[root@localhost ~]# systemctl status crond
● crond.service - Command Scheduler
Loaded: loaded (/usr/lib/systemd/system/crond.service; enabled; vendor preset: enabled)
Active: active (running) since Sat 2020-05-30 15:01:43 JST; 29min ago
Main PID: 31877 (crond)
CGroup: /system.slice/crond.service
└─31877 /usr/sbin/crond -n

May 30 15:01:43 localhost.localdomain systemd[1]: Started Command Scheduler.
May 30 15:01:43 localhost.localdomain crond[31877]: (CRON) INFO (RANDOM_DELAY will be scaled with factor 49% if used.)
May 30 15:01:43 localhost.localdomain crond[31877]: (CRON) INFO (running with inotify support)
May 30 15:01:43 localhost.localdomain crond[31877]: (CRON) INFO (@reboot jobs will be run at computer's startup.)

• Crontab 설정

• Crontab 설정의 경우 두가지 방법으로 설정이 가능합니다.

  1. /etc/crontab 파일을 직접 수정하는 방법

  
  [root@localhost ~]# vi /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
  
  
  

2. Crontab 명령어를 이용하는 방법

[root@localhost ~]# crontab -e # 내 crontab 수정
[root@localhost ~]# crontab -l # 내가 설정한 crontab 확인
[root@localhost ~]# crontab -e -u test # test란 이름의 사용자의 crontab 수정
[root@localhost ~]# crontab -l -u test # test란 이름의 사용자의 crontab 확인

•  Crontab 명령어 예시

*/1 * * * * echo "every 1 min" # 1분마다 실행
*/30 * * * * echo "every 30 min" # 30분마다 실행
30 */6 * * * echo "every 6hours at 30 mins" # 6시간마다 실행 (00:30, 06:30, 12:30, 18:30)
30 1-18/2 * * * echo "every 2hours on 1am - 6pm" # (01 ~ 18)까지 매 2시간마다 실행 01:30, 03:30, ... 17:30)

• 결과 예시 (cat /var/spool/mail/root)
• 1분마다 “every 1 min” 문구가 실행된것을 확인

From root@localhost.localdomain Sat May 30 16:19:01 2020
Return-Path: <root@localhost.localdomain>
X-Original-To: root
Delivered-To: root@localhost.localdomain
Received: by localhost.localdomain (Postfix, from userid 0)
id 417A51B6010A; Sat, 30 May 2020 16:19:01 +0900 (JST)
From: "(Cron Daemon)" <root@localhost.localdomain>
To: root@localhost.localdomain
Subject: Cron <root@localhost> echo "every 1 min"
Content-Type: text/plain; charset=UTF-8
Auto-Submitted: auto-generated
Precedence: bulk
X-Cron-Env: <XDG_SESSION_ID=33>
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>
Message-Id: <20200530071901.417A51B6010A@localhost.localdomain>
Date: Sat, 30 May 2020 16:19:01 +0900 (JST)

every 1 min

From root@localhost.localdomain Sat May 30 16:20:01 2020
Return-Path: <root@localhost.localdomain>
X-Original-To: root
Delivered-To: root@localhost.localdomain
Received: by localhost.localdomain (Postfix, from userid 0)
id 539BA1B6010D; Sat, 30 May 2020 16:20:01 +0900 (JST)
From: "(Cron Daemon)" <root@localhost.localdomain>
To: root@localhost.localdomain
Subject: Cron <root@localhost> echo "every 1 min"
Content-Type: text/plain; charset=UTF-8
Auto-Submitted: auto-generated
Precedence: bulk
X-Cron-Env: <XDG_SESSION_ID=34>
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>
Message-Id: <20200530072001.539BA1B6010D@localhost.localdomain>
Date: Sat, 30 May 2020 16:20:01 +0900 (JST)

every 1 min