rework bash scripts to be completely, or mostly POSIX-compliant

.bash_profile

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/bin/sh
 # shellcheck disable=1091
 #
 # ~/.bash_profile
@@ -13,7 +13,7 @@ export XDG_STATE_HOME="$HOME/var/lib"   # state data that should persist between
 export XDG_CACHE_HOME="$HOME/var/cache" # user-specific non-essential (cached) data.
 export PATH="$XDG_BIN_HOME:$PATH"       # add our bin in front of PATH, making it take precedence
 
-if command -v nvim &>/dev/null; then
+if command -v nvim >/dev/null; then
 	export EDITOR=nvim           # use neovim as an editor
 	export VISUAL=nvim           # use neovim for visual-based editors
 	export MANPAGER='nvim +Man!' # use neovim as a manual pager
@@ -22,14 +22,16 @@ fi
 # coloured GCC warnings and errors
 export GCC_COLORS='error=01;31:warning=01;35:note=01;36:caret=01;32:locus=01:quote=01'
 
-# export Microsoft's bullshit outta here
+# Microsoft bullshit
 export DOTNET_CLI_TELEMETRY_OPTOUT=1
 export VCPKG_DISABLE_METRICS=1
 export VCPKG_ROOT="$XDG_DATA_HOME/vcpkg"
-export PATH+=":$VCPKG_ROOT"
+export PATH="$PATH:$VCPKG_ROOT"
 
 # if executing interactively
 # execute .bashrc in the current login context
-[[ $- != *i* ]] && return
-[[ -z ${PS1-} ]] && retrun
-[[ -f $HOME/.bashrc ]] && . "$HOME/.bashrc"
+case $- in
+*i*) return ;;
+esac
+[ -z "${PS1-}" ] && return
+[ -f "$HOME/.bashrc" ] && . "$HOME/.bashrc"