How to fetch old usernames by UID?

If you want to find usernames that a particular account used to edit with then "Who's That" will help - like this: http://whosthat.osmz.ru/?q=SomeoneElse2. However, that won't necessarily match all names that that account has been called (often users who have been blocked change the account name afterwards). The userid will be constant though, so you can use it e.g. at http://resultmaps.neis-one.org/osm-discussion-comments?uid=490556.

However, often "vandal" users (of which there are thankfully not many) will create multiple accounts, so you sometimes need to look for "edits to the same sort of objects in a similar style".

If you're aware of a vandal account please of course email the Data Working Group at data@osmfoundation.org !

Another option that leads to the same results, but does not depend on a third-party web site, is this:

• get a full history planet file (.osh.pbf)
• install the osmium command line utility
• convert the full planet file into easily parsable "opl" format and check for all lines that contain " i<userid>"
• those lines will also have " u<username>" which you can use to list the user name.

Here's a Linux one-liner (fsvo) that will do it:

curl https://planet.openstreetmap.org/pbf/full-history/history-latest.osm.pbf |
   osmium cat -Fosh.pbf -fopl |
   grep " i490556 " |
   sed -e "s/.* u//" |
   cut -d\  -f1 |
   sort -u

If you are only looking for actions in a particular country or region, you can also download a regional history file instead which speeds up the process.

Note that neither whosthat.osmz.ru nor this process will return user names that the user may have had, and may have used to write messages, but has not used for editing OSM.