On modern Ubuntu, Debian and CentOS/RHEL-family systems, start with journalctl: systemd-journald collects kernel messages, service output and many application events in a structured journal. Traditional text files under /var/log remain important when rsyslog routes messages there or an application writes its own files.
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The commands below show how to query either source, investigate an earlier boot, make the journal survive reboots and safely restore rsyslog routing. Defaults differ by release, so verify paths and service unit names on the host you administer.
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Linux logging at a glance
There are two complementary logging paths:
- systemd-journald: a system service that collects kernel messages, libc syslog calls, native Journal API entries, service standard output and error, and audit records. You query it with
journalctl. - Traditional files: plain-text files in
/var/log, usually written by rsyslog or directly by an application. They are easy to inspect with tools such asless,tailandgrep.
Journald data is binary and indexed, so filters such as a unit name or boot ID are more reliable than searching a large text file. A file may still be the right place to look for a distribution-specific security log or an application’s own access and error records.
Command-first quick reference
| What you need | Command | What it does |
|---|---|---|
| All journal entries | sudo journalctl |
Displays the system journal, oldest entries first. |
| Current boot | sudo journalctl -b |
Limits output to the boot in progress. |
| List boots | sudo journalctl --list-boots |
Shows available boots, their relative indexes and boot IDs. |
| Previous boot | sudo journalctl -b -1 |
Reads the boot immediately before the current one, if it was retained. |
| A specific boot | sudo journalctl -b BOOT_ID |
Uses the ID printed by --list-boots. |
| One service | sudo journalctl -u ssh.service |
Shows entries associated with that systemd unit; substitute the real unit. |
| Follow a service live | sudo journalctl -u ssh.service -f |
Waits for and prints new entries as they arrive. |
| Kernel messages | sudo journalctl -k |
Filters the journal to kernel records. |
| System and kernel for this boot (Debian form) | sudo journalctl -b --system |
Queries system-wide messages for the selected boot. |
| Recent lines in a file | sudo tail -n 100 /var/log/syslog |
Prints the newest 100 lines from a traditional log. |
| Follow a file live | sudo tail -f /var/log/syslog |
Monitors appended text until you stop it with Ctrl+C. |
| Search a file | sudo grep -i 'error|fail|warning' /var/log/syslog |
Finds common failure words without regard to case. |
Use q to leave the pager opened by journalctl or less. Add time, priority or field filters when a broad query is too large; first narrow by boot or unit so that the result remains understandable.
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Where each distribution keeps logs
The logging architecture is similar across the three families, but file names and package defaults are not identical.
| Distribution family | Primary query interface | Common traditional files | Typical rsyslog role | Package manager | Service names to check |
|---|---|---|---|---|---|
| Ubuntu | journalctl |
/var/log/auth.log, /var/log/kern.log, /var/log/syslog |
Routes selected journal/syslog messages into text files when installed and configured. | apt |
systemd-journald, rsyslog |
| Debian | journalctl |
Commonly the same Debian-style files as Ubuntu when rsyslog rules create them. | Optional routing layer; inspect the active configuration rather than assuming every file exists. | apt |
systemd-journald, rsyslog |
| CentOS/RHEL family | journalctl |
/var/log/messages, /var/log/secure, /var/log/maillog, /var/log/cron, /var/log/boot.log |
journald forwards messages to rsyslog, which sorts them into files under /var/log when enabled. |
dnf |
systemd-journald, rsyslog |
Ubuntu’s documented meanings are useful when diagnosing a desktop or server: auth.log records authorization activity such as password prompts, sudo use and remote logins; kern.log contains kernel messages; and syslog contains general system information. Apache commonly writes /var/log/apache2/access.log and /var/log/apache2/error.log. On CentOS/RHEL, the security stream is normally secure rather than auth.log, and general system messages are commonly in messages.
For a distribution comparison, check four things rather than relying on a copied command: whether rsyslog is installed, which rules route messages, whether journald is persistent or volatile, and which account or journal-group policy permits reading. Package names and service unit names are usually consistent within a family (apt on Ubuntu and Debian, dnf on CentOS/RHEL), but an image or minimal installation can omit rsyslog entirely.
Viewing the journal effectively
Inspect the current boot first
Run sudo journalctl -b when a machine has just failed to start a service or behaved unexpectedly. The boot filter prevents older incidents from obscuring the event you are investigating. To inspect only kernel records, add -k; to inspect one unit, add -u UNIT.
Investigate a service
- Find the exact unit name with
systemctl list-units --type=serviceor the service’s package documentation. - Query it with
sudo journalctl -u UNIT, replacingUNITwith a name such asssh.service. - Limit the result to the current boot with
sudo journalctl -b -u UNIT. - Reproduce the problem while watching live output with
sudo journalctl -u UNIT -f.
A distribution may call the same daemon by a different unit name. For example, do not assume an sshd.service unit exists on a host where the installed package registers ssh.service; use the name reported by systemctl.
Compare boots after a reboot
- Run
sudo journalctl --list-boots. - Choose the relative index (for example,
-1for the immediately previous boot) or copy the complete boot ID. - Run
sudo journalctl -b -1orsudo journalctl -b BOOT_ID.
If the list contains only the current boot, the older journal was not retained. The persistence section below explains how to change that behavior.
Read text files without loading them all
Use sudo less /var/log/syslog on Ubuntu or a file that actually exists on the target system. Press / in less to search, and use sudo tail -n 100 for a quick recent sample. sudo tail -f is useful while restarting a daemon that writes directly to a file. A missing file is not proof that logging is broken: rsyslog may be absent, its rules may route elsewhere, or the application may log only to journald.
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Use the Ubuntu graphical viewer
On Ubuntu desktops, GNOME Logs can display, monitor, search, open and close log files. It is a front end to the same kinds of system and application records; use the command line when you need an exact boot, unit or kernel filter.
Understand the two-stage logging path
On a typical system, a daemon writes to standard output and error or to the syslog interface. Journald receives kernel messages, libc syslog calls, native Journal API entries, service output and audit records. On RHEL-family systems, Red Hat documents journald forwarding those messages to rsyslog; rsyslog then sorts them and writes selected classes under /var/log. Ubuntu and Debian can use the same complementary arrangement when rsyslog is installed and its rules create the relevant files.
This explains why the journal and a text file can show different results. A message may be present in journald but not routed to /var/log/syslog or /var/log/messages. Conversely, an application such as Apache may write its own access and error files without producing an equivalent service-unit stream. Check both sources when a unit query is quiet but the application is clearly handling requests.
Make journald logs persist across reboots
Journald stores binary data persistently below /var/log/journal or temporarily below /run/log/journal. The latter is normally a runtime filesystem and is lost during reboot. Whether old boots are available therefore depends on both storage configuration and the existence of the persistent directory.
Enable persistent storage with the distribution defaults
- Create the journal directory:
sudo mkdir -p /var/log/journal. - Apply the systemd ownership and mode rules:
sudo systemd-tmpfiles --create --prefix /var/log/journal. - Move runtime entries into the persistent journal:
sudo journalctl --flush. - Confirm that records are readable with
sudo journalctl --list-bootsand check space consumption withsudo journalctl --disk-usage.
These commands use the default journald policy. If you edit /etc/systemd/journald.conf, set the Storage= policy deliberately (for example, a persistent policy when reboot-to-reboot history is required), then apply it with sudo systemctl restart systemd-journald using the procedure documented for your release. Test with a fresh query and journalctl --disk-usage.
Do not promise a retention period without checking the host
Journald’s size and age limits are controlled by settings such as SystemMaxUse=, RuntimeMaxUse= and MaxRetentionSec=. Their effective values and defaults vary by release and local configuration. Inspect the active journald.conf and related drop-ins before telling an operator that logs will remain for a particular number of days.
Persistence is not the same as unlimited retention. A persistent directory can still contain only the records allowed by the configured maximum-use and retention settings, and a full filesystem can prevent new entries. Check disk usage after enabling persistence and include those limits in capacity planning. If a reboot is needed to verify the change, run journalctl --list-boots again afterward rather than assuming that creating the directory changed historical data that had already been discarded.
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Restore or customize rsyslog text-file routing
Use rsyslog when another tool, compliance workflow or administrator needs traditional files. Its main configuration is /etc/rsyslog.conf; local additions belong in /etc/rsyslog.d/*.conf. Prefer a new drop-in over casually replacing a distribution’s main file.
Install and enable the service
- On Ubuntu or Debian, install it with
sudo apt update && sudo apt install rsyslog. - On CentOS/RHEL-family systems, install it with
sudo dnf install rsyslog. - Enable and start it at boot with
sudo systemctl enable --now rsyslog.
Add a minimal local rule
A minimal modern configuration that loads the local socket and kernel inputs and writes to a single file is:
module(load="imuxsock")
module(load="imklog")
action(type="omfile" file="/var/log/syslog")Save local rules as a file such as /etc/rsyslog.d/20-local.conf. A CentOS/RHEL host may conventionally use /var/log/messages instead; match the naming used by the rest of that host’s configuration.
Validate before restarting
- Check syntax without replacing the running daemon:
sudo rsyslogd -N1. - Fix every reported parse or permission error before proceeding.
- Restart only after validation:
sudo systemctl restart rsyslog. - Generate a harmless test message:
logger -t logtest "hello". - Inspect the destination with
sudo tail -n 20 /var/log/syslogorsudo tail -n 20 /var/log/messages.
Custom destinations must be writable by the rsyslog process and protected with appropriate ownership and mode settings. Check the daemon’s own journal if it fails to start: sudo journalctl -u rsyslog.service -b.
Do not remove existing vendor rules merely to add one destination. A drop-in can coexist with the distribution’s modules and selectors, while overwriting /etc/rsyslog.conf can stop security, kernel or mail messages from reaching files that administrators already expect. After a change, test both the new path and any critical existing path.
Why a log is missing or empty
/var/log/syslog or /var/log/messages does not exist
- Query journald directly with
sudo journalctl -b; the system may be logging only to the journal. - Check whether rsyslog is installed and active:
systemctl status rsyslog. - Inspect
/etc/rsyslog.confand/etc/rsyslog.d/for the actual destination rules. - On Ubuntu, look for
syslog; on CentOS/RHEL, look formessages. Do not assume the other distribution’s filename.
The previous boot cannot be found
Run sudo journalctl --list-boots. If only the current boot appears, the journal was volatile, the persistent directory was absent, or older data was removed by configured limits. Create /var/log/journal, run systemd-tmpfiles --create --prefix /var/log/journal and journalctl --flush, then verify after the next reboot.
The service query returns nothing
- Confirm the unit spelling with
systemctl list-units --type=serviceand, for installed but inactive units,systemctl list-unit-files. - Check whether the program writes a private file, such as an Apache access or error log, instead of standard output.
- Remove an overly narrow boot or time filter and query
sudo journalctl -u UNITagain.
You receive a permission error
Authentication, authorization and audit records are sensitive. Use sudo for administration, or follow the host’s systemd-journal access policy; membership in the system journal group can grant read access. Restrict permissions on copied files and redact passwords, tokens, addresses and other secrets before sharing excerpts.
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Rsyslog fails its syntax check or will not restart
Run sudo rsyslogd -N1 and read the first reported error. Common causes are an invalid directive, a misspelled module, an unwritable custom directory or a conflicting rule in another file under /etc/rsyslog.d/. Correct the drop-in, validate again, restart the service and send a new logger test message.
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Rotation, storage limits and operational safety
Traditional files are normally rotated by the distribution’s logrotate policy, while journal files are rotated and limited by journald. Exact schedules and retention values are release-specific. Inspect /etc/logrotate.conf, /etc/logrotate.d/ and the active journald settings on the target host instead of assuming a universal interval.
Keep enough history to diagnose failures without allowing logs to consume the filesystem. Monitor journalctl --disk-usage, review the configured maximum-use and retention settings, and protect both binary journals and text files because they can reveal login activity, commands, network details and application data. When exporting a log for support, preserve timestamps and unit names but remove secrets.
A practical diagnostic workflow
- Identify the distribution and release, then check whether the failing component is a systemd unit.
- Start with
sudo journalctl -b -u UNIT; add-fwhile reproducing the failure. - For boot or hardware symptoms, compare
sudo journalctl -bwithsudo journalctl -k. - If the event predates the reboot, use
sudo journalctl --list-bootsand select the needed boot. - Check the distribution’s traditional files and the application’s documented log directory.
- If a text file is required but absent, inspect rsyslog installation, rules and service status.
- After any rsyslog change, run
sudo rsyslogd -N1, restart the service and verify withlogger. - Record the host’s actual journald and logrotate limits before designing retention or alerting policies.
Choosing the right source for a question
Use the journal first for questions about a service lifecycle, boot failure, kernel event or messages emitted to standard output and error. Unit and boot filters keep the result tied to the incident. Use a traditional file when a procedure names a distribution-specific stream such as Ubuntu’s auth.log or CentOS/RHEL’s secure, or when an application documents a private directory. For web troubleshooting, for example, Apache’s access and error files can show request and application details that are not obvious in the service journal.
When the two views disagree, do not assume one is corrupt. Confirm the time and boot, determine whether the process wrote to journald, a file, or both, and inspect rsyslog selectors if forwarding is expected. This approach also prevents a common mistake: creating a new empty file with the right name and concluding that the daemon is not logging, when the active rule actually writes to a different destination.
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- Record the host’s distribution, release and active unit names before copying commands from another system.
- Read the existing files in
/etc/rsyslog.confand/etc/rsyslog.d/before adding a rule. - Use a drop-in for local routing, verify destination ownership and permissions, and run
sudo rsyslogd -N1before a restart. - After changing journald storage, run
sudo journalctl --flush, inspect--disk-usageand verify the next boot is listed. - Protect exported logs. Authentication, authorization and audit entries may expose usernames, remote addresses, commands or other sensitive data.
Frequently Asked Questions
Where are Linux logs on Ubuntu, Debian and CentOS?
Query the systemd journal with sudo journalctl. Traditional paths vary: Ubuntu commonly has /var/log/auth.log, kern.log and syslog; CentOS/RHEL commonly has secure, messages, maillog, cron and boot.log. Debian may have Debian-style files when rsyslog rules create them.
How do I view logs from the previous boot?
Run sudo journalctl --list-boots, then use sudo journalctl -b -1 or specify the boot ID shown in the list. If only the current boot appears, older data was not retained.
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Why is /var/log/syslog or /var/log/messages empty?
The host may be logging only to journald, rsyslog may be absent or stopped, or its rules may route messages to another file. Check sudo journalctl -b, rsyslog status and the active files under /etc/rsyslog.conf and /etc/rsyslog.d/.
How do I make journald logs survive a reboot?
Create /var/log/journal, run sudo systemd-tmpfiles --create --prefix /var/log/journal, then run sudo journalctl --flush. Check Storage= and size limits in journald configuration, and verify with journalctl --list-boots after reboot.
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Use sudo journalctl -u UNIT -f, replacing UNIT with the exact unit name reported by systemctl. For a text file, use sudo tail -f /path/to/log.
Do I need rsyslog if journald works?
Not always. Journald is the primary structured interface on these systems. Install or enable rsyslog when another tool or workflow requires traditional files under /var/log, and validate any routing change with sudo rsyslogd -N1.
Why does a service query show no entries?
The unit name may be wrong, the query may be limited to the wrong boot, or the program may write a private application file. Confirm names with systemctl list-units --type=service and inspect the application’s documented log directory.
Can I share authentication or audit logs with support?
Treat them as sensitive. Use appropriate permissions, preserve useful timestamps and unit names, and redact passwords, tokens, addresses and other secrets before sharing.
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Use journalctl for structured system, kernel and service history, and check the distribution’s /var/log files when rsyslog or an application writes traditional logs. Persist the journal deliberately, validate rsyslog with rsyslogd -N1 before restarting it, and verify every path, unit name and retention limit on the exact Ubuntu, Debian or CentOS/RHEL release you administer.
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