[Tex/LaTex] Upright bold greek with condition in math mode

Question

Good day!

I googled the whole stackechange, but was not able to find the answer. I'm quite new to Latex, so my approach is not proper. I try to define a macro, that should format greek letter as upright bold. I know there are several ways to do it, I would like to avoid usage of isomath and stick to upgreek package.

I want to be able to write

\gb{\alpha}
\gb{\Alpha}

and in both cases get upright bold characters. So, here is my code inside:

\newcommand{\gb}[1]{ % Imagine #1=\Psi
\StrGobbleLeft{\detokenize{#1}}{1}[\chrcodet] % variable "\Psi"
\StrGobbleRight{\chrcodet}{1}[\chrcode] % variable "Psi"
\StrLeft{\chrcode}{1}[\chrfirst] % first character "P"
\IfSubStr{ABCDEFGHIJKLMNOPQRSTUVWXYZ}{\chrfirst} % Is it capital?
    {\boldsymbol{#1}} % Yes - no modification
    {\boldsymbol{\csname up\chrcode\endcsname}} % No - glue \up+Psi (that what happens!)
}

I'm trying to get the first character of the control sequence and if it's capital, I call just \boldsymbol with no modifications, otherwise I want to modify control sequence by placing \up at the beginning. So, when I do that, it produces the following error

! Undefined control sequence.
\bm@command ->\upPsi

It looks like the "if" condition always gives false, i.e. "P" is not recognized as capital, thus the following MWE will write twice "lowercase":

\documentclass{article}
\usepackage{bm,upgreek}
\usepackage{etoolbox}
\usepackage{xstring}

\newcommand{\gb}[1]{
    \StrGobbleLeft{\detokenize{#1}}{1}[\chrcodet]
    \StrGobbleRight{\chrcodet}{1}[\chrcode]
    \StrLeft{\chrcode}{1}[\chrfirst]
    #1 - \IfSubStr{ABCDEFGHIJKLMNOPQRSTUVWXYZ}{\chrfirst}{uppercase}{lowercase}
}

\begin{document}

\begin{equation}
    \psi,\Psi,\gb{\psi},\gb{\Psi}
\end{equation}

\end{document}

Can anyone give me an idea, why it does not work as expected?

Answer 1

Does not bm just do what you want without any further redefinitions?

\documentclass{article}
\usepackage{bm,upgreek}


\begin{document}

\begin{equation}
    \psi, \uppsi, \Psi,\bm{\psi},\bm{\uppsi},\bm{\Psi}
\end{equation}

\end{document}