Note_Tech

All technological notes.

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Linux - Shell: Automation

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• Linux - Shell: Automation
  • Cron
    • crond Daemon
    • Crontab Format
    • Special Time Keywords
    • Special Characters
  • Cron Job Definitions
    • System-Wide Crontab: /etc/crontab
    • Modular Configuration: /etc/cron.d/
    • Special Directories for Periodic Tasks
    • crontab: Per-User Crontabs
    • Lab: Create a cron job
    • Lab: Add, List, and Remove a Cron Job
  • Schedule One-time Tasks: at
    • Lab: at
  • Job Scheduling
    • Controlling User Access
    • Scheduler Log File

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Cron

• CRON/command run on notice

  • a time-based job scheduler that automates the execution of tasks (commands or scripts) at specified intervals.
  • It allows users and system administrators to schedule repetitive tasks like backups, system updates, or cleaning temporary files.

• The cron system consists of the crond daemon, configuration files, and user-specific tools like crontab.

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crond Daemon

• crond

  • the cron daemon that runs in the background on Linux
  • starts automatically at boot time (if enabled) and keeps running in the background.

• Service

  systemctl status crond.service
  

• Configuration Files:
  • reads all cron job definitions from:
    • /etc/crontab
    • /etc/cron.d/
    • User-specific crontabs (/var/spool/cron/username)
• Execution:
  • Every minute, crond checks all the defined schedules and matches tasks whose time matches the current time.
• Logging:
  • crond records task executions and errors in the system log file.(e.g., /var/log/cron or journalctl -u crond).

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Crontab Format

• 2 parts:

  • when to run
    • match definition with current date and time.
  • what command to run

• * * * * * command

  • Minute (0-59)
  • Hour (0-23)
  • Day of the Month (1-31)
  • Month of the Year (1-12)
  • Day of the Week (0-6)
    • 0: Sunday

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Special Time Keywords

Keyword	Equivalent Schedule
@reboot	Runs once after the system starts.
@hourly	0 * * * *
@daily	0 0 * * * (Midnight every day).
@weekly	0 0 * * 0 (Midnight every Sunday).
@monthly	0 0 1 * * (Midnight on the 1st).
@yearly/@annually	0 0 1 1 * (Midnight on January 1).

Special Characters

Character	Meaning
*	Matches all values for that field (e.g., every minute, every hour, etc.).
,	Specifies multiple values (e.g., 1,5,10 for minutes).
-	Specifies a range of values (e.g., 1-5 for minutes 1 to 5).
/	Specifies intervals (e.g., */5 for every 5 minutes).

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• Example

# Run every Monday at 07:00.
0 7 * * 1 /opt/sales/bin/weekly-report

# Run at 02:00 every day and
# send output to a log file.
0 2 * * * /root/backupdb > /tmp/db.log 2>&1

# Run every 15 minutes.
0,15,30,45 * * * * /opt/acme/bin/15-min-check
# Run every 15 minutes.
*/15 * * * * /opt/acme/bin/15-min-check

# Run every 30 minutes.
0,30 * * * * /opt/acme/bin/half-hour-check

# Another way to do the same thing.
* */2 * * * /opt/acme/bin/half-hour-check

# Run for the first 5 minutes of the hour
0-4 * * * * /opt/acme/bin/first-five-mins

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Cron Job Definitions

System-Wide Crontab: /etc/crontab

• System-Wide Cron Jobs

  • define jobs for all users and system tasks.
  • allows the specification of environment variables like SHELL, PATH, and MAILTO.

• Format:

MIN HOUR DOM MON DOW USER COMMAND

• /etc/crontab file
  • the system-wide crontab configuration file on Linux systems.
  • It defines scheduled tasks (cron jobs) that are executed by the cron daemon (crond).

# Specifies the default shell used to execute the commands in the crontab.
SHELL=/bin/bash
# Specifies the environment variable PATH, which defines the directories where executable files are searched for when a command is run.
PATH=/sbin:/bin:/usr/sbin:/usr/bin
# Specifies the email address (or user) where the output (both standard output and error) of the cron jobs will be sent.
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

# example
0 5 * * *   root    /usr/local/bin/daily-backup.sh
15 3 * * 7  root    /usr/local/bin/weekly-cleanup.sh

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Modular Configuration: /etc/cron.d/

• /var/spool/cron

• /etc/cron.d/ directory

  • a system directory used for managing cron jobs.
  • It provides a way to define cron jobs without modifying the main /etc/crontab file

• Format of /etc/cron.d/ Files:

m   h   dom   mon   dow   user    command

• How It Works

  • The cron daemon (crond) reads files in /etc/cron.d/ in addition to /etc/crontab and user-specific crontabs.
  • Any valid file inside /etc/cron.d/ will be included in the system’s cron scheduling.

• File requirements:

  • should have a unique name and follow a simple naming convention (e.g., backup_jobs, web_tasks).
  • should have proper permissions to prevent unauthorized modifications (e.g., 644)
  • should be owned by root (root:root)
  • Incorrect syntax in any file in /etc/cron.d/ will cause the cron daemon to skip the entire file.

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Special Directories for Periodic Tasks

• managed automatically by the system and include:
  • /etc/cron.hourly/
  • /etc/cron.daily/
  • /etc/cron.weekly/
  • /etc/cron.monthly/

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crontab: Per-User Crontabs

CMD	DESC
crontab -e	Edit the current user’s crontab file.
crontab -l	List the current user’s crontab entries.
crontab -ir	Prompt for confirmation before removing a crontab
crontab -r	Remove the current user’s crontab file.
crontab -u username	Specify a user whose crontab is to be managed (requires root).
crontab file	Install a new crontab from file.

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Lab: Create a cron job

# create a cron file
vi 5-min-log-cron

# run every 5 minutes
*/5 * * * * echo "$(date) hello world" >> /home/rheladmin/log

# create a cron job
crontab 5-min-log-cron

# list all cron job
crontab -l
# # run every 5 minutes
# */5 * * * * echo "$(date) Hello world" >> /home/rheladmin/log

# remove all cron jobs
crontab -r

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Lab: Add, List, and Remove a Cron Job

useradd user100
passwd user100

su -

# add user
echo "user100" >> /etc/cron.allow

# create job
crontab -e -u user100
*/2 10-23 * * * echo "Hello, this is a cron test." > /tmp/hello.ouot

# confirm
crontab -l -u user100
# */2 10-23 * * * echo "Hello, this is a cron test." > /tmp/hello.out
# confirm in spool
ll /var/spool/cron
# total 8
# -rw-------. 1 root root 27 Feb 16 15:34 root
# -rw-------. 1 root root 68 Feb 16 15:47 user100

ll /var/spool/cron/user100
# -rw-------. 1 root root 68 Feb 16 15:47 /var/spool/cron/user100

# trace by log
grep user100 /var/log/cron
# Feb 16 15:52:01 ServerB CROND[105447]: (user100) CMD (echo "Hello, this is a cron test." > /tmp/hello.out)
# Feb 16 15:52:01 ServerB CROND[105420]: (user100) CMDEND (echo "Hello, this is a cron test." > /tmp/hello.out)
# Feb 16 15:54:01 ServerB CROND[105488]: (user100) CMD (echo "Hello, this is a cron test." > /tmp/hello.out)
# Feb 16 15:54:01 ServerB CROND[105462]: (user100) CMDEND (echo "Hello, this is a cron test." > /tmp/hello.out)

# remove
crontab -l -u user100
# */2 10-23 * * * echo "Hello, this is a cron test." > /tmp/hello.out
crontab -ir -u user100
# crontab: really delete user100's crontab? y
# confirm
crontab -l -u user100
# no crontab for user100

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Schedule One-time Tasks: at

• at command

  • a tool used to schedule one-time tasks for a specific time in the future.
  • at command is used to schedule a one-time execution of a program in the future
• job dir:
  • /var/spool/at

• daemon

  • atd
• Package:

yum install -y at

• Service:
  • atd daemon

systemctl status atd.service

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• Command

CMD	DESC
at -l	List spooled job
at -c jobID	Display content of job file
at -d jobID	Remove content of job file
at -f ~/.bash_profile now +5 hours	Execute file in next 5 hours

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Lab: at

at 03:20pm 2/16/25
# warning: commands will be executed using /bin/sh
# at> date &>/tmp/date.out
# at> <EOT>
# job 2 at Sun Feb 16 15:20:00 2025

# list jobs in dir
ll /var/spool/at
# -rwx------. 1 root root 2670 Feb 16 15:19 a0000201ba7024

# list at job
at -l
# 2       Sun Feb 16 15:20:00 2025 a root

# display content
at -c 2
# #!/bin/sh
# # atrun uid=0 gid=0
# # mail rheladmin 0
# umask 22
# SHELL=/bin/bash; export SHELL
# HISTCONTROL=ignoredups; export HISTCONTROL
# HISTSIZE=1000; export HISTSIZE
# ...

# confirm execution
cat /tmp/date.out
# Sun 16 Feb 2025 03:20:00 PM EST

# confirm after execution
# at job has been removed
at -l
# 1       Sun Mar 16 15:11:00 2025 a root

# confirm
# note: removed
ll /var/spool/at

# confirm by log
tail /var/log/cron
# Feb 16 15:20:00 ServerB atd[6593]: Starting job 2 (a0000201ba7024) for user 'root' (0)

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Job Scheduling

• two service daemons:

  • atd: manages the jobs scheduled to run one time in the future
  • crond: responsible for running jobs repetitively at pre-specified times.

• For any additions or changes, neither daemon needs a restart.

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Controlling User Access

• listing users in the allow or deny file
  • at:
    • /etc/at.allow
    • /etc/at.deny
  • cron
    • /etc/cron.allow
    • /etc/cron.deny
• By default, the deny files exist and are empty, and the allow files are non-existent.

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Scheduler Log File

• All activities for atd and crond services are logged to the /var/log/cron file.

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