There isn't a built-in flag yet, but you can use
pip list --outdated --format=freeze | grep -v '^\-e' | cut -d = -f 1 | xargs -n1 pip install -U
Note: there are infinite potential variations for this. I'm trying to keep this answer short and simple, but please do suggest variations in the comments!
In older version of
pip, you can use this instead:
pip freeze --local | grep -v '^\-e' | cut -d = -f 1 | xargs -n1 pip install -U
grep is to skip editable ("-e") package definitions, as suggested by @jawache. (Yes, you could replace
-n1 flag for
xargs prevents stopping everything if updating one package fails (thanks @andsens).