Use current filename register in bash command ran from VIM

Question

I'm making a command that runs tests and opens coverage report for the currently opened file.

nnoremap cov :!vendor/phpunit/phpunit/phpunit --coverage-html \
/tmp/coverage/ test/ && google-chrome /tmp/coverage/<CR>

How do I put the contents of filename register (%) into the bash command I want to execute?

nnoremap cov :!vendor/phpunit/phpunit/phpunit --coverage-html \
/tmp/coverage/ test/ && google-chrome /tmp/coverage/FILENAME_GOES_HERE<CR>

Also a closer try:

function CoverageHtml(filename)
    execute '!vendor/phpunit/phpunit/phpunit' '--coverage-html' '/tmp/coverage/' 'test/'
    let f = "/tmp/coverage" + a:filename
    execute '!google-chrome' f
endfunction

function CoverageHtmlInit()
    "fails in the next line. 
    let fn = :echo @%
    return CoverageHtml(fn)
endfunction

nnoremap cov :call CoverageHtmlInit()

Answer 1

To refer to the current file, you can use %.

You can even use modified versions of the path, by applying one ore more of the modifiers as documented in :help %:. To highlight just a few interesting ones from the help page:

:p  Make file name a full path.  Must be the first modifier.  Also
    changes "~/" (and "~user/" for Unix and VMS) to the path for
    the home directory.  If the name is a directory a path
    separator is added at the end.  For a file name that does not
    exist and does not have an absolute path the result is
    unpredictable.  On MS-Windows an 8.3 filename is expanded to
    the long name.

:~  Reduce file name to be relative to the home directory, if
    possible.  File name is unmodified if it is not below the home
    directory.

:t  Tail of the file name (last component of the name).  Must
    precede any :r or :e.

:s?pat?sub?
    Substitute the first occurrence of "pat" with "sub".  This
    works like the |:s| command.  "pat" is a regular expression.
    Any character can be used for '?', but it must not occur in
    "pat" or "sub".
    After this, the previous modifiers can be used again.  For
    example ":p", to make a full path after the substitution.

Answer 2

This was the partial solution based on janos' answer. Can't manipulate the %, so needed another test directory structure to make it work.

function CoverageHtml(filename)
    execute '!vendor/phpunit/phpunit/phpunit' '--coverage-html' '/tmp/coverage/src/' 'test/'
    execute '!google-chrome' '/tmp/coverage/%.html'
endfunction

nnoremap cov :let a=@% <CR>:call CoverageHtml(a)<CR>