How do you save an environmental variable? Trying to save an environmental variable.

cd ~
mkdir Projects
export PROJDIR=/Users/David/Projects
cd ~
cd $PROJDIR //can execute.
cd ..
open .bash_profile //was told this was the file you add the variable to.


Then terminal tells me there is nothing with that name.
To check 

ls -a


And there are two files with bash .bash_history and .bashsessions 
.bash_history isn’t executable and when I open .bashsessions I’m not sure how I’d add

export PROJDIR=/Users/$USER/Projects


to the file. So that doesn't seem right either.
‘export’ doesn't write anything to file. It's for making variables available to subprocesses.

If you've never written anything to .bash_profile, it's unsurprising that it doesn't exist, since it's not created for you. You'll need to create it yourself and write that line within.

For example, touch ~/.bash_profile (to create the file) then edit it in an editor of your choice such as TextEdit (open using Finder or open -e ~/.bash_profile). Alternatively, nano ~/.bash_profile. Paste in export PROJDIR=/Users/$USER/Projects and save the file, then reopen your shell or source the profile.