Setting variables in /etc/environment not having an affect but setting them in command line is

Put the export SPARK_HOME=... etc. commands in the startup files of your shell. With bash, that would be either ~/.profile or ~/.bash_profile.

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On Linux, /etc/environment is usually read by pam_env.so during login, and it doesn't support expanding existing variables, so setting PATH=$PATH:/something will result in the literal string $PATH to appear in your PATH. This isn't what you want. (See e.g. this and this, also for fun this.)

Also, setting PATH in /etc/environment might not work, since the global startup scripts for the shell might rewrite them. (They do on Debian by default, on the old CentOS I have handy, the startup scripts only seem to prepend to PATH).

If your system doesn't use pam_env.so, but you only source the script by hand, then these considerations don't matter, of course. But it looks like it's widely used by at least a couple of Linux distributions, so it might be a good idea to use another filename.

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(Because this is completely opposite to what the other answers said, I tested it on an old CentOS.)

I put the following in /etc/environment:

export FOO1=bar
export FOO2=foo:$FOO

After logging in again, set | grep FOO shows:

FOO1=bar
FOO2='foo:$FOO'