[learn shell programming easily]-5. Plan tasks

1、 Planning tasks

Create and manage tasks that are automatically executed at a specified time

matters needing attention ： To make a task schedule run automatically at a specified time , The service that schedules the task must be started .

1.1、 Classification of planned tasks  

• Use at Command invocation atd process Set at a specific time , Perform a task
• Use crontab Command invocation crond process , Set according to a fixed period （ Like every day , Wait a week ） Repeat pre planned tasks

crond Progress is always running

[[email protected] ~]# ps aux|grep crond
root        693  0.0  0.0 126384  1624 ?        Ss   13:16   0:00 /usr/sbin/crond -n

1.2、 Daemon （Daemon）

Definition ： Processes that have been running in memory , Until we stop it artificially , Otherwise, it is always running in memory . Because it's always running in memory , So our users can visit at any time .

If we use "ps aux" Order to see , You will see a lot of bands at the end "d" The process of , So these processes are daemons .

1.2.1、 The five states of a process

The above guard status , Yes, always " function "、" Blocking "、" be ready " These three states are cyclic .

1.3、at command

Definition ： On a specified date 、 Some preset command operations are automatically executed at the time point , This is a one-time scheduled task

• The name of the system service ：/etc/init.d/atd
• Format ：at [HH:MM] [yyyy-mm-dd]

When setting tasks , First of all, we have to start std process , Otherwise, they will not perform scheduled tasks regularly .

[[email protected] rough_book]# service atd start
Redirecting to /bin/systemctl start atd.service

[[email protected] rough_book]# ls
[[email protected] rough_book]# at 21:58
at> mkdir fan$RANDOM
at> <EOT>    #  Use ctrl+d, Submit scheduled tasks 
job 1 at Thu Jun 23 21:58:00 2022    

#  View the current scheduled task list 
[[email protected] rough_book]# at -l
1	Thu Jun 23 21:58:00 2022 a root

#  Wait to 21:58, You can see that there is a new file .
[[email protected] rough_book]# ls
fan28985



#  Delete the scheduled task with No 2 The task of ,atrm Equivalent to at -d
[[email protected] rough_book]# atrm 2

1.3.1、 Plan where tasks are stored

Where the task is stored ：/var/spool/at

[[email protected] rough_book]# at -l
4	Thu Jun 23 22:03:00 2022 a root
[[email protected] rough_book]# ls /var/spool/at
a0000401a5240b  spool


#  The principle is atd It's time for this process , They will execute /var/spool/at In the corresponding a Opening file .. because linux All the documents inside . Once the task is completed , The files will disappear .

1.3.2、batch

batch Is and at It's about the same plan Make .batch It will automatically run when the system load is not high . So when not busy ？ The average load is less than 0.8

[[email protected] rough_book]# top
top - 22:10:28 up  8:54,  5 users,  load average: 0.00, 0.01, 0.05

#  Check how long the system has been powered on , And the average load of the system  
[[email protected] rough_book]# uptime
 22:10:51 up  8:54,  5 users,  load average: 0.00, 0.01, 0.05

# load average The following three numbers represent ： stay 1、5、15 Average load in minutes 

#  Both commands can view the load of the system 

Average load ： It's a period of time (1、5、15 minute ) The average number of runnable or non interruptible processes . It can be understood as ready team and operation 、 The number of processes in the block . The value is the same as CPU The nucleus of .

man uptime: see uptime The following information about the average load of the system can be found in the manual

System  load  averages  is the average number of processes that are either in a runnable or uninter‐ ruptable state.  A process in a runnable state is either using the CPU or waiting to use the CPU.  A process  in uninterruptable state is waiting for some I/O access, eg waiting for disk.  The averages are taken over the three time intervals.  Load averages are not normalized for the number of CPUs in a  system,  so a load average of 1 means a single CPU system is loaded all the time while on a 4 CPUsystem it means it was idle 75% of the time.

The average load of a system is the average number of processes that are in an operational or non interruptible state .  A process in a runnable state is using CPU, Or waiting to be used CPU.  The process in a non interruptible state is waiting for some I/O visit , Like waiting for disk .  Take the average of the three time intervals .  The average load is not based on CPU The number of , So the average load is 1 It means a single CPU The system is always loaded , and CPU The number of 4 individual CPU System , It means that the 75% My time is free .

2、crond process

crond Process is every 1 Check the user's scheduled tasks in minutes ;crond It is installed by default and will run automatically after startup . See the following blog for details .

[linux] 3、 ... and 、 Planning tasks （crontab、 How hackers use planned tasks to obtain our things and how to check them 、 Front and back 、anacron service ）_FanMY_71 The blog of -CSDN Blog