Linux find error log

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Невозможно представить себе пользователя и администратора сервера, или даже рабочей станции на основе Linux, который никогда не читал лог файлы. Операционная система и работающие приложения постоянно создают различные типы сообщений, которые регистрируются в различных файлах журналов. Умение определить нужный файл журнала и что искать в нем поможет существенно сэкономить время и быстрее устранить ошибку.

Журналирование является основным источником информации о работе системы и ее ошибках. В этом кратком руководстве рассмотрим основные аспекты журналирования операционной системы, структуру каталогов, программы для чтения и обзора логов.

Основные лог файлы

Все файлы журналов, можно отнести к одной из следующих категорий:

• приложения;
• события;
• службы;
• системный.

Большинство же лог файлов содержится в директории /var/log.

• /var/log/syslog или /var/log/messages содержит глобальный системный журнал, в котором пишутся сообщения с момента запуска системы, от ядра Linux, различных служб, обнаруженных устройствах, сетевых интерфейсов и много другого.
• /var/log/auth.log или /var/log/secure — информация об авторизации пользователей, включая удачные и неудачные попытки входа в систему, а также задействованные механизмы аутентификации.
• /var/log/dmesg — драйвера устройств. Одноименной командой можно просмотреть вывод содержимого файла. Размер журнала ограничен, когда файл достигнет своего предела, старые сообщения будут перезаписаны более новыми. Задав ключ --level= можно отфильтровать вывод по критерию значимости.

Поддерживаемые уровни журналирования (приоритеты):
   emerg - система неиспользуемая
   alert - действие должно быть произведено немедленно
    crit - условия критичности
     err - условия ошибок
    warn - условия предупреждений
  notice - обычные, но значимые условия
    info - информационный
   debug - отладочные сообщения

(5:520)$ dmesg -l err
[1131424.604352] usb 1-1.1: 2:1: cannot get freq at ep 0x1
[1131424.666013] usb 1-1.1: 1:1: cannot get freq at ep 0x81
[1131424.749378] usb 1-1.1: 1:1: cannot get freq at ep 0x81

• /var/log/alternatives.log — Вывод программы update-alternatives, в котором находятся символические ссылки на команды или библиотеки по умолчанию.
• /var/log/anaconda.log — Записи, зарегистрированные во время установки системы.
• /var/log/audit — Записи, созданные службой аудита auditd.
• /var/log/boot.log — Информация, которая пишется при загрузке операционной системы.
• /var/log/cron — Отчет службы crond об исполняемых командах и сообщения от самих команд.
• /var/log/cups — Все, что связано с печатью и принтерами.
• /var/log/faillog — Неудачные попытки входа в систему. Очень полезно при проверке угроз в системе безопасности, хакерских атаках, попыток взлома методом перебора. Прочитать содержимое можно с помощью команды faillog.
• var/log/kern.log — Журнал содержит сообщения от ядра и предупреждения, которые могут быть полезны при устранении ошибок пользовательских модулей встроенных в ядро.
• /var/log/maillog/ или /var/log/mail.log — Журнал почтового сервера, используемого на ОС.
• /var/log/pm-powersave.log — Сообщения службы экономии заряда батареи.
• /var/log/samba/ — Логи файлового сервера Samba, который используется для доступа к общим папкам Windows и предоставления доступа пользователям Windows к общим папкам Linux.
• /var/log/spooler — Для представителей старой школы, содержит сообщения USENET. Чаще всего бывает пустым и заброшенным.
• /var/log/Xorg.0.log — Логи X сервера. Чаще всего бесполезны, но если в них есть строки начинающиеся с EE, то следует обратить на них внимание.

Для каждого дистрибутива будет отдельный журнал менеджера пакетов.

• /var/log/yum.log — Для программ установленных с помощью Yum в RedHat Linux.
• /var/log/emerge.log — Для ebuild-ов установленных из Portage с помощью emerge в Gentoo Linux.
• /var/log/dpkg.log — Для программ установленных с помощью dpkg в Debian Linux и всем семействе родственных дистрибутивах.

И немного бинарных журналов учета пользовательских сессий.

• /var/log/lastlog — Последняя сессия пользователей. Прочитать можно командой last.
• /var/log/tallylog — Аудит неудачных попыток входа в систему. Вывод на экран с помощью утилиты pam_tally2.
• /var/log/btmp — Еже один журнал записи неудачных попыток входа в систему. Просто так, на всякий случай, если вы еще не догадались где следует искать следы активности взломщиков.
• /var/log/utmp — Список входов пользователей в систему на данный момент.
• /var/log/wtmp — Еще один журнал записи входа пользователей в систему. Вывод на экран командой utmpdump.

(5:535)$ sudo utmpdump /var/log/wtmp
[5] [02187] [l0  ] [        ] [4.0.5-gentoo     ] [0.0.0.0     ] [Вт авг 11 16:50:07 2015]
[1] [00000] [~~  ] [shutdown] [4.0.5-gentoo     ] [0.0.0.0     ] [Вт авг 11 16:50:08 2015]
[2] [00000] [~~  ] [reboot  ] [3.18.12-gentoo   ] [0.0.0.0     ] [Вт авг 11 16:50:57 2015]
[8] [00368] [rc  ] [        ] [3.18.12-gentoo   ] [0.0.0.0     ] [Вт авг 11 16:50:57 2015]
[1] [20019] [~~  ] [runlevel] [3.18.12-gentoo   ] [0.0.0.0     ] [Вт авг 11 16:50:57 2015]

И другие журналы

Так как операционная система, даже такая замечательная как Linux, сама по себе никакой ощутимой пользы не несет в себе, то скорее всего на сервере или рабочей станции будет крутится база данных, веб сервер, разнообразные приложения. Каждое приложения или служба может иметь свой собственный файл или каталог журналов событий и ошибок. Всех их естественно невозможно перечислить, лишь некоторые.

• /var/log/mysql/ — Лог базы данных MySQL.
• /var/log/httpd/ или /var/log/apache2/ — Лог веб сервера Apache, журнал доступа находится в access_log, а ошибки — в error_log.
• /var/log/lighthttpd/ — Лог веб сервера lighttpd.

В домашнем каталоге пользователя могут находится журналы графических приложений, DE.

• ~/.xsession-errors — Вывод stderr графических приложений X11.

Initializing  "kcm_input" :  "kcminit_mouse"
Initializing  "kcm_access" :  "kcminit_access"
Initializing  "kcm_kgamma" :  "kcminit_kgamma"
QXcbConnection: XCB error: 3 (BadWindow), sequence: 181, resource id: 10486050, major code: 20 (GetProperty), minor code: 0
kf5.kcoreaddons.kaboutdata: Could not initialize the equivalent properties of Q*Application: no instance (yet) existing.
QXcbConnection: XCB error: 3 (BadWindow), sequence: 181, resource id: 10486050, major code: 20 (GetProperty), minor code: 0
Qt: Session management error: networkIdsList argument is NULL

• ~/.xfce4-session.verbose-log — Сообщения рабочего стола XFCE4.

Чем просматривать — lnav

Почти все знают об утилите less и команде tail -f. Также для этих целей сгодится редактор vim и файловый менеджер Midnight Commander. У всех есть свои недостатки: less неважно обрабатывает журналы с длинными строками, принимая их за бинарники. Midnight Commander годится только для беглого просмотра, когда нет необходимости искать по сложному шаблону и переходить помногу взад и вперед между совпадениями. Редактор vim понимает и подсвечивает синтаксис множества форматов, но если журнал часто обновляется, то появляются отвлекающие внимания сообщения об изменениях в файле. Впрочем это легко можно обойти с помощью <:view /path/to/file>.

Недавно я обнаружил еще одну годную и многообещающую, но слегка еще сыроватую, утилиту — lnav, в расшифровке Log File Navigator.

Установка пакета как обычно одной командой.

$ aptitude install lnav #Debian/Ubuntu/LinuxMint
$ yum install lnav #RedHat/CentOS
$ dnf install lnav #Fedora
$ emerge -av lnav #Gentoo, нужно добавить в файл package.accept_keywords
$ yaourt -S lnav #Arch

Навигатор журналов lnav понимает ряд форматов файлов.

• Access_log веб сервера.
• CUPS page_log
• Syslog
• glog
• dpkg.log
• strace
• Произвольные записи с временными отметками
• gzip, bzip
• Журнал VMWare ESXi/vCenter

Что в данном случае означает понимание форматов файлов? Фокус в том, что lnav больше чем утилита для просмотра текстовых файлов. Программа умеет кое что еще. Можно открывать несколько файлов сразу и переключаться между ними.

(5:471)$ sudo lnav /var/log/pm-powersave.log /var/log/pm-suspend.log

Программа умеет напрямую открывать архивный файл.

(5:471)$ lnav -r /var/log/Xorg.0.log.old.gz

Показывает гистограмму информативных сообщений, предупреждений и ошибок, если нажать клавишу <i>. Это с моего syslog-а.

Mon May 02 20:25:00        123 normal         3 errors         0 warnings         0 marks
Mon May 02 22:40:00          2 normal         0 errors         0 warnings         0 marks
Mon May 02 23:25:00         10 normal         0 errors         0 warnings         0 marks
Tue May 03 07:25:00         96 normal         3 errors         0 warnings         0 marks
Tue May 03 23:50:00         10 normal         0 errors         0 warnings         0 marks
Wed May 04 07:40:00         96 normal         3 errors         0 warnings         0 marks
Wed May 04 08:30:00          2 normal         0 errors         0 warnings         0 marks
Wed May 04 10:40:00         10 normal         0 errors         0 warnings         0 marks
Wed May 04 11:50:00        126 normal         2 errors         1 warnings         0 marks

Кроме этого поддерживается подсветка синтаксиса, дополнение по табу и разные полезности в статусной строке. К недостаткам можно отнести нестабильность поведения и зависания. Надеюсь lnav будет активно развиваться, очень полезная программа на мой взгляд.

Использованные материалы

1. lnav — An Advanced Log File viewer for Linux
2. What Are Linux Logs? How to View Them, Most Important Directories, and More
3. Как посмотреть логи в Linux

What are Linux Log Files?

All Linux systems create and store information log files for boot processes, applications, and other events. These files can be a helpful resource for troubleshooting system issues.

Most Linux log files are stored in a plain ASCII text file and are in the /var/log directory and subdirectory. Logs are generated by the Linux system daemon log, syslogd or rsyslogd.

This tutorial will walk you through how to find and read Linux log files, and configure the system logging daemon.

Prerequisites

• Access to Linux
• A user account with root user privileges

1. First, open the Linux terminal as a root user. This will enable root privileges.

2. Use the following command to see the log files:

cd /var/log

3. To view the logs, type the following command:

ls

The command displays all Linux log files, such as kern.log and boot.log. These files contain the necessary information for the proper function of the operating system.

Log files are accessed using root privileges. By definition, root is the default account that has access to all Linux files.

Use the following example line command to access the respective file:

sudo less [log name here].log

This command displays a timeline of all information related to that operation.

Note that log files are stored in plain text so they can be viewed by using the following standard commands:

zcat – Displays all the contents of logfile.gz

zmore – See the file in pages, without decompressing the files

zgrep – Search inside a compressed file

grep – Find all occurrences of a search term in a file or filter a log file

tail – Output the last few lines of files

head– View the very beginning of text files

Important Linux System Logs

Logs can tell a lot about the operations of a system. A good understanding of each type of file will help how to distinguish the respective logs.

Most directories can be grouped into one of four categories:

1. System Logs
2. Event Logs
3. Application Logs
4. Service Logs

Many of these logs can be located in the var/log subdirectory.

System Logs

Systems log files are needed for Linux to work. On its own, it contains the most significant amount of information about system functionality. The most common log files are:

• /var/log/boot.log: System Boot log (the boot log stores all information related to booting operations)
• /var/log/auth.log: Auth logs (the authentication log stores all authentication logs, including successful and failed attempts)
• /var/log/httpd/: Apache access and error logs
• /var/log/mysqld.log: MySQL database server log file
• /var/log/debug: Debug logs (the debug log stores detailed messages related to debugging and is useful for troubleshooting specific system operations)
• /var/log/daemon.log: Daemon logs (the daemon log contains information about events related to running the Linux operation)
• /var/log/maillog: Mail server logs (the mail log stores information related to mail servers and archiving emails)
• /var/log/kern.log: Kernel logs (the kernel log stores information from the Ubuntu Linux kernel)
• /var/log/yum.log: Yum command logs

System Logging Daemon

A daemon log is a program that runs in the background and is essential for system operations. These logs have their own category of logs and are seen as the heart of the logging operations for any system.

The path for the system login daemon’s configuration is /etc/syslog.conf.

Each file consists of a selector and an action entry field. The syslogd daemon can forward log messages as well. This can be useful for debugging purposes.

Application Logs

Application logs store information relevant to any application that is executed. This can include error messages, signs of system compromise, and browser identification string.

Log files that fall into this category include CUPS Print System logs, Rootkit Hunter log, Apache HTTP server logs, Samba SMB server logs, and X11 server log.

Non-Human-Readable Logs

Not all logs are designed in a human-readable format. Some are designed only to be read by system applications. Such files are often related to login information. They include login failure logs, last logins logs, and login records.

There are tools and software for reading Linux log files. They are not necessary for reading files as most can be read directly from the Linux terminal.

Supplemental GUIs for Viewing Linux Log Files

System Log Viewer is a GUI that can be used to monitor system logs.

The interface provides several functions for managing logs, including a log statistics display. It is a user-friendly log monitoring GUI.

Useful features include:

• A live view of logs
• Number of lines in the log
• Log size
• Most recent log dates
• Modifications made to logs
• Filters
• Keyboard Shortcuts

Alternatively, use Xlogmaster which can monitor a considerable number of log files. It features three different modes:

• Run mode: Starts a specified program and obtains stdout
• Cat mode: Cats files within specified intervals
• Tail mode: Checks log files within regular intervals

Xlogmaster is useful for increasing security. It translates all data for highlighting, hiding lines, and displays this information for taking user requested action.

How to Configure Log Files on Ubuntu and CentOS

This section explains different mechanisms for configuring log files. Let’s start with a CentOS example.

To view users currently logged onto a Linux server, enter the who command as a root user:

who

This also lists the login history of users. To view the login history of the system administrator, enter the following command:

last reboot

To view information of the last login, enter:

lastlog

Execute Log Rotation

Log files that have zeroes appended at the end are rotated files. That means log file names have automatically been changed within the system.

The purpose of log rotation is to compress outdated logs that are taking up space. Log rotation can be done using the logrotate command. This command rotates, compresses, and mails system logs.

logrotate handles systems that create significant amounts of log files. The command is used by the cron scheduler and reads the logrotate configuration file /etc/logrotate.conf. It’s also used to read files in the logrotate configuration directory.

To include additional functionality to logrotate, start by entering the following command:

var/log/log name here].log {
Missingok
Notifempty
Compress
Size 20k
Daily
Create 0600 root root
}

It compresses and resizes the desired log file.

The commands perform the actions as follows:

missingok – Tells logrotate not to output an error if a log file is missing

notifempty – Does not rotate the log file if it is empty. It reduces the size of the log file with gzip

size – Ensures that the log file does not exceed the specified dimension and rotates it otherwise

daily – Rotates the log files on a daily schedule. This can also be done on a weekly or monthly schedule

create – Instantiates a log file where the owner and group are a root user

Conclusion

A thorough understanding of how to view and read Linux logs is necessary for troubleshooting a Linux system.

Using the proper commands and tools can simplify this process.

This tutorial explains the basic administration of a Linux server through system
logs. A system log is a file that contains information about the events that
happened on the system during runtime.

Prerequisites

Before proceeding with the rest of this tutorial, ensure that you have a basic
knowledge of working with the Linux command line. While many of the concepts
discussed in this article are general applicable to all Linux distributions,
we’ll be demonstrating them in Ubuntu only so ensure to set up an Ubuntu 20.04
server that includes a non-root user with sudo access.

Step 1 — Finding Linux system logs

All Ubuntu system logs are stored in the /var/log directory. Change into this
directory in the terminal using the command below:

You can view the contents of this directory by issuing the following command:

You should see a similar output to the following:

alternatives.log  auth.log       btmp            cloud-init-output.log  dmesg    dpkg.log  journal/  landscape/  private/  ubuntu-advantage-license-check.log  ubuntu-advantage-timer.log  unattended-upgrades/
apt/              bootstrap.log  cloud-init.log  dist-upgrade/          dmesg.0  faillog   kern.log  lastlog     syslog    ubuntu-advantage.log                ufw.log                     wtmp

Let’s look at a few of the essential system log files that may be present in the
/var/log directory and what they contain:

• /var/log/syslog: stores general information about any global activity in the
  system.
• /var/log/auth.log: keeps track of all security-related actions (login,
  logout, or root user activity).
• /var/log/kern.log: stores information about events originating from the
  Linux kernel.
• /var/log/boot.log: stores system startup messages.
• /var/log/dmesg: contains messages related to device drivers.
• /var/log/faillog: keeps track of failed logins, which comes in handy when
  investigating attempted security breaches.

The /var/log directory is also used to store various application logs. For
example, if your distribution is bundled with Apache or MySQL, or installed
later, their log files will also be found here.

Step 2 — Viewing Linux log file contents

Log files contain a large amount of information that are useful for monitoring
or analyzing activities performed by the system or a specific application.
Therefore, a Linux server administrator must learn the art of reading and
understanding the various messages present in log files to effectively diagnose
or troubleshoot an issue.

Before we can read log files, we ought to know how they are formatted. Let’s
review two basic approaches to log file formatting and storage: plain text and
binary files.

Plaintext log files

These logs are plain text files with a standardized content format. Ubuntu uses
a log template called
RSYSLOG_TraditionalFileFormat.
This log format consists of four main fields with a space delimiter:

1. The timestamp indicates the time when a log entry was created in the
   format MMM dd HH:mm:ss (e.g. Sep 28 19:00:00). Notice that this format
   does not include a year.
2. Hostname is the host or system that originally create the message.
3. Application is the application that created the message.
4. Message contains the actual details of an event.

Let’s go ahead and review some log files in the plaintext format. Run the
command below to print the contents of the /var/log/syslog file with the
tail utility:

sudo tail /var/log/syslog

This outputs the last 10 lines of the file:

Mar 23 12:38:09 peter dbus-daemon[1757]: [session uid=1000 pid=1757] Activating via systemd: service name='org.freedesktop.Tracker1' unit='tracker-store.service' requested by ':1.1' (uid=1000 pid=1754 comm="/usr/libexec/tracker-miner-fs " label="unconfined")
Mar 23 12:38:09 peter systemd[1743]: Starting Tracker metadata database store and lookup manager...
Mar 23 12:38:09 peter dbus-daemon[1757]: [session uid=1000 pid=1757] Successfully activated service 'org.freedesktop.Tracker1'
Mar 23 12:38:09 peter systemd[1743]: Started Tracker metadata database store and lookup manager.
Mar 23 12:38:40 peter tracker-store[359847]: OK
Mar 23 12:38:40 peter systemd[1743]: tracker-store.service: Succeeded.
Mar 23 12:39:01 peter CRON[359873]: (root) CMD (  [ -x /usr/lib/php/sessionclean ] && if [ ! -d /run/systemd/system ]; then /usr/lib/php/sessionclean; fi)
Mar 23 12:39:23 peter systemd[1]: Starting Clean php session files...
Mar 23 12:39:23 peter systemd[1]: phpsessionclean.service: Succeeded.
Mar 23 12:39:23 peter systemd[1]: Finished Clean php session files.

You’ll notice that that each record in this file is formatted in the manner
described earlier. For example, the last record has its timestamp as Mar 23
12:39:23, hostname as peter, application as systemd[1] and message as
Finished Clean php session files.

If you want to view the entire log file, you can use the cat utility or any
text editor such as nano or vim.

Binary log files

While plaintext is the dominant storage format for log files, you will also
encounter binary log files that cannot be read with a normal text editor. The
/var/log directory contains multiple binary files that are related to the user
authorization:

• /var/log/utmp: tracks users that are currently logged into the system.
• /var/log/wtmp: tracks previously logged in users. It contains a past data
  from utmp.
• /var/log/btmp: tracks failed login attempts.

For these binary logs, special command-line tools are used to display the
relevant information in human-readable form. For example, to review the contents
of the /var/log/utmp file, run the who utility with -H option (this option
causes column labels to be printed in the output table):

You’ll see the program’s output appear on the screen:

NAME   LINE         TIME             COMMENT
george pts/0        2021-03-21 15:29 (2001:67c:1220:80c:b1:a84e:69ee:f530)
willie pts/1        2021-03-21 07:20 (adsl-dyn22.78-98-29.t-com.sk)
bonnie pts/2        2021-03-21 10:31 (2001:67c:1220:80c:b1:a84e:69ee:f530)
peter  pts/6        2021-03-21 14:37 (100.64.97.50)
...

The output above describes all the currently logged in users, the time of login
and their host machine’s IP address.

You can also review the contents of the /var/log/wtmp binary file through the
last command as shown below:

You’ll see the program’s output appear on the screen:

peter    :1           Sat Mar 13 08:06   still logged in
reboot   system boot  Sat Mar 13 08:06   still running
peter    :1           Fri Mar 12 07:42 - down  (1+00:22)
reboot   system boot  Fri Mar 12 07:42 - 08:05 (1+00:23)
peter    :1           Sun Mar  7 11:20 - down  (4+20:21)
reboot   system boot  Sun Mar  7 11:20 - 07:41 (4+20:21)
peter    :1           Fri Mar  5 08:02 - crash (2+03:17)
reboot   system boot  Fri Mar  5 08:01 - 07:41 (6+23:39)
peter    :0           Tue Mar  2 08:38 - crash (2+23:23)
reboot   system boot  Tue Mar  2 08:38 - 07:41 (9+23:03)
peter    :1           Thu Feb 25 11:44 - down  (4+20:53)
reboot   system boot  Thu Feb 25 11:44 - 08:37 (4+20:53)

wtmp begins Thu Feb 25 11:43:23 2021

The output shows a table where the first column refers to the user name (the
pseudo-user reboot is recorded each time when the system is rebooted). The third
field refers to the login timestamp, and the last column shows the session
duration.

To review the /var/log/btmp file (containing failed login attempts), execute
the lastb command with sudo privileges:

You’ll see the program’s output appear on the screen:

falcon   tty3                  Thu Feb 12 07:10 - 07:10  (00:00)
ruby     tty1                  Thu Feb 12 07:09 - 07:09  (00:00)
sergio   tty1                  Thu Feb 12 07:09 - 07:09  (00:00)

btmp begins Thu Feb 25 11:43:32 2021

The output shows users that failed to login with the corresponding timestamp.

Step 3 — Examining the syslog deamon configuration

All system logs are created and maintained by a background process called a
daemon. The traditional Linux daemon for logging is syslogd. However, Ubuntu
20.04 uses a daemon called rsyslogd which is a superset of syslogd. It uses
a special configuration file (/etc/rsyslog.conf) that specifies the logging
rules.

Go ahead and print the contents of the /etc/rsyslog.conf file with the cat
command:

This command prints the entire content of this configuration file, but we’re
only going to show a truncated output here:

[label /etc/rsyslog.conf]
. . .

###########################
#### GLOBAL DIRECTIVES ####
###########################

#
# Use traditional timestamp format.
# To enable high precision timestamps, comment out the following line.
#
$ActionFileDefaultTemplate RSYSLOG_TraditionalFileFormat

# Filter duplicated messages
$RepeatedMsgReduction on

#
# Set the default permissions for all log files.
#
$FileOwner syslog
$FileGroup adm
$FileCreateMode 0640
$DirCreateMode 0755
$Umask 0022
$PrivDropToUser syslog
$PrivDropToGroup syslog

#
# Where to place spool and state files
#
$WorkDirectory /var/spool/rsyslog

#
# Include all config files in /etc/rsyslog.d/
#
$IncludeConfig /etc/rsyslog.d/*.conf

This file contains a lot of information, but we’ll focus on two configuration
details. Firstly, a variable called $ActionFileDefaultTemplate defines the
syslog record format as described in Step 2. You can change the value of this
variable if the default log format is unsuitable for you. Secondly, the last
line in the file defines a variable called $IncludeConfig that specifies the
directory for additional configuration files.

In Ubuntu, all additional Rsyslog rules are placed in the
/etc/rsyslog.d/50-default.conf file by default. Go ahead and examine the
contents of this file with the head utility (the -n 15 option specifies that
only the first 15 lines should be printed):

head -n 15 /etc/rsyslog.conf/50-default.conf

You’ll see the program’s output appear on the screen:

#  Default rules for rsyslog.
#
#           For more information see rsyslog.conf(5) and /etc/rsyslog.conf

#
# First some standard log files.  Log by facility.
#
auth,authpriv.*         /var/log/auth.log
*.*;auth,authpriv.none      -/var/log/syslog
#cron.*             /var/log/cron.log
#daemon.*           -/var/log/daemon.log
kern.*              -/var/log/kern.log
#lpr.*              -/var/log/lpr.log
mail.*              -/var/log/mail.log
#user.*             -/var/log/user.log

The output contains rsyslogd configuration rules. Each non-empty line (or line
that does not start with the # character) defines a rule. The rule definition
starts with a selector followed by one or more spaces and an action field:

• The selector specifies the facility with a corresponding priority. For
  example, the * selector refers to all facilities or priorities.
• The action field of a selector usually references a log file.

Step 4 — Rotating log files in Linux

The size of log file must be controlled because they always grow over time. Each
system has limited resources and logs that are too large can lead to performance
and memory problems, not to mention the loss of storage space. Linux
distributions typically solve this problem through the concept of log rotation
which continuously repeats the following actions:

1. Instead of continuously writing to log file as it grows larger, the file name
   is changed to one with a version suffix, and creates a brand new file is
   created for new log entries. This means a log file has multiple backups which
   are optionally compressed.
2. When the backup files reaches a specified number, the system deletes the
   oldest ones.

Let’s view an example of rotating log files in Linux. Execute the ls command
with following options:

ls -l -h -t /var/log/syslog*

The -l option outputs a listing that includes various metadata about a file,
the -h option prints the file size in human-readable form, and the -t option
sorts the listing by modification time (newest first). The /var/log/syslog*
argument specifies that only files in the /var/log directory with the syslog
prefix should be included in the output.

You’ll see the program’s output appear on the screen:

-rw-r----- 1 syslog adm  47K mar 30 09:49 /var/log/syslog
-rw-r----- 1 syslog adm 3,5G mar 30 07:45 /var/log/syslog.1.gz
-rw-r----- 1 syslog adm 1,6M mar 29 10:06 /var/log/syslog.2.gz
-rw-r----- 1 syslog adm  29K mar 28 07:49 /var/log/syslog.3.gz
-rw-r----- 1 syslog adm  54K mar 27 08:08 /var/log/syslog.4.gz
-rw-r----- 1 syslog adm 6,4M mar 26 07:35 /var/log/syslog.5.gz
-rw-r----- 1 syslog adm  31K mar 25 08:01 /var/log/syslog.6.gz

This output describes all the versions of the syslog file. Typically, it is
the biggest log file on the system because, as explained earlier, it records
almost every event that occurs in the system. The older versions are labelled
with a version suffix (e.g. syslog.6.gz is the oldest syslog backup).

Notice that the backup files are all compressed with the standard GNU zip
compression algorithm. (as evidenced by the .gz extension). This helps with
space savings since log files can grow to the size of gigabytes (in our example
the biggest file is 3.5 GB). You’ll also notice that these files cover a time
interval of only six days.

Step 5 — Configuring the logrotate daemon

Log rotation is maintained by a system daemon called logrotate. Similar to the
rsyslogd utility, this daemon uses a special configuration file called
logrotate.conf in the /etc directory.

Go ahead and display the contents of the logrotate config file with cat
utility:

This command prints the entire contents of the configuration file to the screen:

# see "man logrotate" for details
# rotate log files weekly
weekly

# use the adm group by default, since this is the owning group
# of /var/log/syslog.
su root adm

# keep 4 weeks worth of backlogs
rotate 4

# create new (empty) log files after rotating old ones
create

# use date as a suffix of the rotated file
#dateext

# uncomment this if you want your log files compressed
#compress

# packages drop log rotation information into this directory
include /etc/logrotate.d

# system-specific logs may be also be configured here.

The output describes the global configuration for logrotate. In our example,
the log files are rotated weekly, the system keeps four rotation backups, and
compression is turned off. However, this is the most general configuration for
any log. We can also set up a more specific configuration for a particular log
file. In Ubuntu, the configuration for specific logs are placed in the
/etc/logrotate.d directory by default (you’ll notice the presence of this
directory in /etc/logrotate.conf).

Let’s view the contents of the /etc/logrotate.d directory by executing the
ls command:

The output below describes all the utilities that have a specific log rotation
config:

alternatives  bootlog      dpkg               ubuntu-advantage-tools
apache2       btmp         ppp                ufw
apport        certbot      rsyslog            unattended-upgrades
apt           cups-daemon  speech-dispatcher  wtmp

You’ll notice that the rsyslog daemon has its own log rotation configuration
file. Go ahead and display the first 15 lines of the rsyslog file with the
head utility:

head -n 15 /etc/logrotate.d/rsyslog

You’ll see the following output on the screen:

/var/log/syslog
{
    rotate 7
    daily
    missingok
    notifempty
    delaycompress
    compress
    postrotate
        /usr/lib/rsyslog/rsyslog-rotate
    endscript
}

/var/log/mail.info
/var/log/mail.warn

This output shows that the syslog file is rotated daily, and that seven
compressed backups are kept before older ones are deleted.

You can force logrotate to rotate a log file immediately by executing
following the command:

sudo logrotate -fv /etc/logrotate.conf

The -f option forces immediate rotation and the -v option turns on verbose
mode (it will display messages during rotation). The execution of this command
shows the following output (truncated to save space):

reading config file /etc/logrotate.conf
including /etc/logrotate.d
reading config file alternatives
reading config file apache2
reading config file apport
reading config file apt
reading config file bootlog
reading config file btmp
reading config file certbot
reading config file cups-daemon
reading config file dpkg
reading config file ppp
reading config file rsyslog
reading config file speech-dispatcher
reading config file ubuntu-advantage-tools
reading config file ufw
reading config file unattended-upgrades
reading config file wtmp
. . .

The beginning of the output shows that the logrotate daemon reads all its
configuration files first before proceeding. The entire output is very long
because it prints every single detail of the rotation process.

Conclusion

In this tutorial, you’ve learnt the basics of system logs in Linux and how to
read and understand them. We discussed where the logs are typically placed and
the different log formats you will likely encounter. We also covered the
rsyslogd utility, which is responsible for maintaining log files in Ubuntu
before discussing log rotation and how to use the logrotate utility to keep
log files small and manageable. To learn more about all the utilities described
in this article, explore the
Rsyslog server article and the
Logrotate article in
the Logging guides.

Thanks for reading!

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