Linuxシェルスクリプトプログラミングにおけるif文の使い方（条件判断）

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