re-use '~/.profile` for Fish?

You can use bash to parse /etc/profile and ~/.profile, and then start fish.

1. Create /usr/local/bin/fishlogin with contents

    #!/bin/bash -l
    exec -l fish "$@"
   

2. Make it executable

    sudo chmod a+rx /usr/local/bin/fishlogin
   

3. Check that it works by running fishlogin and checking that you end up in a Fish shell. Press Control+D to exit the Fish shell.

4. Add it to /etc/shells

    echo /usr/local/bin/fishlogin | sudo tee -a /etc/shells
   

5. Set it as your default shell.

   Under Linux:

    sudo usermod -s /usr/local/bin/fishlogin $USER
   

   Under macOS:

    chsh -s /usr/local/fishlogin $USER
   

For a much cleaner solution, you can use the foreign env plugin:

fenv source ~/.profile

My current solution (see here for a maybe more recent version):

egrep "^export " ~/.profile | while read e
    set var (echo $e | sed -E "s/^export ([A-Z_]+)=(.*)\$/\1/")
    set value (echo $e | sed -E "s/^export ([A-Z_]+)=(.*)\$/\2/")

    # remove surrounding quotes if existing
    set value (echo $value | sed -E "s/^\"(.*)\"\$/\1/")

    if test $var = "PATH"
        # replace ":" by spaces. this is how PATH looks for Fish
        set value (echo $value | sed -E "s/:/ /g")

        # use eval because we need to expand the value
        eval set -xg $var $value

        continue
    end

    # evaluate variables. we can use eval because we most likely just used "$var"
    set value (eval echo $value)

    set -xg $var $value
end