if 語句格式

if  條件 then  Command else  Command fi                             別忘了這個結尾

If語句忘了結尾fi test.sh: line 14: syntax error: unexpected end of fi

    if 的三種條件表示式

if command then if  函式 then	命令執行成功，等於返回0 （比如grep ,找到匹配） 執行失敗，返回非0 （grep,沒找到匹配）
if [ expression_r_r_r  ] then	表示式結果為真，則返回0，if把0值引向then
if test expression_r_r_r then	表示式結果為假，則返回非0，if把非0值引向then

     
   [ ]&& ——快捷if

[ -f "/etc/shadow" ] && echo"This computer uses shadow passwors"

&&可以理解為then    如果左邊的表示式為真則執行右邊的語句

 

    shell的if與c語言if的功能上的區別

shell if	c語言if
0為真，走then	正好相反，非0走then
不支援整數變數直接if 必須:if [ i –ne 0 ] 但支援字串變數直接if if [ str ] 如果字串非0	支援變數直接if if (i )

 
=================================以command作為if條件===================================
  
    以多條command或者函式作為if 條件

echo –n “input:” read user if 多條指令,這些命令之間相當於“and”（與） grep $user /etc/passwd>/tmp/null       who -u | grep $user then            上邊的指令都執行成功,返回值$?為0，0為真，執行then  echo "$user has logged" else    指令執行失敗，$?為1，執行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

    以函式作為if條件 (函式就相當於command,函式的優點是其return值可以自定義)

if 以函式作為if條件， getyn then   函式reture值0為真，走then echo " your answer is yes" else  函式return值非0為假，走else echo "your anser is no" fi

   if command  等價於 command+if$?

$ vi testsh.sh #!/bin/sh if cat 111-tmp.txt | grepting1 then echo found else echo "no found" fi	$ vi testsh.sh #!/bin/sh cat 111-tmp.txt | grepting1 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

   
========================================以條件表示式作為if條件=============================

    傳統if 從句子——以條件表示式作為 if條件
if [ 條件表示式 ]
then
 command
 command
 command
else
 command
 command
fi
   
  條件表示式

• 檔案表示式

if [ -f  file]   如果檔案存在
if [ -d...  ]   如果目錄存在
if [ -s file ]   如果檔案存在且非空 
if [ -r file ]   如果檔案存在且可讀
if [ -w file  ]   如果檔案存在且可寫
if [ -x file  ]   如果檔案存在且可執行  

• 整數變量表達式

if [ int1 -eq int2]   如果int1等於int2   
if [ int1 -ne int2]    如果不等於   
if [ int1 -ge int2]      如果>=
if [ int1 -gt int2]      如果>
if [ int1 -le int2]      如果<=
if [ int1 -lt int2]      如果<
   

•    字串變量表達式

If  [ $a = $b]                如果string1等於string2
                               字串允許使用賦值號做等號
if  [ $string1 !=  $string2]  如果string1不等於string2       
if  [ -n$string ]           如果string 非空(非0），返回0(true) 
if  [ -z $string ]            如果string 為空
if  [ $sting]                如果string非空，返回0 (和-n類似)   


    條件表示式引用變數要帶$

if [ a = b ];then     echo equal else echo no equal fi

[[email protected] ~]$ sh test.sh input a: 5 input b: 5 no equal （等於表示式沒比較$a和$b,而是比較和a和b,自然a!=b)

改正：

if [ $a = $b ];then        echo equal else echo no equal fi

[[email protected] ~]$ sh test.sh input a: 5 input b: 5 equal

                                                                                   
  -eq  -ne -lt  -nt只能用於整數，不適用於字串，字串等於用賦值號=

[[email protected] ~]$ vi test.sh echo -n "input your choice:" read var if  [ $var -eq "yes" ] then echo $var fi [[email protected] ~]$ sh -x test.sh input your choice: y test.sh: line 3: test: y: integerexpression_r_r_r expected                       期望整數形式，即-eq不支援字串

    =放在別的地方是賦值,放在if [ ]裡就是字串等於,shell裡面沒有==的,那是c語言的等於


   無空格的字串，可以加" ",也可以不加

[[email protected] ~]$ vi test.sh echo "input a:" read a echo "input is $a" if [ $a = 123 ] ; then echo equal123 fi

[[email protected] ~]$ sh test.sh input a: 123 input is 123 equal123

    =作為等於時，其兩邊都必須加空格，否則失效
等號也是操作符，必須和其他變數，關鍵字，用空格格開 (等號做賦值號時正好相反，兩邊不能有空格）

[[email protected] ~]$ vi test.sh echo "input your choice:" read var if [ $var="yes" ] then echo $var echo "input is correct" else echo $var echo "input error" fi	[[email protected] ~]$ vi test.sh echo "input your choice:" read var if [ $var = "yes"]   在等號兩邊加空格 then echo $var echo "input is correct" else echo $var echo "input error" fi
[[email protected] ~]$ sh test.sh input your choice: y y input is correct [[email protected] ~]$ sh test.sh input your choice: n     n input is correct  輸錯了也走then,都走then,為什麼? 因為if把$var="yes"連讀成一個變數，而此變數為空，返回1，則走else	[[email protected] ~]$ sh test.sh input your choice: y y input error [[email protected] ~]$ sh test.sh input your choice: no                        no input error 一切正常

    If [  $ANS ]    等價於  if [ -n $ANS ]
     如果字串變數非空（then） , 空(else)

echo "input your choice:" read ANS if [ $ANS ] then echo no empty else echo empth fi

[[email protected] ~]$ sh test.sh input yourchoice:                   回車                                                  empth                                   說明“回車”就是空串 [[email protected] ~]$ sh test.sh input your choice: 34 no empty

 
    整數條件表示式，大於，小於，shell裡沒有> 和<,會被當作尖括號，只有-ge,-gt,-le,lt

[[email protected] ~]$ vi test.sh echo "input a:" read a if  [ $a -ge 100 ] ; then echo 3bit else echo 2bit fi

[[email protected] ~]$ sh test.sh input a: 123 3bit [[email protected] ~]$ sh test.sh input a: 20 2bit

 整數操作符號-ge,-gt,-le,-lt, 別忘了加-

if  test $a  ge 100 ; then [[email protected] ~]$ sh test.sh test.sh: line 4: test: ge: binary operator expected

if  test $a -ge 100 ; then [[email protected] ~]$ sh test.sh input a: 123 3bit

============================邏輯表示式=========================================
   邏輯非!                  條件表示式的相反
if [ ! 表示式 ]
if [ ! -d $num]                      如果不存在目錄$num


   邏輯與–a                   條件表示式的並列
if [ 表示式1  –a  表示式2 ]


    邏輯或-o                   條件表示式的或
if [ 表示式1  –o 表示式2 ]

  
   邏輯表示式

•    表示式與前面的=  != -d –f –x -ne -eq -lt等合用

•    邏輯符號就正常的接其他表示式，沒有任何括號（ ），就是並列

if [ -z "$JHHOME" -a -d $HOME/$num ]

•    注意邏輯與-a與邏輯或-o很容易和其他字串或檔案的運算子號搞混了

  最常見的賦值形式，賦值前對=兩邊的變數都進行評測
左邊測變數是否為空，右邊測目錄(值)是否存在（值是否有效）

[[email protected] ~]$ vi test.sh : echo "input the num:" read num echo "input is $num" if [ -z "$JHHOME" -a -d $HOME/$num]  如果變數$JHHOME為空，且$HOME/$num目錄存在 then JHHOME=$HOME/$num                    則賦值 fi echo "JHHOME is$JHHOME"

----------------------- [[email protected] ~]$ sh test.sh input the num: ppp input is ppp JHHOME is 目錄-d$HOME/$num  不存在，所以$JHHOME沒被then賦值

[[email protected] ~]$ mkdir ppp [[email protected] ~]$ sh test.sh input the num: ppp input is ppp JHHOME is /home/macg/ppp

   一個-o的例子，其中卻揭示了”=”必須兩邊留空格的問題

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

[[email protected] ~]$ sh test.sh input your choice: n y [[email protected] ~]$ sh test.sh input your choice: no y 為什麼輸入不是yes,結果仍是y(走then） 因為=被連讀了，成了變數$ANS="Yes"，而變數又為空，所以走else了

[[email protected] ~]$ 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

[[email protected] ~]$ sh test.sh input your choice: no n [[email protected] ~]$ sh test.sh input your choice: yes y [[email protected] ~]$ sh test.sh input your choice: y y

===================以  test 條件表示式作為if條件===================================

    if test $num -eq0     等價於   if [ $num –eq 0]

    test 表示式,沒有 [  ]
if test $num -eq0                
then
echo "try again"
else
echo "good"
fi

   man test

[[email protected] ~]$ man test [(1)                            UserCommands                           [(1) SYNOPSIS       test EXPRESSION       [ EXPRESSION ]       [-n] STRING              the length of STRING isnonzero         -n和直接$str都是非0條件       -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              FILEexists                                檔案存在       -f FILE              FILE exists and is a regularfile    檔案存在且是普通檔案       -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簡化語句=================================

    最常用的簡化if語句

&&如果是“前面”，則“後面” [ -f /var/run/dhcpd.pid ] && rm/var/run/dhcpd.pid   檢查 檔案是否存在，如果存在就刪掉

||  如果不是“前面”，則後面 [ -f /usr/sbin/dhcpd ] || exit0   檢驗檔案是否存在，如果存在就退出

 
    用簡化 if 和$1,$2,$3來檢測引數，不合理就呼叫help
[ -z "$1" ]&&help               如果第一個引數不存在（-z  字串長度為0 ）
[ "$1" = "-h" ] &&help                      如果第一個引數是-h,就顯示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

轉載自：http://blog.sina.com.cn/s/blog_6151984a0100ekl6.html