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  

if [ 条件表达式 ] then  command  command  command else  command  command fi

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 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) 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                                                                                      -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 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，shell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

[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: 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: 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: 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 [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

[ -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

[ -f /usr/sbin/dhcpd ] || exit 0    检验文件是否存在，如果存在就退出 

[ -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

#!/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