Getting the current username, it sounds like a very easy task:

1
2

$ bash -c 'echo "$USER"'
chao

But depending on the environment variable is not reliable, it can be easily overridden:

1
2

$ USER=foo bash -c 'echo "$USER"'
foo

Or just simply fail to work:

1

$ docker run --rm bash:4.4.23 bash -c 'USER=foo echo "$USER"'

The USER environment was not set in /etc/profile in this case:

1
2
3
4
5
6

$ docker run --rm bash:4.4.23 bash -c 'cat /etc/profile'
export CHARSET=UTF-8
export PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
export PAGER=less
export PS1='\h:\w\$ '
umask 022

This is especially important when writing shell script. You do not want to end up with a wrong username.

One solution is to use whoami, which prints the effective user:

1
2

$ USER=foo bash -c 'whoami'
chao

Another solution is to use id command:

1
2

$ USER=foo bash -c 'id -un'
chao

This will eliminate the overridden environment variable issue. Also, check out this StackOverflow post for others and POSIX compatible means:

https://stackoverflow.com/questions/19306771/get-current-users-username-in-bash

Users frequently have custom domain for their blogs in Tumblr. If you need to know the [Tumblr] username from the custom domain, just need to grab the value from the header field X-Tumblr-User:

$ curl -Is blog.foursquare.com | grep -i x-tumblr-user
X-Tumblr-User: foursquare

Another example:

$ curl -Is blog.birchbox.com | grep -i x-tumblr-user
X-Tumblr-User: birchbox

Now you can use the username to make some API calls to Tumblr.