if 语句格式
if 的三种条件表达式
[ ] && ——快捷if
shell的if与c语言if的功能上的区别
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)
一个-o的例子,其中却揭示了”=”必须两边留空格的问题
===================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
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 )
[ "$1" = "-h" ] && help
如果第一个参数是-h,就显示help
例子
#!/bin/sh
[ -f "/etc/sysconfig/network-scripts/ifcfg-eth0" ] && rm -f /etc/sysconfig/network-scripts/ifcfg-eth0
cp ifcfg-eth0.bridge /etc/sysconfig/network-scripts/ifcfg-eth0
if then else fi |
If语句忘了结尾fi test.sh: line 14: syntax error: unexpected end of fi |
if command then if then | 执行失败,返回非0 (grep,没找到匹配) |
if [ expression_r_r_r then | |
if test expression_r_r_r then | |
[ -f "/etc/shadow" ] && echo "This computer uses shadow passwors" |
|
| |
0为真,走then | |
必须:if [ i –ne 0 ] 但支持字符串变量直接if if [ str ] 如果字符串非0 | if (i ) |
=放在别的地方是赋值,放在if [ ] 里就是字符串等于,shell里面没有==的,那是c语言的等于
[macg@machome ~]$ 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 |
============================逻辑表达式=========================================
if [ ! 表达式 ]
if [ ! -d $num ]
if [ 表达式1
if [ 表达式1
-
表达式与前面的= != -d –f –x -ne -eq -lt等合用
-
逻辑符号就正常的接其他表达式,没有任何括号( ),就是并列
左边测变量是否为空,右边测目录(值)是否存在(值是否有效)
|
[macg@mac-home ~]$ vi test.sh : echo "input the num:" read num echo "input is $num" if [ -z "$JHHOME" -a -d $HOME/$num ] then JHHOME=$HOME/$num 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 |
[macg@mac-home ~]$ mkdir ppp [macg@mac-home ~]$ sh test.sh input the num: ppp input is ppp JHHOME is /home/macg/ppp |
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 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
if test $num -eq 0
then
echo "try again"
else
echo "good"
fi
======================if简化语句=================================
[ -f /var/run/dhcpd.pid ] && rm /var/run/dhcpd.pid |
[ -f /usr/sbin/dhcpd ] || exit 0 |
[ -z "$1" ] && help
[ "$1" = "-h" ] && 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