Shell中的流程控制

shell裡面的流程控制不能為空，因此如果else裡面沒有語句就不要寫else，貌似也可以在else裡面寫乙個":"表示什麼都不做。源自實驗樓的高階 bash 指令碼程式設計指南課程。

if語句的語法格式為：

if condition

then
command 1
command 2
...command n
fi

if condition

then
command 1
command 2
...command n
else
command 1
command 2
...command m
fi

if condition1

then
command 1
command 2
...command n
elif condition2
then
command 1
command 2
...command m
else
command 1
command 2
...command p
fi

a=10

b=20
if[$a -eq $b
]then
echo
"a == b"
elif
[$a -gt $b
]then
echo
"a > b"
elif
[$a -lt $b
]then
echo
"a < b"
else
echo
"ineligible"
fi

for var in item1 item2 ... itemn

docommand 1
command 2
...command n
done

for loop in 1 2 3 4 5

doecho
"the value is: $loop"
done

the value is: 1

the value is: 2
the value is: 3
the value is: 4

for str in this is a string

doecho
$str
done

thisisa

string

while condition

docommand 1
command 2
...command n
done

a=1

while[
$a -lt 6 ]
doecho
$alet
"a++"
#用於指定算數運算子，即 let expretion，let只能執行整數的相關操作，也可以寫為 let a=$a+1
done

123

45

echo

'press exit'
echo -n 'who do you think is the most handsome: '
while
read man
doecho
"yes！$man is really handsome"
done

無限迴圈的語法格式為：

while

:# ":"在shell中表示空命令，也可以視為true。
docommand 1
command 2
...command n
done

while

true
docommand 1
command 2
...command n
done

for((;

;))

until conditon

docommand 1
command 2
...command n
done

a=1

until[
$a -gt 6 ]
doecho
$alet
"a++"
#用於指定算數運算子，即 let expretion，let只能執行整數的相關操作，也可以寫為 let a=$a+1
done

123

456

case var in

item 1)
command 1
command 2
...command n ;;
item 2)
command 1
command 2
...command m ;;
esac

c=

"ccc"
read man
case
$man
in"aaa"
)echo
"aaa";;
"bbb"
)echo
"bbb";;
$c)echo
"ccc";;
*)echo
"else";;
esac

break命令允許跳出所有迴圈（終止執行後面的所有迴圈）。

例如：

while:do

echo -n "enter a number between 1 and 5: "
read anum
case
$anum
in 1|2|3|4|5)
echo
"the number you entered is $anum!";;
*)echo
"the number you entered is not between 1 and 5! game over!!!"
break;;
esac
done

enter a number between 1 and 5: 5

the number you entered is 5!
enter a number between 1 and 5: 6
the number you entered is not between 1 and 5! game over!
!!

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