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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 的三種條件表達式

if
command
then if
函數
then
命令執行成功，等于返回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    表達式結果為真，則返回0，if把0值引向then
if test expression_r_r_r
then   表達式結果為假，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
    如果左邊的表達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 )

以多條command或者函數作為if 條件

復制代碼代碼如下:

echo –n “input:”
read user if
多條指令,這些命令之間相當于“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，運行then
echo "$user has logged"
else     指令執行失敗，$?為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       以函數作為if條件 (函數就相當于command,函數的優點是其return值可以自定義)

if
以函數作為if條件，
getyn
then 函數reture值0為真，走then
echo " your answer is yes"
else 函數return值非0為假，走else
echo "your anser is no"
fi if command 等價于 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

傳統if 從句子——以條件表達式作為 if條件

復制代碼代碼如下:

if [ 條件表達式 ]
then
 command
 command
 command
else
 command
 command
fi

條件表達式

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
if [ int1 -eq int2 ]    如果int1等于int2
if [ int1 -ne int2 ]    如果不等于
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

復制代碼代碼如下:

If [ $a = $b ]                 如果string1等于string2
                                字符串允許使用賦值號做等號
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 為空
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 （等于表達式沒比較$a和$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只能用于整數，不適用于字符串，字符串等于用賦值號=
[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 [ ] 里就是字符串等于,shell里面沒有==的,那是c語言的等于
   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 = 作為等于時，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
input is correct
輸錯了也走then,都走then,為什么?
因為if把$var="yes"連讀成一個變量，而此變量為空，返回1，則走else   [macg@machome ~]$ 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 ]     等價于 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，大于，小于，shell里沒有> 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      條件表達式的并列
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（），就是并列
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
左邊測變量是否為空，右邊測目錄(值)是否存在（值是否有效）

復制代碼代碼如下:

[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$JHHOME為空，且$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:

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:

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:

y
[macg@machome ~]$ sh test.sh
input your choice:
no
y
為什么輸入不是yes,結果仍是y(走then）
因為=被連讀了，成了變量$ANS="Yes"，而變量又為空，所以走else了
[macg@machome ~]$ 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 條件表達式作為if條件===================================
    if test $num -eq 0      等價于   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

最常用的簡化if語句

如果是“前面”，則“后面”

復制代碼代碼如下:

[ -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”，則后面

復制代碼代碼如下:

[ -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

復制代碼代碼如下:

[ -z "$1" ]  help                 如果第一個參數不存在（-z  字符串長度為0 ）
[ "$1" = "-h" ]  help                        如果第一個參數是-h,就顯示help 

例子

復制代碼代碼如下:

#!/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

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