Compile part of file

\ifdefined\myflag
  Your code here
\else
  Another code
\fi

Then, compile with

xelatex or pdflatex -job-name=myfilexx "\def\myflag{}\input{myfile}"

\newif\ifsoln \solnfalse
\solntrue  %% Comment out if not compile the block..

\begin{document}

\ifsoln 
  Your code here
\fi

===========================

\usepackage{ifthen}
\newboolean{prob}
\setboolean{prob}{false} % boolvar=true or false

\begin{document}

\ifthenelse {\boolean{prob}}{Code if true}{Code if false}

============================

\usepackage{comment}

\includecomment{soln}
\excludecomment{soln}

\begin{soln}
  xxxx 
\end{soln}

Alternatively, 

xelatex '\AtBeginDocument{\includecomment{soln}}\input{syllabus}'

or create a makefile with the following contents

student:
     latex '\AtBeginDocument{\excludecomment{soln}}\input{syllabus}'
     
teacher:
     latex '\AtBeginDocument{\includecomment{soln}}\input{syllabus}'

Put the following at the first line will also set the output filename
%& -job-name=somename


%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

-- Using if then in the preamble
\
newcommand{\isitthree}[1]
{
  \ifnum#1=3
    number #1 is 3
  \else
    number #1 is not 3
  \fi
}


\newif\ifismonday
\newif\ifistuesday

\ismondayfalse
\istuesdaytrue

\ifismonday
  sucks!
\else
  \ifistuesday
    almost sucks.
  \else
    is \textbf{great}
  \fi
\fi

=======================================================

\usepackage{ifthen}
\newboolean{includethis}
\setboolean{includethis}{false}
\newcommand{\ifinclude}[1]{\ifthenelse{\boolean{includethis}}{#1}{}}


Then if you do
\ifinclude{%
\begin{frame}{Slide title}
 This slide will be hidden.
\end{frame}
}

the frame will be included only if you change the setting of 
the boolean variable includethis to 'true' in the preamble. 

======================================================
In your source file, write

\providecommand{\comment}[1]{\emph{#1}}% fallback definition

and then compile the LaTeX document ("myfile.tex") as


pdflatex (whatever options you need) "\newcommand\comment[1]{\textbf{#1}}\input{myfile}"