for 循环

• 使用while循环读取文件

    cat file.txt |while read line
    do
    echo $line
    done
    
    或者：
    
    while read line
    do
    echo $line
    done < file.txt
  

注意:由于使用while来读入文件里的行时，会整行读入，不会关注行的内容(空格..)，所以比for读文件有更好的适用性，推荐使用while循环读取文件

linux shell循环示例

for循环示例

for循环语法：

 
1
2
3
4
5
6
for VARIABLE in 1 2 3 4 5 .. N
do
         command1
         command2
         commandN
done

1

2

3

4

5

6

7

8

9

10

<i>#!/bin/bash

for i in 1 2 3 4 5

do

echo "Welcome $i times"

done

</i>

bash version 3.0+版本

 
#!/bin/bash
 for i in {1..5}
do
   echo "Welcome $i times"
done

bash version 4版本

 
#!/bin/bash
echo "Bash version ${BASH_VERSION}..."
for i in {0..10..2}
  do
     echo "Welcome $i times"
 done

含有“seq”命令的语法示例

 
#!/bin/bash
for i in $(seq 1 2 20)
do
   echo "Welcome $i times"
done

for循环的三个表达式

语法如下：

 
for (( EXP1; EXP2; EXP3 ))
do
         command1
         command2
         command3
done

示例如下：

#!/bin/bash

for (( c=1; c<=5; c++ ))

do

echo "Welcome $c times..."

done

效果：

 
Welcome 1 times
Welcome 2 times
Welcome 3 times
Welcome 4 times
Welcome 5 times

for的无限循环

 
#!/bin/bash
for (( ; ; ))
do
   echo "infinite loops [ hit CTRL+C to stop]"
done

break条件语句

 
for I in 1 2 3 4 5
do
  statements1      #Executed for all values of ''I'', up to a disaster-condition if any.
  statements2
  if (disaster-condition)
  then
         break                #Abandon the loop.
  fi
  statements3          #While good and, no disaster-condition.
done

下面的shell脚本将通过在/ etc目录中存储的所有文件。 for循环将放弃当/ etc / resolv.conf的文件中找到。

 
#!/bin/bash
for file in /etc/*
do
         if [ "${file}" == "/etc/resolv.conf" ]
         then
                 countNameservers=$(grep -c nameserver /etc/resolv.conf)
                 echo "Total  ${countNameservers} nameservers defined in ${file}"
                 break
         fi
done

continue条件语句

 
for I in 1 2 3 4 5
do
  statements1      #Executed for all values of ''I'', up to a disaster-condition if any.
  statements2
  if (condition)
  then
         continue   #Go to next iteration of I in the loop and skip statements3
  fi
  statements3
done

利用这个脚本在命令行中指定的所有文件名的备份。如果。bak文件存在，它会跳过cp命令。

 
#!/bin/bash
FILES="$@"
for f in $FILES
do
        # if .bak backup file exists, read next file
         if [ -f ${f}.bak ]
         then
                 echo "Skiping $f file..."
                 continue  # read next file and skip cp command
         fi
        # we are hear means no backup file exists, just use cp command to copy file
         /bin/cp $f $f.bak
done

linux中shell编程for in循环语句的用法:
for in语句的格式:
for 无$变量 in 字符串
do
$变量
done
一简单的字符串枚举遍历法，利用for in格式对字符串按空格切份的功能
SERVICES="22 80 25 110 8000 23 20 21 3306 "
for x in $SERVICES
do
iptables -A INPUT -p tcp --dport $x -m state --state NEW -j ACCEPT
done
for variable in values --------字符串数组依次赋值
#!/bin/sh
for i in a b c 字符串列表A B C
字符串用空格分隔,没有括号,没有逗号, 然后循环将其依次赋给变量i
变量没有$
do
echo "i is $i"
done
[macg@machome ~]$ sh test.sh
i is a
i is b
i is c
for in 里，变量和*不等价
#!/bin/bash
for i in *.h ;#将list设置为当前目录下pwd的所有.h结尾文件，不包括以.开头的隐藏文件
do
cat ${i}.h
done
[macg@vm test]$ ./tip.sh
cat: *.h.h: No such file or directory
$i代表的是整个路径，而不是*.h里的.h前面的部分
改正
#!/bin/bash
for i in *.h
do
cat $i
done
[macg@vm test]$ echo hahaha >>1.h
[macg@vm test]$ echo ha >>2.h
[macg@vm test]$ ./tip.sh
hahaha
ha
for i in /etc/profile.d/*.sh
do
$i
done
$i代表的是/etc/profile.d/color.sh,
/etc/profile.d/alias.sh, /etc/profile.d/default.sh
for in 对(命令行，函数)参数遍历
test()
{
local i
for i in $* ; do
echo "i is $i"
done
}
$*是字符串：以"参数1 参数2 ... " 形式保存所有参数
$i是变量i的应用表示
[macg@machome ~]$ sh test.sh p1 p2 p3 p4
i is p1
i is p2
i is p3
i is p4
for in语句与通配符*合用,批量处理文件
批量改文件名
[root@vm testtip]# ls
aaa.txt ccc.txt eee.txt ggg.txt hhh.txt jjj.txt lll.txt nnn.txt
bbb.txt ddd.txt fff.txt go.sh iii.txt kkk.txt mmm.txt ooo.txt
[root@vm testtip]# cat go.sh
for i in *.txt #*.txt相当于一个字符串数组，依次循环赋值给i
do
mv "$i" "$i.bak"
done
[root@vm testtip]# sh go.sh
[root@vm testtip]# ls
aaa.txt.bak ccc.txt.bak eee.txt.bak ggg.txt.bak hhh.txt.bak jjj.txt.bak lll.txt.bak nnn.txt.bak bbb.txt.bak ddd.txt.bak fff.txt.bak go.sh iii.txt.bak kkk.txt.bak mmm.txt.bak ooo.txt.bak
for in语句与` `和$( )合用，利用` `或$( )的将多行合为一行的缺陷，实际是合为一个字符串数组
for i in $(ls *.txt)
do
echo $i
done
[macg@machome ~]$ sh test
111-tmp.txt
111.txt
22.txt
33.txt
或者说，利用for in克服` `和$( ) 的多行合为一行的缺陷
用for in语句自动对字符串按空格遍历的特性，对多个目录遍历
LIST="rootfs usr data data2"
for d in $LIST; do
mount /backup/$d
rsync -ax --exclude fstab --delete /$d/ /backup/$d/
umount /backup/$d
done