Log file
Log files store details about the run of Factorio and contain information that help when solving bugs. Log files are created from version 0.10.0 onwards.
Log file location
Factorio keeps log files in the user data directory, next to the saves and mods directories. It is a .log file that can be opened with any text editor.
The file you need most of the time is factorio-current.log. Backup copy of the log file from previous run is in factorio-previous.log.
File layout
The file is a long list of lines. The first column is the amount of seconds the game has been running. Typically, game starting will take anywhere from 10-60 secs.
The first line is a print about your build of the game. For the Steam version for Linux, this will be:
0.000 2017-04-24 21:39:48; Factorio 0.15.1 (build 29050, linux64, steam)
It will vary by version, OS, and origin of the game.
Next follows information about the install of the game itself, like the specific version of the OS, and arguments provided to the game launch. It will also print read/write data paths, which can give clues about where the game stores the files it creates, and it's install location.
It then prints off a bunch of information about graphics, hardware, and starts loading mods. The base game is loaded first, then all the mods the user has installed. It will print the order it loads mods in, this can be useful for debugging when a mod causes the game to fail to init. The log file also gives some insight to the interior code of Factorio, right after checking checksums it will load the PlayerData.cpp file, which compares the player-data.json to the cloud version (if running on steam) and checks for changes.
After initializing mods, it will checksum them (used for the mod portal, to detect new versions) and start loading sprites, creating atlas bitmaps. At some point, it will print Sprites loaded and start compressing the atlas depending on graphics settings. It will then load the sound engine, check custom inputs, and then finish.
As soon as the user loads a map, it will state so, and specify the process of loading the map, loading from a few .dat files. The user is now free to play.
Bug Reporting
Log files contain information helpful to diagnose any type of bug, accordingly they are required in any official bug report to be made in the factorio forums. Please follow the bug report instructions closely when making a report.
Privacy
Log files (in 0.10.5 and later) contain the system date and time of your Factorio session, your updater login name and full paths for some files belonging to Factorio or mods. Before 0.10.5 the log also could contain your updater token (not a password) as a part of update request URLs. This has since been removed. Typically all of the information in a log file is not sensitive.