Detailed explanation of scheduled tasks for ordinary users in Linux

Preface

Ordinary users define crontab scheduled tasks: For example, an oracle user defines a scheduled task: print the current directory every minute

[oracle@node2 ~]$ crontab -e

*/1 * * * * /bin/ls -al > /tmp/ls.log

How to determine whether a scheduled task has been executed? First, the crond service must be running

[oracle@node2 ~]$ service crond status
Redirecting to /bin/systemctl status crond.service
● crond.service - Command Scheduler
Loaded: loaded (/usr/lib/systemd/system/crond.service; enabled; vendor preset: enabled)
Active: active (running) since Sun 2018-07-15 08:27:38 EDT; 2min 43s ago
Main PID: 6189 (crond)
CGroup: /system.slice/crond.service
└─6189 /usr/sbin/crond -n
[oracle@node2 ~]$

1. Use the mail command as the root user

[root@node2 ~]# mail
Heirloom Mail version 12.5 7/5/10. Type ? for help.
"/var/spool/mail/root": 801 messages 9 new 94 unread
N795 (Cron Daemon) Sun Jul 15 08:25 35/932 "Cron <root@node2> /bin/ls"
N796 (Cron Daemon) Sun Jul 15 08:26 35/932 "Cron <root@node2> /bin/ls"
N797 (Cron Daemon) Sun Jul 15 08:27 35/932 "Cron <root@node2> /bin/ls"
N798 (Cron Daemon) Sun Jul 15 08:28 35/932 "Cron <root@node2> /bin/ls"
N799 (Cron Daemon) Sun Jul 15 08:29 35/932 "Cron <root@node2> /bin/ls"
N800 (Cron Daemon) Sun Jul 15 08:30 35/932 "Cron <root@node2> /bin/ls"
& file
"/var/spool/mail/root": 801 messages 9 new 94 unread
& h
>N793 (Cron Daemon) Sun Jul 15 08:23 35/932 "Cron <root@node2> /bin/ls"
N794 (Cron Daemon) Sun Jul 15 08:24 35/932 "Cron <root@node2> /bin/ls"
N795 (Cron Daemon) Sun Jul 15 08:25 35/932 "Cron <root@node2> /bin/ls"
N796 (Cron Daemon) Sun Jul 15 08:26 35/932 "Cron <root@node2> /bin/ls"
N797 (Cron Daemon) Sun Jul 15 08:27 35/932 "Cron <root@node2> /bin/ls"
N798 (Cron Daemon) Sun Jul 15 08:28 35/932 "Cron <root@node2> /bin/ls"
N799 (Cron Daemon) Sun Jul 15 08:29 35/932 "Cron <root@node2> /bin/ls"
N800 (Cron Daemon) Sun Jul 15 08:30 35/932 "Cron <root@node2> /bin/ls"
&800
Message 800:
From [email protected] Sun Jul 15 08:30:01 2018
Return-Path: <[email protected]>
X-Original-To: root
Delivered-To: [email protected]
From: "(Cron Daemon)" <[email protected]>
To: [email protected]
Subject: Cron <root@node2> /bin/ls
Content-Type: text/plain; charset=UTF-8
Auto-Submitted: auto-generated
Precedence: bulk
X-Cron-Env: <XDG_SESSION_ID=278>
X-Cron-Env: <XDG_RUNTIME_DIR=/run/user/0>
X-Cron-Env: <LANG=en_US.UTF-8>
X-Cron-Env: <SHELL=/bin/sh>
X-Cron-Env: <HOME=/root>
X-Cron-Env: <PATH=/usr/bin:/bin>
X-Cron-Env: <LOGNAME=root>
X-Cron-Env: <USER=root>
Date: Sun, 15 Jul 2018 08:30:01 -0400 (EDT)
Status: R
\
anaconda-ks.cfg
Desktop
Documents
Downloads
initial-setup-ks.cfg
Music
Pictures
Public
Templates
Videos

In the mial interactive environment, use the file command to display the total number of current emails and other information. Use head to view the most recent email and enter the number to view the detailed information of the email.

2. View the log: tail -n 10 /var/log/cron

[root@node2 ~]# tail -n 10 /var/log/cron
Jul 15 08:31:01 node2 CROND[6297]: (root) CMD (/bin/ls)
Jul 15 08:31:01 node2 CROND[6298]: (oracle) CMD (/bin/ls -al > /tmp/ls.log )
Jul 15 08:32:01 node2 CROND[6321]: (oracle) CMD (/bin/ls -al > /tmp/ls.log )
Jul 15 08:32:01 node2 CROND[6322]: (root) CMD (/bin/ls)
Jul 15 08:33:02 node2 CROND[6342]: (root) CMD (/bin/ls)
Jul 15 08:33:02 node2 CROND[6343]: (oracle) CMD (/bin/ls -al > /tmp/ls.log )
Jul 15 08:34:01 node2 CROND[6362]: (root) CMD (/bin/ls)
Jul 15 08:34:01 node2 CROND[6363]: (oracle) CMD (/bin/ls -al > /tmp/ls.log )
Jul 15 08:35:01 node2 CROND[6382]: (oracle) CMD (/bin/ls -al > /tmp/ls.log )
Jul 15 08:35:01 node2 CROND[6383]: (root) CMD (/bin/ls)
[root@node2 ~]#

The log file records the execution of scheduled tasks for all users

3. Use mail to view under ordinary user

(In the scheduled task, the user who executes the scheduled task must be clearly defined, otherwise even if the scheduled task is defined under the oracle user, the email may not be received)

[oracle@node2 ~]$ crontab -l
*/1 * * * * oracle /bin/ls -al > /tmp/ls.log
[oracle@node2 ~]$
[oracle@node2 ~]$ mail
Heirloom Mail version 12.5 7/5/10. Type ? for help.
"/var/spool/mail/oracle": 6 messages 1 unread
1 (Cron Daemon) Mon Mar 5 14:10 52/2476 "Cron <oracle@localhost> ls -al"
2 (Cron Daemon) Sun Jul 15 06:10 57/2777 "Cron <oracle@node2> /bin/ls -al"
3 (Cron Daemon) Sun Jul 15 06:11 57/2777 "Cron <oracle@node2> /bin/ls -al"
4 (Cron Daemon) Sun Jul 15 06:12 57/2778 "Cron <oracle@node2> /bin/ls -al"
5 (Cron Daemon) Sun Jul 15 06:13 57/2778 "Cron <oracle@node2> /bin/ls -al"
>U 6 (Cron Daemon) Sun Jul 15 08:38 26/929 "Cron <oracle@node2> oracle /bin/ls -al > /tmp/ls.log"
&

The /var/log/cron file cannot be directly viewed by ordinary users

[oracle@node2 ~]$ tail -n 10 /var/log/cron
tail: cannot open '/var/log/cron' for reading: Permission denied
You have new mail in /var/spool/mail/oracle
[oracle@node2 ~]$

There is another way to write scheduled tasks in the /etc/crontab file, but in centos7, writing and executing in this file will result in an error

[oracle@node2 ~]$ cat /etc/crontab
SHELL=/bin/bash
PATH=/sbin:/bin:/usr/sbin:/usr/bin
MAILTO=root
# For details see man 4 crontabs
# Example of job definition:
# .---------------- minute (0 - 59)
# | .------------- hour (0 - 23)
# | | .---------- day of month (1 - 31)
# | | | .------- month (1 - 12) OR jan,feb,mar,apr ...
# | | | | .---- day of week (0 - 6) (Sunday=0 or 7) OR sun,mon,tue,wed,thu,fri,sat
# | | | | |
# * * * * * user-name command to be executed
[oracle@node2 ~]$

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