bash編程的循環：while和until
while 測試條件; do
語句1
語句2
...
done
Count=2
if [ $Count -le 5 ]; then
fi
while [ $Count -le 5 ]; do
let Count++
done
條件滿足時就循環，直到條件不再滿足，則退出循環；
for如何退出循環？遍歷列表中的元素完成；
while循環呢？在循環體改變測試條件相應變量等的值。
補充：算術運算符
Sum=$[$Sum+$I]
Sum+=$I
+=
-=
*=
/=
%=
Sum=$[$Sum+1]
Sum+=1
let Sum++
let Count--
and --> AND
exit --> EXIT
quit
計算100以內所有正整數的和；
Sum=0
for I in {1..100}; do
Sum+=$I
done
echo $Sum
#!/bin/bash
Sum=0
Count=1
while [ $Count -le 100 ]; do
let Sum+=$Count
let Count++
done
echo $Sum
計算100以內所有偶數的和；
#!/bin/bash
#
EvenSum=0
Count=0
while [ $Count -le 100 ]; do
let Sum+=$Count
let Count+=2
done
while [ $Count -le 100 ]; do
if [ $[$Count%2] -eq 0 ]; then
Sum+=$Count
fi
let Count++
done
while循環遍歷文件的每一行：
while read LINE; do
statement1
statement2
...
done < /path/to/somefile
練習：如果用戶的ID號爲偶數，則顯示其名稱和shell；對所有用戶執行此操作；
while read LINE; do
Uid=`echo $LINE | cut -d: -f3`
if [ $[$Uid%2] -eq 0 ]; then
echo $LINE | cut -d: -f1,7
fi
done < /etc/passwd
練習：轉換用戶輸入的字符爲大寫，除了quit（遇見quit退出）；
read -p "A string: " String
while [ "$String" != 'quit' ]; do
echo $String | tr 'a-z' 'A-Z'
read -p "Next [quit for quiting]: " String
done
until 測試條件; do
語句1
語句2
...
done
條件不滿足就循環，直到滿足時，退出循環；
例子：計算100以內所有正整數的和；
Sum=0
Count=1
until [ $Count -gt 100 ]; do
let Sum+=$Count
let Count++
done
echo $Sum
練習：轉換用戶輸入的字符爲大寫，除了quit（遇見quit退出）；
read -p "A string: " String
until [ "$String" == 'quit' ]; do
echo $String | tr 'a-z' 'A-Z'
read -p "Next [quit for quiting]: " String
done
練習：每隔5秒查看hadoop用戶是否登錄，如果登錄，顯示其登錄並退出；否則，顯示當前時間，並說明hadoop尚未登錄：
who | grep "^hadoop" &> /dev/null
RetVal=$?
until [ $RetVal -eq 0 ]; do
date
sleep 5
who | grep "^hadoop" &> /dev/null
RetVal=$?
done
echo "hadoop is here."
until who | grep "^hadoop" &> /dev/null; do
date
sleep 5
done
echo "hadoop is here."
who | grep "^hadoop" &> /dev/null
RetVal=$?
while [ $RetVal -ne 0 ]; do
date
sleep 5
who | grep "^hadoop" &> /dev/null
RetVal=$?
done
echo "hadoop is here."
練習：使用while循環統計/etc/rc.d/init.d/functions中#開頭的行數；
let Count=0
declare -i Count=0
#!/bin/bash
Count=0
while read LINE ; do
if echo $LINE | grep "^[[:space:]]*#" &> /dev/null ; then
let Count++
fi
done < /etc/rc.d/init.d/functions
echo -e "\n\ttotal $Count lines start with #.\n"
#!/bin/bash
#
Num=0
while read LINE; do
echo $LINE |grep "^#.*" &> /dev/null && let Num++
done < /etc/rc.d/init.d/functions
echo "#Number: $Num"
計算100以內所有偶數的和以及奇數的和；在一個循環中實現；
要求：分別使用while和until各自獨立實現。
#!/bin/bash
Evensum=0
Oddsum=0
Count=0
while [ $Count -le 100 ];do
if [ $[$Count%2] -eq 0 ];then
let Evensum+=$Count
else
let Oddsum+=$Count
fi
let Count++
done
echo "The Evensum is : $Evensum"
echo "The Oddsum is : $Oddsum"
#!/bin/bash
Evensum=0
Oddsum=0
Count=0
until [ $Count -gt 100 ]; do
if [ $[$Count%2] -eq 0 ];then
let Evensum+=$Count
else
let Oddsum+=$Count
fi
let Count++
done
echo "The Evensum is : $Evensum"
echo "The Oddsum is : $Oddsum"