20.10 for cycle

Syntax: for variable name in condition; do... ; done

Case 1: Calculate the sum of 1 to 100

#!/bin/bash
sum=0
for i in `seq 1 100`
do
    sum=$[$sum+$i]
    echo $i
done
echo $sum

Case 2 (File List Loop):

#!/bin/bash
cd /etc/
for a in `ls /etc/`
do
    if [ -d $a ]
    then
       ls -d $a
    fi
done

 

Note: for loops, spaces and carriage returns are used as separators

 

20.11 while cycle

Grammar: while condition; do... ; done

Case 1: View the system load for no more than 30 seconds and send more than 10 emails

#!/bin/bash
while :                 //Represents a dead cycle
do 
load=`w|head -1|awk -F 'load average: ' '{print $2}'|cut -d . -f1`
if [ $load -gt10 ]
then
/usr/local/sbin/mail.py 125455@qq.com "load high" "$load"
fi
sleep 30
done

Case 2: Human Interactive Input of a Number in the Cycle

#!/bin/bash
while :
do
read -p "please input a number:" n
if [ -z "$n" ]
then
echo "You need to enter something"
continue                            //Continue to execute previous orders
fi
n1=`echo $n|sed 's/[ 0-9 ]//g'`
if [ ! -z "$n1" ]
then 
echo "You can only enter a number."
continue                                        
fi
break                               //Exit loop 
done
echo $n                           

20.13 break out of the loop

Case study:

#!/bin/bash
for i in `seq 1 5`
do
echo $i
if [ $i == 3 ]
then
break
fi
echo $i
done
echo aaaaa       

The best way to compare numbers is to use eq

Comparing strings==

 

20.14 continue ends this cycle

Ignore the code under continue and go straight to the next loop

#!/bin/bash
for i in `seq 1 5`
do
echo $i
if [ $i == 3 ]
then
continue
fi
echo $i
done
echo aaaaa

Contrast:

continue

 

break

20.15 exit exits the entire script

#!/bin/bash
for i in `seq 1 5`
do
echo $i
if [ $i==3 ]
then
exit
fi
echo $i
done
echo aaaaa

Continue: End this cycle and continue it

break: Exit the entire cycle

exit: exit the entire script