Added command substitution

chapters/ch04-system.tex

@@ -220,4 +220,43 @@ command 1> file.txt
 command 2> file.txt
 \end{Verbatim}
 
+\section{Command Substitution}\index[concepts]{command substitution}
 
+\lettrine{O}{ccasionally} you may need to use the output of one command as the argument of another command.
+
+For example, suppose you have a folder full of pdfs that you want to combine into a single pdf using \texttt{pdftk}\index{pdftk}, and you want to join them in alphabetical order. You could do something like this:
+
+\begin{Verbatim}[frame=lines]
+pdftk a.pdf b.pdf c.pdf d.pdf e.pdf f.pdf cat output merged.pdf
+\end{Verbatim}
+
+However, if you have a lot of pdf files, that becomes very tedious. Using command substitution, we can use the \texttt{ls} command to sort that list of files and then use the output of that command as the input to \texttt{pdftk}:
+
+\begin{Verbatim}[frame=lines]
+pdftk $(ls -v *.pdf) cat output merged.pdf
+\end{Verbatim}
+%$
+
+In place of the dollar sign/parentheses construction, you can also use grave accents. Here is an example where we can use the \texttt{date} command as a variable in scripts:
+
+\begin{Verbatim}[frame=lines]
+echo "Today is `date`"
+\end{Verbatim}
+
+\noindent{}which returns 
+
+\begin{Verbatim}[]
+Today is Thu Sep 17 09:39:44 AM EDT 2026
+q\end{Verbatim}
+
+And of course, you can pass options to the substituted command as well:
+
+\begin{Verbatim}[frame=lines]
+echo "Today is day #`date +%j` of the year"
+\end{Verbatim}
+
+\noindent{}which returns
+
+\begin{Verbatim}[]
+Today is day #260 of the year
+\end{Verbatim}