Linux Log Files

Log files locations

There are many different log files that all serve different purposes. When trying to find a log about something, you should start by identifying the most relevant file. Below is a list of common log file locations.

System logs

System logs deal with exactly that - the Ubuntu system - as opposed to extra applications added by the user. These logs may contain information about authorizations, system daemons and system messages.

Authorization log

Location: /var/log/auth.log

Keeps track of authorization systems, such as password prompts, the sudo command and remote logins.

Daemon Log

Location: /var/log/daemon.log

Daemons are programs that run in the background, usually without user interaction. For example, display server, SSH sessions, printing services, bluetooth, and more.

Debug log

Location: /var/log/debug

Provides debugging information from the Ubuntu system and applications.

Kernel log

Location: /var/log/kern.log

Logs from the Linux kernel.

System log

Location: /var/log/syslog

Contains more information about your system. If you can’t find anything in the other logs, it’s probably here.

Application logs

Some applications also create logs in /var/log. Below are some examples.

Apache logs

Location: /var/log/apache2/ (subdirectory)

Apache creates several log files in the /var/log/apache2/ subdirectory. The access.log file records all requests made to the server to access files. error.log records all errors thrown by the server.

X11 server logs

Location: /var/log/Xorg.0.log

The X11 server creates a seperate log file for each of your displays. Display numbers start at zero, so your first display (display 0) will log to Xorg.0.log. The next display (display 1) would log to Xorg.1.log, and so on.

Non-human-readable logs

Not all log files are designed to be read by humans. Some were made to be parsed by applications. Below are some of examples.

Login failures log

Location: /var/log/faillog

Contains info about login failures. You can view it with the faillog command.

Last logins log

Location: /var/log/lastlog

Contains info about last logins. You can view it with the lastlog command.

Login records log

Location: /var/log/wtmp

Contains login info used by other utilities to find out who’s logged in. To view currently logged in users, use the who command.

This is not an exhaustive list! You can search the web for more locations relevant to what you’re trying to debug. There is also a longer list here.

Viewing and monitoring logs from the command line

It is also important to know how to view logs in the command line. This is especially useful when you’re remotely connected to a server and don’t have a GUI.

The following commands will be useful when working with log files from the command line.

Viewing files

The most basic way to view files from the command line is using the cat command. You simply pass in the filename, and it outputs the entire contents of the file: cat file.txt.

This can be inconvenient when dealing with large files (which isn’t uncommon for logs!). We could use an editor, although that may be overkill just to view a file. This is where the less command comes in. We pass it the filename (less file.txt), and it will open the file in a simple interface. From here, we can use the arrow keys (or j/k if you’re familiar with Vim) to move through the file, use / to search, and press q to quit. There are a few more features, all of which are described by pressing h to open the help.

Viewing the start or end of a file

We may also want to quickly view the first or last n number of lines of a file. This is where the head and tail commands come in handy. These commands work much like cat, although you can specify how many lines from the start/end of the file you want to view. To view the first 15 lines of a file, we run head -n 15 file.txt, and to view the last 15, we run tail -n 15 file.txt. Due to the nature of log files being appended to at the bottom, the tail command will generally be more useful.

Monitoring files

To monitor a log file, you may pass the -f flag to tail. It will keep running, printing new additions to the file, until you stop it (Ctrl + C). For example: tail -f file.txt.

Searching files

Use the grep command. We specify what we want to search for in double quotes, along with the filename, and grep will print all the lines containing that search term in the file. For example, to search for lines containing “test” in file.txt, you would run grep "test" file.txt.

If the result of a grep search is too long, you may pipe it to less, allowing you to scroll and search through it: grep "test" file.txt | less.

Editing files

The simplest way to edit files from the command line is to use nano. nano is a simple command line editor, which has all the most useful keybindings printed directly on screen. To run it, just give it a filename (nano file.txt). To close or save a file, press Ctrl + X. The editor will ask you if you want to save your changes. Press y for yes or n for no. If you choose yes, it will ask you for the filename to save the file as. If you are editing an existing file, the filename will already be there. Simply leave it as it is and it will save to the proper file.