Vim - how to run a command immediately when starting vim?

I have a plugin (FindFile.vim) that needs to run :FindFileCache . whenever I start vim to gather a file cache for quick opening.. I have to run this every time I start vim though.

How might I write a command that runs once, every time that vim starts up?

The best place to keep your configuration stuff is in your .vimrc file. However, it's sourced too early, check :h startup:

At startup, Vim checks environment variables and files and sets values
accordingly.  Vim proceeds in this order:

1. Set the 'shell' and 'term' option                *SHELL* *COMSPEC* *TERM*
2. Process the arguments
3. Execute Ex commands, from environment variables and/or files *vimrc* *exrc*
4. Load the plugin scripts.                                 *load-plugins*
5. Set 'shellpipe' and 'shellredir'
6. Set 'updatecount' to zero, if "-n" command argument used
7. Set binary options
8. Perform GUI initializations
9. Read the viminfo file
10. Read the quickfix file
11. Open all windows
12. Execute startup commands

As you can see, your .vimrc will be loaded before plugins. If you put :FindFileCache . in it an error will occur, since that command does not exist yet. (It will exist once the plugin is loaded in step 4.)

To solve this, instead of executing the command directly, create an auto-command. Auto-commands execute some command when an event occurs. In this case, the VimEnter event looks appropriate (from :h VimEnter):

                                                    *VimEnter*
VimEnter                    After doing all the startup stuff, including
                            loading .vimrc files, executing the "-c cmd"
                            arguments, creating all windows and loading
                            the buffers in them.

Then, just place this line in your .vimrc:

autocmd VimEnter * FindFileCache .

There is also the -c flag of vim. I do this in my tmuxp config to have vim start with a vertical split:

vim -c "vnew"

At least with neovim you can also open a file at the same time:

nvim -c "colorscheme mustang" some_file

Create a file named ~/.vim/after/plugin/whatever_name_you_like.vim and fill it with

FindFileCache .

The order in which scripts are read and executed in the vim directories is described in :help 'runtimepath'

To get even later than other answers but still just after startup, use a timer in .vimrc. For example, this code in .vimrc waits half a second after startup before setting a variable.

function DelayedSetVariables(timer)
    let g:ycm_filetype_blacklist['ignored'] = 1
endfunction
let timer=timer_start(500,'DelayedSetVariables')

(The variable in the example is the blacklist from YouCompleteMe plugin. I assume, the plugin starts up some other process asynchronously which then creates variables, but is not quite ready by the time vim has started. The variable does not exist when I try to set it in .vimrc, an after file, or VimEnter event. This is specific to my Windows system somehow, YCM documentation says .vimrc should work for setting options.)

Put FindFileCache in your .vimrc.

Autload commands are different and will not work for your scenario.

You can run vim file.txt "+:FindFileCache ."