Composer killed while updating

The "Killed" message usually means your process consumed too much memory, so you may simply need to add more memory to your system if possible. At the time of writing this answer, I've had to increase my virtual machine's memory to at least 768MB in order to get composer update to work in some situations.

However, if you're doing this on a live server, you shouldn't be using composer update at all. What you should instead do is:

1. Run composer update in a local environment (such as directly on your physical laptop/desktop, or a docker container/VM running on your laptop/desktop) where memory limitations shouldn't be as severe.
2. Upload or git push the composer.lock file.
3. Run composer install on the live server.

composer install will then read from the .lock file, fetching the exact same versions every time rather than finding the latest versions of every package. This makes your app less likely to break, and composer uses less memory.

Read more here: https://getcomposer.org/doc/01-basic-usage.md#installing-with-composer-lock

Alternatively, you can upload the entire vendor directory to the server, bypassing the need to run composer install at all, but then you should run composer dump-autoload --optimize.

If like me, you are using some micro VM lacking of memory, creating a swap file does the trick:

#Check free memory before
free -m

mkdir -p /var/_swap_
cd /var/_swap_

#Here, 2G ~ 2GB of swap memory. Feel free to add MORE
sudo fallocate -l 2G swapfile

chmod 600 swapfile
mkswap swapfile
swapon swapfile
#Automatically mount this swap partition at startup
echo "/var/_swap_/swapfile none swap sw 0 0" >> /etc/fstab

#Check free memory after
free -m

As several comments pointed out, don't forget to add sudo if you don't work as root.

btw, feel free to select another location/filename/size for the file.
/var is probably not the best place, but I don't know which place would be, and rarely care since tiny servers are mostly used for testing purposes.