for循环总结：

1、 for((i=1;i<=10;i++));do echo $(expr $i \* 4);done   C语法的for循环
2、在shell中常用的是 for i in $(seq 10)
3、for i in `ls`
4、for i in ${arr[@]}
#!/bin/bash
arr=("a" "b" "c")
echo "arr is (${arr[@]})"
echo "item in array"

for i in ${arr[@]}
do
echo "$i"
done
5、for i in $*
#!/bin/bash
for i in $*
do
echo $i
done
6、for File in /proc/sys/net/ipv4/conf/*/accept_redirects; do
7、for i in f1 f2 f3 ;do
8、for i in *.txt
9、for i in $(ls *.txt)

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

awk 'BEGIN{for(i=1; i<=10; i++) print i}'  AWK中的for循环写法和C语言一样的

#数字递增循环
for i in `seq 1 1000`;do xxxx;done
for ((i=1;i<1000;i++));do xxxx;done
for i in {1..1000};do  xxxx;done

i=1
while(($i<1000))
do
echo $i
i=`expr $i+1` 或i=$(($i+1))
done