[fbt@cetaganda ~]$ cat .xinitrc
bash "$HOME/.xinitrc.d/xinitrc"
[fbt@cetaganda ~]$ cat .xinitrc.d/xinitrc
#!/bin/bash

cfg_dir="$HOME/.xinitrc.d"
cfg_default_session='fvwm'

xinit.msg() { echo "[xinit] $1"; }
xinit.err() { xinit.msg "(error) $1"; }

xinit.start_session() {
    [[ -f "$cfg_dir/sessions/$session" ]] || {
        xinit.err "$session not found in $cfg_dir/sessions"
    }

    source "$cfg_dir/sessions/$session"
}

session="$1"

[[ "$session" ]] || { session="$cfg_default_session"; }

source "$cfg_dir/common"
xinit.start_session
[fbt@cetaganda ~]$ tree .xinitrc.d/
.xinitrc.d/
├── common
├── sessions
│   ├── fluxbox
│   └── fvwm
└── xinitrc