【轉】shell編程if語句

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 的三種條件表達式

if command then if 函數 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	表達式結果爲真，則返回0，if把0值引向then
if test expression_r_r_r then	表達式結果爲假，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"

&& 可以理解爲then 如果左邊的表達式爲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爲真，走then	正好相反，非0走then
不支持整數變量直接if 必須:if [ i –ne 0 ] 但支持字符串變量直接if if [ str ] 如果字符串非0	支持變量直接if if (i )

=================================以command作爲if 條件=================================== 以多條command或者函數作爲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 $user then 上邊的指令都執行成功,返回值$?爲0，0爲真，運行then

　　echo "$user has logged"

else 指令執行失敗，$?爲1，運行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

以函數作爲if條件 (函數就相當於command,函數的優點是其return值可以自定義)

if 以函數作爲if條件， getyn then 函數reture值0爲真，走then echo " your answer is yes" else 函數return值非0爲假，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 | 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

========================================以條件表達式作爲 if條件=============================
傳統if 從句子——以條件表達式作爲 if條件 if [ 條件表達式 ] then command command command else command command fi 條件表達式

• 文件表達式

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

• 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<

• 字符串變量表達式

If [ $a = $b ]   如果string1等於string2 字符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 爲空 if [ $sting ]           如果string 非空，返回0 (和-n類似)  
條件表達式引用變量要帶$

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

[macg@machome ~]$ sh test.sh input a: 5 input b: 5 no equal （等於表達式沒比較$a和$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

[macg@machome ~]$ sh test.sh input a: 5 input b: 5 equal

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

[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
期望整數形式，即-eq不支持字符串

=放在別的地方是賦值,放在if [ ] 裏就是字符串等於,shell裏面沒有==的,那是c語言的等於
無空格的字符串，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

= 作爲等於時，其兩邊都必須加空格，否則失效等號也是操作符，必須和其他變量，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）

[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" ] 在等號兩邊加空格 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 輸錯了也走then,都走then,爲什麼? 因爲if把$var="yes"連讀成一個變量，而此變量爲空，返回1，則走else	[macg@machome ~]$ 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 ] 等價於 if [ -n $ANS ]如果字符串變量非空（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:                 回車 empth 說明“回車”就是空串[macg@machome ~]$ sh test.sh input your choice: 34 no empty

整數條件表達式，大於，小於，shell裏沒有> 和< ,會被當作尖括號，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

整數操作符號-ge,-gt,-le,-lt, 別忘了加-

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

============================邏輯表達式========================================= 邏輯非 ! 條件表達式的相反 if [ ! 表達式 ] if [ ! -d $num ] 如果不存在目錄$num
邏輯與 –a 條件表達式的並列 if [ 表達式1 –a 表達式2 ]
邏輯或 -o 條件表達式的或 if [ 表達式1 –o 表達式2 ]
邏輯表達式

• 表達式與前面的= != -d –f –x -ne -eq -lt等合用

• 邏輯符號就正常的接其他表達式，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很容易和其他字符串或文件的運算符號搞混了

最常見的賦值形式，賦值前對=兩邊的變量都進行評測左邊測變量是否爲空，右邊測目錄(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 ] 如果變量$JHHOME爲空，且$HOME/$num目錄存在 then
JHHOME=$HOME/$num     則賦值 fi
echo "JHHOME is $JHHOME"

----------------------- [macg@mac-home ~]$ 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

[macg@mac-home ~]$ mkdir ppp [macg@mac-home ~]$ 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

[macg@machome ~]$ sh test.sh input your choice: n y [macg@machome ~]$ sh test.sh input your choice: no y 爲什麼輸入不是yes,結果仍是y(走then） 因爲=被連讀了，成了變量$ANS="Yes"，而變量又爲空，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 條件表達式作爲if條件===================================
if test $num -eq 0 等價於 if [ $num –eq 0 ]
test 表達式,沒有 [ ] 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 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 文件是否存在，如果存在就刪掉

|| 如果不是“前面”，則後面 [ -f /usr/sbin/dhcpd ] || exit 0 檢驗文件是否存在，如果存在就退出

用簡化 if 和$1,$2,$3來檢測參數，不合理就調用help [ -z "$1" ] && help 如果第一個參數不存在（-z 字符串長度爲0 ） [ "$1" = "-h" ] && help 如果第一個參數是-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