\documentclass{article}

\usepackage{filecontents}
\begin{filecontents*}{fcommand.tex}
\newcommand{\mycommand}[1]
  { test:command arg=##1 environment arg=#1}
\end{filecontents*}

\newenvironment{myenvir}[1]{%
  \input{fcommand}
  \begin{center}
}{%
  \end{center}%
}

\begin{document}

\begin{myenvir}{abc}
  \mycommand{def}
\end{myenvir}

\end{document}