shell编程——if语句
原文地址：shell编程——if语句
作者：macg
if 语句格式
| if 条件 then Command else Command fi 别忘了这个结尾 |
| If语句忘了结尾fi test.sh: line 14: syntax error: unexpected end of fi |
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 |
[ ] && ——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if |
| 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 –ne 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 ) |
=================================以command作为if 条件===================================
以多条command或者函数作为if 条件
| echo –n “input:” read user if 多条指令,这些命令之间相当于“and”（与） grep $user /etc/passwd >/tmp/null who -u | grep $user then 上边的指令都执行成功,返回值$?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.sh input : macg macg pts/0 May 15 15:55 . 2075 (192.168.1.100) macg has logged # sh test.sh input : ddd ddd 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， getyn then 函数reture值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 |
if command 等价于 command+if $?
| $ vi testsh.sh #!/bin/sh if cat 111-tmp.txt | grep ting1 then echo found else echo "no found" fi |
$ vi testsh.sh #!/bin/sh cat 111-tmp.txt | grep ting1 if [ $? -eq 0 ] then echo $? echo found else 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 |
========================================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
command
else
command
command
fi
条件表达式
- 文件表达式
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 -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
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 equal else echo no equal fi |
| [macg@machome ~]$ sh test.sh input a: 5 input b: 5 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) |
| if [ $a = $b ] ;then echo equal else echo no equal fi |
| [macg@machome ~]$ sh test.sh input a: 5 input b: 5 equal |
-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 test.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.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 |
= 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.sh echo "input your choice:" read var if [ $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 [ $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? 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 |
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 ANS if [ $ANS ] then echo no empty else echo empth fi |
| [macg@machome ~]$ sh test.sh input your choice: 回车 empth 说明“回车”就是空串 [macg@machome ~]$ sh test.sh input your choice: 34 no empty |
整数条件表达式，大于，小于，s 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 |
整数操作符号-ge,-gt,-le,-lt, 别忘了加-
| if test $a ge 100 ; then [macg@machome ~]$ sh test.sh test.sh: line 4: test: ge: binary operator expected |
| if test $a -ge 100 ; then [macg@machome ~]$ sh test.sh input a: 123 3bit |
============================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
- 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 num echo "input is $num" if [ -z "$JHHOME" -a -d $HOME/$num ] 如果变量$JHHOME为空，且$HOME/$num目录存在 then JHHOME=$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-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 |
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 [ $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$ANS="Yes"，而变量又为空，所以走else了 |
| [macg@machome ~]$ vi test.sh echo "input your choice:" read ANS echo "input your choice:" read ANS if [ $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 |
===================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 $num -eq 0
then
echo "try again"
else
echo "good"
fi
man test
| [macg@machome ~]$ 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 FILE FILE 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-eth0
cp ifcfg-eth0.bridge /etc/sysconfig/network-scripts/ifcfg-eth0
[ -f "/etc/sysconfig/network-scripts/ifcfg-eth1" ] && rm -f /etc/sysconfig/network-scripts/ifcfg-eth1
cp 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