empty else echo empth fi |
[macg@machome ~]$ sh test.sh input your choice:回车 empth@H_502_14@说明“回车”就是空串 [macg@machome ~]$ sh test.sh input your choice: 34 no empty |
@H_502_14@
@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@整数条件表达式,大于,小于@H_502_14@,s@H_502_14@hell里没有> 和<,会被当作尖括号,只有-ge,-gt,-le,lt
[macg@machome ~]$ vi test.sh echo "input a:" read a if[ $a -ge 100 ] ; then echo 3bit else echo 2bit fi |
[macg@machome ~]$ sh test.sh input a: 123 3bit [macg@machome ~]$ sh test.sh input a: 20 2bit |
@H_502_14@@H_502_14@整数操作符号-ge,-lt,别忘了加-
iftest $a@H_502_14@ge100 ; then [macg@machome ~]$ sh test.sh test.sh: line 4: test: ge: binary operator expected |
iftest $a -ge 100 ; then [macg@machome ~]$ sh test.sh input a: 123 3bit |
@H_502_14@============================逻辑表达式=========================================
@H_502_14@
@H_502_14@逻辑非 !条件表达式的相反
@H_502_14@if [ ! 表达式 ]
@H_502_14@if [ ! -d $num ]@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@如果不存在目录$num
@H_502_14@逻辑与 �Ca@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@条件表达式的并列
@H_502_14@if [ 表达式1@H_502_14@@H_502_14@�Ca@H_502_14@@H_502_14@表达式2 ]
@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@逻辑或 -o@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@条件表达式的或
@H_502_14@if [ 表达式1@H_502_14@@H_502_14@�Co 表达式2 ]
@H_502_14@@H_502_14@@H_502_14@@H_502_14@
逻辑表达式
@H_502_14@if [ -z "$JHHOME" -a -d $HOME/$num ]
@H_502_14@@H_502_14@@H_502_14@最常见的赋值形式,赋值前对=两边的变量都进行评测
@H_502_14@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 num echo "input is $num" @H_502_14@if [ -z "$JHHOME" -a -d $HOME/$num ]如果变量$JHHOME为空,且$HOME/$num目录存在 @H_502_14@then @H_502_14@JHHOME=$HOME/$num则赋值 fi echo "JHHOME is $JHHOME" |
----------------------- [macg@mac-home ~]$ sh test.sh input the num: ppp input is ppp JHHOME is 目录@H_502_14@-d $HOME/$num不存在,所以$JHHOME没被then赋值 |
[macg@mac-home ~]$ mkdir ppp [macg@mac-home ~]$ sh test.sh input the num: ppp input is ppp JHHOME is /home/macg/ppp |
@H_502_14@@H_502_14@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 ANS if [@H_502_14@$ANS="Yes"-o $ANS="yes" -o $ANS="y" -o $ANS="Y" ] then ANS="y" else ANS="n" fi echo $ANS |
[macg@machome ~]$ sh test.sh input your choice: n y [macg@machome ~]$ sh test.sh input your choice: no y 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@H_502_14@$ANS="Yes",而变量又为空,所以走else了 |
[macg@machome ~]$ vi test.sh echo "input your choice:" read ANSecho "input your choice:" read ANS if [@H_502_14@$ANS = "Yes"-o $ANS = "yes" -o $ANS = "y" -o $ANS = "Y" ] then ANS="y" else ANS="n" fi echo $ANS |
[macg@machome ~]$ sh test.sh input your choice: no n [macg@machome ~]$ sh test.sh input your choice: yes y [macg@machome ~]$ sh test.sh input your choice: y y |
@H_502_14@===================以@H_502_14@@H_502_14@test 条件表达式 作为if条件===================================
@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@if test $num -eq 0等价于if [ $num �Ceq 0 ]
@H_502_14@@H_502_14@@H_502_14@test表达式,没有 []
@H_502_14@if test $num -eq 0@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@
@H_502_14@then
@H_502_14@echo "try again"
@H_502_14@else
@H_502_14@echo "good"
@H_502_14@fi
@H_502_14@man test
[macg@machome ~]$ 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 FILE FILE exists and is executable |
@H_502_14@======================if简化语句=================================
@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@最常用的简化if语句
@H_502_14@&& 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 |
@H_502_14@||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 |
@H_502_14@
@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@用简化 if 和$1,$2,$3来检测参数,不合理就调用help
@H_502_14@[ -z "$1" ] && help如果第一个参数不存在(-z字符串长度为0 )
@H_502_14@[ "$1" = "-h" ] && help@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@如果第一个参数是-h,就显示help
@H_502_14@例子
@H_502_14@#!/bin/sh
@H_502_14@[ -f "/etc/sysconfig/network-scripts/ifcfg-eth0" ] && rm -f /etc/sysconfig/network-scripts/ifcfg-eth0
@H_502_14@cp ifcfg-eth0.bridge /etc/sysconfig/network-scripts/ifcfg-eth0
@H_502_14@[ -f "/etc/sysconfig/network-scripts/ifcfg-eth1" ] && rm -f /etc/sysconfig/network-scripts/ifcfg-eth1
@H_502_14@cp ifcfg-eth1.bridge /etc/sysconfig/network-scripts/ifcfg-eth1
@H_502_14@[ -f "/etc/sysconfig/network-scripts/ifcfg-eth0:1" ] && rm -f /etc/sysconfig/network-scripts/ifcfg-eth0:1