linux for循环怎么理解

for循环是Linux shell 中最常用的结构。for 循环有三种结构：一种结构是列表for循环；第二种结构是不带列表for循环；第三种结构是类C风格的for循环。

for var in list
do
commands
done

#!/bin/bash
for test in alabama alaska arizona arkansas california colorado
do
echo the next state is $test
done
>
the next state is alabama
the next state is alaska
the next state is arizona
the next state is arkansas
the next state is california
the next state is colorado

#!/bin/bash
for test in alabama alaska arizona arkansas california colorado
do
echo "the next state is $test"
done
echo "The las state we visited was $test"
test=connecticut
echo "wait, now we're visiting $test"
>
the next state is alabama
the next state is alaska
the next state is arizona
the next state is arkansas
the next state is california
the next state is colorado
The las state we visited was colorado
wait, now we're visiting connecticut

#!/bin/bash
for test in I don't know if this'll work
do
echo "word:$test"
done
>
word:I
word:dont know if thisll
word:work

#!/bin/bash
for test in nevada "New Hampshire" "New Mexico" "New York"
do
echo "Now going to $test"
done
>
Now going to nevada
Now going to New Hampshire
Now going to New Mexico
Now going to New York

#!/bin/bash
list="Alabama ALaska Arizona Arkansas Colorado"
list=$list" Connecticut"
for state in $list
do
echo "Have you ever visited $state?"
done
>
Have you ever visited Alabama?
Have you ever visited ALaska?
Have you ever visited Arizona?
Have you ever visited Arkansas?
Have you ever visited Colorado?
Have you ever visited Connecticut?

#!/bin/bash
file="states"
for state in `cat $file`
do
echo "Visit beautiful  $state"
done
#cat states
>
Alabama
Alaska
dddfac
dfsase
cvcvss
dfqzcvc
ddddwq
bnnn
qqojfaso
>
Visit beautiful  Alabama
Visit beautiful  Alaska
Visit beautiful  dddfac
Visit beautiful  dfsase
Visit beautiful  cvcvss
Visit beautiful  dfqzcvc
Visit beautiful  ddddwq
Visit beautiful  bnnn
Visit beautiful  qqojfaso
 
#!/bin/bash
IFS=$'\n'
file="states"
for state in `cat $file`
do
echo "visit beautiful $state"
done
>
#cat states
>
A           labama
Alaska
dddfac
dfsase
cvcvss
dfqzcvc
ddddwq
bnnn
qqojfaso
>
visit beautiful A           labama
visit beautiful Alaska
visit beautiful dddfac
visit beautiful dfsase
visit beautiful cvcvss
visit beautiful dfqzcvc
visit beautiful ddddwq
visit beautiful bnnn
visit beautiful qqojfaso

在处理长脚本时，可能在一个地方需要修改IFS的值，然后忘掉它并在脚本中其他地方以为还是默认的值。一个可参考的简单实践：
IFS.OLD=$IFS
IFS=$'\n'

IFS=$IFS.OLD

#!/bin/bash
for file in /root/*
do
if  [ -d "$file" ]then
echo " $file is a directory "
elif [ -f "$file" ]then
echo "$file is a file"
fi
done

#!/bin/bash
for file in /root/.b* /home/rich/badtest
do
if [ -d "$file" ]then
echo "$file is a directory"
elif [ -f "$file" ]then
echo "$file is a file"
else
echo "$file doesn't exist"
fi
done

类C语言风格
#!/bin/bash
for (( i=1; ido
echo "The next nuber is $i"
done
>
The next nuber is 1
The next nuber is 2
The next nuber is 3
The next nuber is 4
The next nuber is 5
The next nuber is 6
The next nuber is 7
The next nuber is 8
The next nuber is 9
The next nuber is 10

#!/bin/bash
for (( a=1, b=10; a do
echo "$a - $b"
done
>
1 - 10
2 - 9
3 - 8
4 - 7
5 - 6
6 - 5
7 - 4
8 - 3
9 - 2
10 - 1
11 - 0
12 - -1