UNIX Shell控制結構—IF
流控制（Decision Making）
IF語句有三種格式：
第一種：if ... fi statement
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下面是一個實例：
[plain]
cat if1.sh
#!/bin/sh
a=10
b=20
#①
if [ $a -eq $b ]; then
echo "a is equal to b";
fi
if [ $a -gt $b ]; then
echo "a is great than b";
fi
#②
if [ $a -lt $b ]
then
echo "a is less than b";
fi
# the EOF
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①條件和處理命令分開的一種寫法：
if 條件; then
處理命令
fi
②條件和處理命令分開的另一種寫法：
if 條件
then
處理命令
fi
這裡需要根據個人習慣去選擇。
上面的例子中，變量的值是賦死了的，若要給此腳本傳遞兩個參數，可做如下修改：
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cat if1.sh
#!/bin/sh
# a=10
# b=20
if [ $1 -eq $2 ]; then
echo "the first number is equal to the second";
fi
if [ $1 -gt $2 ]; then
echo "the first number is great than the second";
fi
if [ $1 -lt $2 ]
then
echo "the first number is less than the second";
fi
# the EOF
給腳本傳遞參數，只需要在sh命令後面添加即可，使用空格隔開：
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sh if1.sh 1 2
the first number is less than the second
sh if1.sh 12 1
the first number is great than the second
sh if1.sh 1 1
the first number is equal to the second
第二種：if ... else ... fi，具體如下：
if [ expression ]
then
statement(s) to be sxecuted if expression is true
else
statement(s) to be sxecuted if expression is not true
fi
一個簡單的實例
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cat ifparam.sh
#!/bin/sh
if [ $# -lt 3 ]; then
echo "Usage:`basename $0` arg1 arg2 arg3" >&2
exit 1
fi
#EOF
echo "arg1:$1"
echo "arg1:$2"
echo "arg1:$3"
腳本注解：
1.$#表示參數輸入的個數
2.basename $0打印文件的名稱
3.若輸入小於三個參數則，將輸出一個信息，這個信息被當做是錯誤信息（>&2）。
執行腳本：
sh ifparam.sh scott tom
Usage:ifparam.sh arg1 arg2 arg3
sh ifparam.sh scott tom jim
arg1:scott
arg1:tom
arg1:jim
再來看一個測試：
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cat ifeditor.sh
#!/bin/csh
#if [ -z $EDITOR ]①
#if [ -z "`echo $EDITOR`" ]③
#下面這種寫法不能正確的計算出環境的值
#因為wc -c計算包括新的空的行
#if [ `echo $EDITOR | wc -c` -eq 0 ]②
if [ -z "`echo $EDITOR`" ]
then
echo "Your EDITOR environment is not set"
else
echo "Using $EDITOR as the default editor"
fi
#EOF
sh ifeditor.sh
Your EDITOR environment is not set
這裡使用三種方式去檢測環境變量是否設置；
①直接報錯：語法錯誤
②正確寫法，使用test檢測，-z表示如果為設置則長度為0返回值為true
③wc -c計算包括了空行，所以不准確
第三種：if ... elif ... fi，具體如下：
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if [ expression 1 ]; then
statement(s) to be sxecuted if expression 1 is true
elif [ expression 2 ]; then
statement(s) to be sxecuted if expression 2 is true
elif [ expression 3 ]; then
statement(s) to be sxecuted if expression 3 is true
else
statement(s) to be sxecuted if no expression is true
fi
下面是一個比較兩個數字大小的例子：
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cat elif.sh
#!/bin/sh
if [ $1 == $2 ]; then
echo "the first number is equal to the next"
elif [ $1 -gt $2 ]; then
echo "the first number is great than the next"
elif [ $1 -lt $2 ]; then
echo "the first number is great than the next"
else
echo "None of the condition met"
fi
#EOF
當不輸入任何數字的時候，也就是為空，結果如下：
sh elif.sh
the first number is equal to the next
當輸入數字的時候，如下：
sh elif.sh 10 20
elif.sh: test: unknown operator ==
這種情況，我們可以將其錯誤信息輸入到一個文件當中，如下：
sh elif.sh 10 20 > log.txt 2>&1
cat log.txt
elif.sh: test: unknown operator ==
當將“==”號修改為-eq，結果如下：
sh elif.sh 10 20
the first number is less than the next
下面是一個if的實例，包括這三種命令格式；
腳本的作用是創建一個目錄，如果不輸入任何值，則打印腳本的作用說明；
輸入則提示是否創建，輸入非提示，則報錯誤，否則按提示走。
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#!/bin/sh
DIR=$1
if [ "$DIR" = "" ]; then
echo "Usage:`basename $0` directory to create" >&2
exit 1
fi
if [ -d $DIR ]; then
echo "Directory $DIR exists"
else
echo "The Directory does exist"
echo -n "Create it now?[y..n]:"
read ANS
if [ "$ANS" = "y" ] || [ "$ANS" = "Y" ]; then
echo "creating now"
mkdir $DIR > log.txt 2>&1
if [ $? -ne 0 ]; then
echo "Errors creating the directory $DIR" >&2
exit 1
fi
echo "Creating successful"
elif [ "$ANS" = "n" ] || [ "$ANS" = "N" ]; then
echo "Giving up creating directory $DIR"
else
echo "Bad input"
fi
fi
#EOF
1.不輸入參數
sh ifmkdir.sh
Usage:ifmkdir.sh directory to create
2.輸入一個存在的目錄，提示目錄已經存在：
sh ifmkdir.sh test
Directory test exists
查看確實有test目錄：
[ -d test ]
echo $?
0
創建test1目錄：sh ifmkdir.sh test1
The Directory does exist
Create it now?[y..n]:y
creating now
Creating successful
3.執行腳本，但不想創建目錄：
sh ifmkdir.sh test2
The Directory does exist
Create it now?[y..n]:n
Giving up creating directory test2
4.執行腳本時，不按照提示輸入：
sh ifmkdir.sh test2
The Directory does exist
Create it now?[y..n]:d
Bad input
