RH134 - ch02s03

Jobs scheduled to run repeatedly are called recurring jobs. Red Hat Enterprise Linux systems ship with the crond daemon, provided by the cronie package, enabled and started by default specifically for recurring jobs. The crond daemon reads multiple configuration files: one per user (edited with the crontab command), and a set of system-wide files. These configuration files give users and administrators fine-grained control over when their recurring jobs should be executed.

If a scheduled command produces any output or error that is not redirected, the crond daemon attempts to email that output or error to the user who owns that job (unless overridden) using the mail server configured on the system. Depending on the environment, this may need additional configuration. The output or error of the scheduled command can be redirected to different files.

Scheduling Recurring User Jobs

Normal users can use the crontab command to manage their jobs. This command can be called in four different ways:

Table 2.1. Crontab Examples

Command	Intended use
crontab -l	List the jobs for the current user.
crontab -r	Remove all jobs for the current user.
crontab -e	Edit jobs for the current user.
crontab `filename`	Remove all jobs, and replace with the jobs read from `filename`. If no file is specified, `stdin` is used.

Note

The superuser can use the -u option with the crontab command to manage jobs for another user. You should not use the crontab command to manage system jobs; instead, use the methods described in the next section.

Describing User Job Format

The crontab -e command invokes Vim by default, unless the EDITOR environment variable has been set to something different. Enter one job per line. Other valid entries include: empty lines, typically for ease of reading; comments, identified by lines starting with the number sign (#); and environment variables using the format NAME=value, which affects all lines below the line where they are declared. Common variable settings include the SHELL variable, which declares which shell to use to interpret the remaining lines of the crontab file; and the MAILTO variable, which determines who should receive any emailed output.

Important

Sending email may require additional configuration of the local mail server or SMTP relay on a system.

Fields in the crontab file appear in the following order:

Minutes
Hours
Day of month
Month
Day of week
Command

Important

When the Day of month and Day of week fields are both other than *, the command is executed when either of these two fields are satisfied. For example, to run a command on the 15th of every month, and every Friday at 12:15, use the following job format:

15 12 15 * Fri command

The first five fields all use the same syntax rules:

* for “Do not Care”/always.
A number to specify a number of minutes or hours, a date, or a weekday. For weekdays, 0 equals Sunday, 1 equals Monday, 2 equals Tuesday, and so on. 7 also equals Sunday.
x-y for a range, x to y inclusive.
x,y for lists. Lists can include ranges as well, for example, 5,10-13,17 in the Minutes column to indicate that a job should run at 5, 10, 11, 12, 13, and 17 minutes past the hour.
*/x to indicate an interval of x, for example, */7 in the Minutes column runs a job every seven minutes.

Additionally, 3-letter English abbreviations can be used for both months and weekdays, for example, Jan, Feb, and Mon, Tue.

The last field contains the command to execute using the default shell. The SHELL environment variable can used to change the shell for the scheduled command. If the command contains an unescaped percentage sign (%), then that percentage sign is treated as a newline character, and everything after the percentage sign is passed to the command on stdin.

Example Recurring User Jobs

This section describes some examples of recurring jobs.

The following job executes the command /usr/local/bin/yearly_backup at exactly 9 a.m. on February 2nd, every year.
```
0 9 2 2 * /usr/local/bin/yearly_backup
```
The following job sends an email containing the word Chime to the owner of this job, every five minutes between 9 a.m. and 5 p.m., on every Friday in July.
```
*/5 9-16 * Jul 5 echo "Chime"
```
The preceding 9-16 range of hours means that the job timer starts at the ninth hour (09:00) and continues until the end of the sixteenth hour (16:59). The job starts executing at 09:00 with the last execution at 16:55 because five minutes from 16:55 is 17:00 which is beyond the given scope of hours.
The following job runs the command /usr/local/bin/daily_report every weekday at two minutes before midnight.
```
58 23 * * 1-5 /usr/local/bin/daily_report
```
The following job executes the mutt command to send the mail message Checking in to the recipient boss@example.com on every workday (Monday to Friday), at 9 a.m.
```
0 9 * * 1-5 mutt -s "Checking in" boss@example.com % Hi there boss, just checking in.
```

References

crond(8), crontab(1), and crontab(5) man pages

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Is It possible to reset root password when the system is on multi-user.target?

jennifer1987

27 lip 2023

Hi thereWell, i have a question, Is It possible to reset root password when the system is on multi-user.target? I was not be able to found a foro o some information related with this and another question that I have, If the grub menu dont show up, is it posible to reset the root password?

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Red Hat System Administration II (RH134)

Haley_Ruccio

21 lip 2023

Build the skills to perform the key tasks needed to become a full-time Linux administratorRed Hat System Administration II (RH134) is the second part of the RHCSA training track for IT professionals who have already attended Red Hat System Administration I. The course goes deeper into core Linux system administration skills in storage configuration and management, installation and deployment of Red Hat Enterprise Linux, management of security features such as SELinux, control of recurring system tasks, management of the boot process and troubleshooting, basic system tuning, and command-line automation and productivity. This course assumes that students have attended Red Hat System Administration I (RH124).Experienced Linux administrators who seek rapid preparation for the RHCSA certification should instead start with RHCSA Rapid Track (RH199).This course is based on Red Hat Enterprise Linux 9.0.Course summaryInstall Red Hat Enterprise Linux using scalable methodsAccess security files, file systems, and networksExecute shell scripting and automation techniquesManage storage devices, logical volumes, and file systemsManage security and system accessControl the boot process and system servicesRun containersAudience for this courseThis course is geared toward Windows system administrators, network administrators, and other system administrators who are interested in supplementing current skills or backstopping other team members, in addition to Linux system administrators who are responsible for these tasks:Configuring, installing, upgrading, and maintaining Linux systems using established standards and proceduresProviding operational supportManaging systems for monitoring system performance and availabilityWriting and deploying scripts for task automation and system administrationRecommended trainingSuccessful completion of Red Hat System Administration I (RH124) is recommended.Experienced Linux administrators seeking to accelerate their path toward becoming a Red Hat Certified System Administrator should start with the RHCSA Rapid Track course (RH199).Take our free assessment to gauge whether this offering is the best fit for your skills.Technology considerationsThere are no special technology requirements.This course is not designed for bring your own device (BYOD).Internet access is not required, but is recommended to allow students to research and access Red Hat online resources.

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18 lip 2023

We are excited to launch a space dedicated to the Red Hat Training course Red Hat System Administration II! To gain the most value from this group - click the "Join Group" button in the upper right hand corner.We encourage group members to collaborate in this group to discuss topics, ask questions, share best practices and tips, provide course feedback, and share their accomplishments as it relates to RH134.Read more about Red Hat System Administration II here.

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Revision: rh134-8.2-f0a9756