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Schedule tasks using at and cron
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Job scheduling and execution is handled by the atd and crond daemons. While atd manages jobs scheduled to run once in the future, crond is responsible for running jobs repetitively at pre-specified times. At start-up, crond reads schedules in files located in the
/var/spool/cronand/etc/cron.ddirectories, and loads them in memory for later execution. -
There are 4 files that control permissions for setting scheduled jobs. These are at.allow, at.deny, cron.allow and cron.deny. These files are in the
/etcdirectory. The syntax of the files is identical, with each file taking 1 username per line. If no files exist, then no users are permitted. By default, the deny files exist and are empty, and the allow files do not exist. This opens up full access to using both tools for all users. -
All activities involving atd and crond are logged to the
/var/log/cronfile. -
The at command is used to schedule one-time execution of a program by the atd daemon. All submitted jobs are stored in the
/var/spool/atdirectory. -
To schedule a job using at the below syntax is used:
at 11:30pm 6/30/15
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The commands to execute are defined in the terminal, press ctrl+d when finished. The added job can be viewed with at and can be removed with the -d option.
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A shell script can also be provided:
at -f ~/script1.sh 11:30pm 6/30/15 -
The
/etc/crontabfile has the following columns:- 1: Minutes of hour (0-59), with multiple comma separated values, or * to represent every minute.
- 2: Hours of day (0-23), with multiple comma separated values, or * to represent every hour.
- 3: Days of month (1-31), with multiple comma separated values, or * to represent every day.
- 4: Month of year (1-12, jan-dec), with multiple comma separated values, or * to represent every month.
- 5: Day of week (0-6, sun-sat), with multiple comma separated values, or * to represent every day.
- 6: Full path name of the command or script to be executed, along with any arguments.
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Step values can be used with */2 meaning every 2nd minute.
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The crontab command can be used to edit the file. Common options are e (edit), l (view), r (remove):
crontab -e
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Start and stop services and configure services to start automatically at boot
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To check the status of a service:
systemctl status <service>
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To start a service:
systemctl start <service>
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To stop a service:
systemctl stop <service>
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To make a service reload its configuration:
systemctl reload <service>
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To make a service reload its configuration or restart if it can't reload:
systemctl reload-or-restart <service>
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To make a service start on boot:
systemctl enable <service>
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To stop a service starting on boot:
systemctl disable <service>
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To check if a service is enabled:
systemctl is-enabled <service>
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To check if a service has failed:
systemctl is-failed <service>
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To view the configuration file for a service:
systemctl cat /usr/lib/sysdtemd/system/<service>
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To view the dependencies for a service:
systemctl list-dependencies <service>
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To stop a service from being run by anyone but the system and from being started on boot:
systemctl mask <service>
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To remove a mask:
systemctl unmask <service>
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Configure systems to boot into a specific target automatically
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To get the default target:
systemctl get-default
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To list available targets:
systemctl list-units --type target --all
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To change the default target:
systemctl set-default <target>
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The change will take affect after a reboot.
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Configure time service clients
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To print the date:
date +%d%m%y-%H%M%S
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To set the system clock as per the hardware clock:
hwclock -s
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To set the hardware clock as per the system clock:
hwclock -w
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The timedatectl command can also be used to view the date and time.
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To change the date or time:
timedatectl set-time 2020-03-18 timedatectl set-time 22:43:00
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To view a list of time zones:
timedatectl list-timezones
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To change the time zone:
timedatectl set-timezone <timezone>
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To enable NTP:
timedatectl set-ntp yes
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To start the chronyd service:
systemctl start chronyd
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Install and update software packages from Red Hat Network, a remote repository, or from the local file system
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The .rpm extension is a format for files that are manipulated by the Redhat Package Manager (RPM) package management system. RHEL 8 provides tools for the installation and administration of RPM packages. A package is a group of files organised in a directory structure and metadata that makes up a software application.
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An RPM package name follows the below format:
openssl-1.0.1e-34.el7.x86_64.rpm # package name = openssl # package version = 1.0.1e # package release = 34 # RHEL version = el7 # processor architecture = x86_64 # extension = .rpm
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The subscription-manager command can be used to link a Red Hat subscription to a system.
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The dnf command is the front-end to rpm and is the preferred tool for package management. The yum command has been superseded by dnf in RHEL 8. It requires that the system has access to a software repository. The primary benefit of dnf is that it automatically resolves dependencies by downloading and installing any additional required packages.
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To list enabled and disabled repositories:
dnf repolist all dnf repoinfo
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To search for a package:
dnf search <package> dnf list <package>
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To view more information about a particular package:
dnf info <package>
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To install a package:
dnf install <package>
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To remove a package:
dnf remove <package>
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To find a package from a file:
dnf provides <path>
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To install a package locally:
dnf localinstall <path>
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To view available groups:
dnf groups list
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To install a group (e.g. System Tools):
dnf group "System Tools" -
To remove a group (e.g. System Tools):
dnf group remove "System Tools" -
To see the history of installations using dnf:
dnf history list -
To undo a particular installation (e.g. ID=22):
dnf history undo 22 -
To redo a particular installation (e.g. ID=22):
dnf history redo 22 -
To add a repository using the dnf config manager:
dnf config-manager --add-repo <url>
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To enable a repository using the dnf config manager:
dnf config-manager --enablerepo <repository>
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To disable a repository using the dnf config manager:
dnf config-manager --disablerepo <repository>
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To create a repository:
dnf install createrepo mkdir <path> createrepo --<name> <path> yum-config-manager --add-repo file://<path>
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Work with package module streams
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RHEL 8 introduced the concept of Application Streams. Components made available as Application Streams can be packaged as modules or RPM packages and are delivered through the AppStream repository in RHEL 8. Module streams represent versions of the Application Stream components. Only one module stream can be active at a particular time, but it allows multiple different versions to be available in the same dnf repository.
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To view modules:
dnf module list
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To get information about a module:
dnf module info --profile <module-name>
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To install a module:
dnf module install <module-name>
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To remove a module:
dnf module remove <module-name>
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To reset a module after removing it:
dnf module reset <module-name>
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To be specific about the module installation:
dnf module install <module-name>:<version>/<profile>
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To check the version of a module:
<module-name> -v
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Modify the system bootloader
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The GRUB2 configuration can be edited directly on the boot screen. The configuration can also be edited using the command line.
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To view the grub2 commands:
grub2
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To make a change to the configuration:
vi /etc/default/grub # Change a value grub2-mkconfig -o /boot/grub2/grub.cfg # View changes vi /boot/grub2/grub.cfg
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crontab -e
Edits the current crontab using the editor specified by the VISUAL or EDITOR environment variables.
The atd service needs to be running to run one-time only jobs.
at 11:25AM to schedule job.
You enter an interactive shell and you use Ctrl-D to close it.
Use at -l to list all pending jobs.
To see the contents of a scheduled job. at -c [job_number]
Use atrm to remove jobs from the list.
dnf repolist
dnf provides htop or dnf provides */Containerfile
dnf search htop
dnf search all htop
dnf update The same as dnf upgrade. The "update" is an alias for "upgrade".
dnf group install Only mandatory and default packages are installed, to see optional packages use dnf group info and to install with all optional packages do dnf group install --with-optional
dnf group list See group packages that you can install.
dnf group list hidden Some groups are normally only installed through environment groups and not separately, and for that reason don't show when using dnf group list
dnf list installed List installed software on the machine.
dnf history
dnf history info 10
dnf history undo 10
sudo dnf history rollback 10 Let's say you want to undo everything that was installed after number 10. This command seen below would remove mutt, emacs and powertop.
To reinstall something that was removed.
dnf history redo 15
During the exam your virtual machine will not have access to the internet. Hence, we cannot use subscription-manager and associated repos. No repositories will be available by default. This means we cannot install any packages by default.
Red Hat will tell you that a repository is available at a certain location, and you will have to configure the repository for that manually.
You need to be capable of configuring repository access, or you will fail the exam.
This is just and FYI. You will not have access to these repositories during the exam!.
To access repositories that are offered through subscription manager, use dnf config-manager --enable name-of-the-repository to add repository access.
To enable third party repositories, create a repo files in "/etc/yum.repos.d/".
Let's create a local repository from the RHEL 9 ISO file. I mount the RHEL 9 ISO in the virtual cdrom drive. For me, it's mounted on "/run/media/armann/RHEL-9-0-0-BaseOS-x86_64".
If you need to mount the cdrom manually. mount /dev/sr0 /mnt
Let's copy the iso file to our computer. Make sure you have around 9GB available
on the root of your hard disk. dd if=/dev/sr0 of=/rhel9.iso bs=1M status=progress
Let's edit "/etc/fstab" so it's mounted automatically for us. You can see the last line, that's how we mount the rhel9.iso automatically after boot.
Let's mount it. mount -a
Now we need to manually create the repository file.
Go under "/etc/yum.repos.d/".
Create a file, vim baseos.repo
Let's add the following lines into baseos.repo.
[baseos]
name=baseos
baseurl=file:///opt/iso/BaseOS
gpgcheck=0
Create another repo file.
Call it appstream.repo
[appstream]
name=appstream
baseurl=file:///opt/iso/AppStream
gpgcheck=0
Now we can check to see if our repository file is okay.
dnf repolist
We should see baseos and appstream and no errors.
BaseOS repo is for packages that don't change during the lifecycle of the OS. The AppStream repo is for packages that do change major versions during the lifecycle period of the Os.
To see all the software you are entitled to use with the subscription attached to the machine.
rct cat-cert /etc/pki/entitlement/5715597599610761455.pem
/usr/lib/systemd/system - systemd unit files that are distributed with the RPM package manager are found here
/run/systemd/system - systemd unit files that are created at runtime -takes priority over files in /usr/lib/systemd/system
/etc/systemd/system - systemd unit files created by systemctl enable {servicename}
/etc/systemd/system.com - systemd Configuration file
systemctl status
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List currently running services
systemctl list-units -t service -
List all services
systemctl list-units -t service --all -
List dependencies (A good overview of what is "on" and what is "off". )
systemctl list-dependencies -
List dependencies that start after the service starts
systemctl list-dependencies --aftersystemctl list-dependencies --after name.service -
List dependencies that start before the service starts (that this service relies upon)
systemctl list-dependencies --before -
To enable the service to start at boot
systemctl enable name.service -
To manually start a service
systemctl start name.service -
To restart a service
systemctl restart name.service
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To restart a service only if it's already running
systemctl try-restart name.service -
To reload a service (attempts to keep the service active while reloading config changes)
systemctl reload name.service -
To mask a service (prevents a service from being able to be started until unmasked)
systemctl mask name.service
To unmask a service
systemctl unmask name.service
Reload config
systemctl reload name.service
reload will reload a specific service. That means that the systemd will send a SIGHUP signal to a service, and that signal will tell the service to reload its configuration files, which has nothing to do with systemd config files.
Reload custom service files
systemctl daemon-reload
Location of custom systemd units is "/etc/systemd/system". This command reloads the service files in that directory.
Check the configuration of a unit file
systemctl cat sshd.service
Edit unit file
systemctl edit sshd.service
By default it uses Nano. To change that to Vim.
export SYSTEMD_EDITOR=/usr/bin/vim
This creates a drop-in-file in "/etc/systemd/system/"
If it does not create the drop-in-file automatically, do systemctl daemon-reload
To see tunables for a service do systemctl show httpd.service
/user/lib/systemd/system/ is for configuration files provided by packages. Do not edit those directly since they can be overwritten by newer packages.
/etc/tuned/ and /usr/lib/tuned/ need an explanation for it here.
Mask
To prevent certain units from starting up, use systemctl mask. It links a unit to the /dev/null device, which ensures it cannot be started. For instance systemctl mask nginx
systemctl unmask removes the unit mask.
Show all boots that have been logged. Needs to have persistent journalling.
journalctl --list-boots
journalctl -xrb
-x = Augment log lines with explanation texts from the message catalog. This will add explanatory help texts to log messages
-r = Reverse output so that the newest entries are displayed first.
-b = Show messages from a specific boot. This will add a match for "_BOOT_ID=".
The argument may be empty, in which case logs for the current boot will be shown.
You could use rsyslog to make the journal persistent.
"/etc/systemd/journal.conf" The setting "Storage=auto" ensures that persistent storage is happening automatically after manually creating the directory "/var/log/journal"
Then we need to restart the journal service.
systemctl restart systemd-journal-flush.service
View only messages with a priority error and higher.
journalctl -p err
View the last 10 lines, and adds new messages when they are added.
journalctl -f
Show messages for the sshd service only.
journalctl -u sshd.service
See current settings for growth and what it's currently using.
journalctl | grep -E 'Runtime Journal|System Journal'
When using systemd timers, the timer should be started, and not the service unit.
systemctl list-units -t timer
systemctl list-unit-files dnf-makecache*
systemctl status dnf-makecache.timer
Checkout the "Trigger and Triggers".
To schedule and activate a timer you use the "OnCalendar" option.
OnCalendar=*:00/20 runs every 20 minutes.
You can use "OnUnitActiveSec" to start the unit at a specific time after the unit was last activated.
You can use "OnBootSec" or "OnStartupSec" to start the unit a specific time after booting.
Install tuned. dnf install tuned
To see available commands, type this in and press double tab. tuned-adm
To see available profiles. tune-adm list
Config files for Tuned are located in "/usr/lib/tuned/".
The time zone configured on the server is found in /etc/localtime. This is a symbolic link that points to one of the time zones files in /usr/share/zoneinfo.
hwclock sets the hardware time.
If the hardware clock is not correct but the system time is you can sync the hardware time to the
system time with hwclock --systohc
Use date so show current date and time.
Use timedatectl to manage time and time zone configuration.
timedatectl status show all time properties in use.
timedatectl list-timezones show all available timezones.
timedatectl set-timezone Europe/Rome
timedatectl set-time
timedatectl set-timezone
timedatectl set-ntp enables or disables NTP sync.
An NTP service must be configured. The default for RHEL is Chrony. If your server is running systemd-timesyncd.service you must disable that service before enabling Chrony.
Chronyd is the default RHEL 9 NTP service. Use "/etc/chrony.conf" to change sync parameters.
Use iburst to permit fast synchronization.
After changing the conf file restart the chronyd service.
chronyc sources -v to see the servers you are synchronizing with.