bash脚本中如何使用while
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脚本编程：
顺序结构
选择结构
if
case
循环结构
for
while
until
while循环：适用于循环次数未知的场景，要有退出条件
语法：
while CONDITION; do
statement
...
done
练习：计算100以内所有正整数的和
[root@localhost ~]# bash sum3.sh
the sum is: 5050
[root@localhost ~]# cat sum3.sh
#!/bin/bash
declare -i i=1
declare -i sum=0
while [ $i -le 100 ]; do
let sum+=$i
let i++
done
echo "the sum is: $sum"
[root@localhost ~]#
将小写转换为大写
[root@localhost ~]# bash translate.sh
input string:asdf
ASDF
input string:bad
BAD
input string:quit
[root@localhost ~]# cat translate.sh
#!/bin/bash
read -p "input string:" string
while [ $string != 'quit' ]; do
echo $string | tr 'a-z' 'A-Z'
read -p "input string:" string
done
[root@localhost ~]#
练习：每隔5秒查看hadoop用户是否登录，如果登录，显示其登录并退出；否则，显示当前时间，并说明hadoop尚未登录：
[root@localhost ~]# vim checkhadoop.sh
[root@localhost ~]# bash +x checkhadoop.sh
Thu Feb 23 20:53:33 PST 2017
Thu Feb 23 20:53:38 PST 2017
Thu Feb 23 20:53:43 PST 2017
Thu Feb 23 20:53:48 PST 2017
Thu Feb 23 20:53:53 PST 2017
Thu Feb 23 20:53:58 PST 2017
Thu Feb 23 20:54:03 PST 2017
Thu Feb 23 20:54:08 PST 2017
Thu Feb 23 20:54:13 PST 2017
Thu Feb 23 20:54:18 PST 2017
Thu Feb 23 20:54:23 PST 2017
hadoop is logged in
[root@localhost ~]# cat checkhadoop.sh
#!/bin/bash
who | grep "hadoop" &> /dev/null
ret=$?
while [ $ret -ne 0 ] ; do
sleep 5
echo "`date`"
who | grep "hadoop" &> /dev/null
ret=$?
done
echo "hadoop is logged in"
[root@localhost ~]#
写一个脚本：
1) 显示一个菜单给用户：
d|D) show disk usages.
m|M) show memory usages.
s|S) show swap usages.
*) quit.
2) 当用户给定选项后显示相应的内容；
扩展：
当用户选择完成，显示相应信息后，不退出；而让用户再一次选择，再次显示相应内容；除了用户使用quit；
[root@localhost ~]# bash showdisk.sh
d|D) show disk usages.
m|M) show memory usages.
s|S) show swap usages.
*) quit.
your choice:d
disk usage:
Filesystem Size Used Avail Use% Mounted on
/dev/sda2 18G 3.1G 14G 19% /
tmpfs 931M 72K 931M 1% /dev/shm
/dev/sda1 283M 33M 236M 13% /boot
your choice:m
memory usage:
Mem: 1861 452 1409 1 68 153
your choice:s
swap usage:
Swap: 2047 0 2047
your choice:quit
[root@localhost ~]# cat showdisk.sh
#!/bin/bash
cat << EOF
d|D) show disk usages.
m|M) show memory usages.
s|S) show swap usages.
*) quit.
EOF
read -p "your choice:" choice
while [ $choice != 'quit' ]; do
case $choice in
d|D)
echo "disk usage:"
df -Ph
;;
m|M)
echo "memory usage:"
free -m | grep Mem
;;
s|S)
echo "swap usage:"
free -m | grep Swap
;;
*)
echo "unkonw"
exit 9
;;
esac
read -p "your choice:" choice
done
[root@localhost ~]#
循环控制：
continue
中断当前这一次循环，提前进入下一软循环
[root@localhost ~]
# bash +x sum4.sh
the sum is: 2550
[root@localhost ~]# cat sum4.sh
#!/bin/bash
let sum=0
let i=0
while [ $i -le 100 ]; do
let i++
if [ $[$i%2] -eq 1 ];then
continue
fi
let sum+=$i
done
echo "the sum is: $sum"
[root@localhost ~]#
break
中断循环，而后执行循环后面的语句；
[root@localhost ~]# cat sum5.sh
#!/bin/bash
declare -i sum=0
for i in {1..10000};do
let sum+=$i
if [ $sum -gt 5000 ];then
break
fi
let i++
done
echo "the sum is:$sum"
[root@localhost ~]# bash sum5.sh
the sum is:5050
[root@localhost ~]#
while特殊用法1--无限循环
[root@localhost ~]# bash while1.sh
file path:hello
hello not exist
file path:/etc/inittab
/etc/inittab exits
file path:quit
[root@localhost ~]# cat while1.sh
#!/bin/bash
while :;do
read -p "file path:" filepath
[ $filepath == "quit" ] && break
if [ -e $filepath ];then
echo "$filepath exits"
else
echo "$filepath not exist"
fi
done
[root@localhost ~]#
while特殊用法2--按行读取文件
[root@localhost ~]# cat while2.sh
#!/bin/bash
file=/etc/passwd
while read line;do
[ `echo $line | awk -F : '{print $7}'` ] && echo $line | awk -F : '{print $1}'
done < $file
[root@localhost ~]# bash while2.sh
root
bin
daemon
adm
lp
sync
shutdown
halt
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