We have already introduced the basic use of while and other methods, so as to introduce you another way to follow the way and while, until special use.
1. until format
until condition ;do
Circulatory body
done
Entry Conditions: If the judgement condition is false, it will enter the loop body for execution.
Exit condition: If the judgment condition is true, it will not enter the loop body for execution.
Note: This is different from while, which is easy to confuse for beginners.
Example: Find the sum of all positive integers within 100
#!/bin/bash
#
declare -i i=1
declare -i sum=0
until [ $i -gt 100 ] ;do
let sum+=$i
let i++
done
echo "sum:$sum"
Example: Print 99 multiplication table
#!/bin/bash
#
declare -i i=1
declare -i j=1
until [ -j -gt 9 ];do
until [ $i -gt $j ];do
echo -n -e "$i*$J=$[$i*$J]\t"
let i++
done
echo
let i=1
let j++
done
2. Special Usage in Loop Control Statements (Used in Loop Body)
Continu [n]: End the current cycle of Layer N ahead of time and go directly to the next round of judgment
while condition；do
CMD1
CMD2
...
if condition2;then
continue
fi
COM
done
Brea [n]: End Layer N loops ahead of time
while condition；do
CMD1
CMD2
...
if condition2;then
break
fi
COM
done
Example: Find the sum of all even numbers within 100, requiring a loop to traverse all positive integers within 100
#!/bin/bash
#
declare -i i=0
declare -i sum=0
until [ $i -gt 100 ];do
let i++
if [ $[$i%2] -eq 1 ];then
continue
fi
let sum+=$i
done
echo "Sum $sum;"
3. Create a Dead Cycle
If we want to judge a loop, when the loop meets a certain condition, we can combine the continue or break commands.
while true;do
Circulatory body
done
until false；do
Circulatory body
done
Example: Every 3 seconds to the system to get the information of the user who has logged in, if docker logged in, it is recorded in the log, and exit;
#!/bin/bash
#
read -p "enter a user name "username
while true;do
if who |grep "^$username" &>/dev/null;then
break
fi
sleep 3
done
echo "$username is logged." >>/tmp/usr.log
The second method
#!/bin/bash
#
read -p "enter a user name "username
until who |grep "^$username" &>/dev/null;do
sleep 3
done
echo "$username is logged." >>/tmp/usr.log
4. Special use of while loops (traversing every line of a file)
while read line ;do
Circulatory body
done </PATH/FROM/SOMEFILE
Read every line in the / PATH/FROM/SOMEFILE file once and assign the line to the variable line.
Example: Find out all users whose ID number is even and display their user and ID number
#!/bin/bash
#
while read line ;do
if [ $[`echo $line |cut -d: -f3`%2] -eq 0 ];then
echo -e -n "userName:`echo $line |cut -d: -f1`\t"
echo "uid:`echo $line |cut -d: -f3`"
fi
done < /etc/passwd
5. Special formats for loops:
For ((initialization of control variables; conditional judgment expression; modified expression of control variables); do)
Circulatory body
done
Initialization of control variables: only once when running to the loop code segment;
Control variable modification expression: At the end of each cycle, the control variable modification operation will be carried out first, and then the conditional judgment will be made.
Example: Find the sum of all positive integers within 100
declare -i sum=0
for((i=1;i<=100;i++));do
let sum+=$i
done
echo "Sum $sum"
Example: 99 multiplication table
#!/bin/bash
#
for((j=1;j<=9;j++));do
for((i=1;i<=j;i++));do
echo -e -n "${i}*${j}=$[$i*$j]\t"
done
done
Write a script to accomplish the following tasks
(1) Display a menu as follows:
cpu)show cpu information;
mem)show memory information;
disk)show disk information;
quit)quit
(2) prompting users to select items;
(3) Display the content selected by the user;
Further, it shows that the script does not exit after completion, but prompts the user to continue displaying other content, and quits only when quit is used.
#!/bin/bash
#
while true ;do
while [ "$option" != 'cpu' -a "$option" != 'mem' -a "$option" != 'disk' -a
"$option" != 'quit' ];do
read -p "wrong option,enter a option: " option
done
case "$option" in
cpu)
lscpu
;;
mem)
cat /proc/meminfo
;;
disk)
fdisk -l
;;
quit)
echo "quit ..."
break
;;
esac
read -p "Please enter option again: " option
done
5. Conditional Judgment: case Statement
case variable reference in
pat1)
Branch 1
；；
pat2)
Branch 2
；；
...
*)
Default branch
esac
Exercise: Write a script to fulfill the following requirements
(1) script acceptable parameters: start, stop, restart, status
(2) If the parameter is not one of the four, it is prompted to exit after using the format.
(3) If it is start: create / var/lock/subsys/script_name and display "Start Successfully"
If the implementation has stopped, how to deal with it
(4) If stop: delete / var/lock/subsys/script_name and display "stop successful"
If there was no start, how to deal with it
(5) If it is status, then
If the file / var/lock/subsys/script_name exists, it shows "script_name is running..."
If / var/lock/subsys/script_name does not exist, then "script_name is stopped..."
Where: script_name is the current script name
#!/bin/bash
#
script=$0
read -p "Enter a option: " option
echo "wrong option."
read -p "Enter a option: " option
done
case $option in
start)
touch /var/lock/subsys/$script
echo "$script is running..."
;;
stop)
rm -rf /var/lock/subsys/$script
echo "$script is stopped..."
;;
status)
if [ -e $script ];then
echo "$script is running..."
else
echo "$script is stopped..."
fi
;;
*)
echo "quit..."
;;
esac