title: What is the different between PATH=:$PATH and PATH="$PATH:" and other export lines
body: <p>I have a question about following lines related to adding PATH to enviroment.</p>
  
  <pre><code>export PATH=/usr/loca/cuda/bin:$PATH
  export PATH=/usr/local/cuda-9.1/bin${PATH:+:${PATH}}
  export PATH="/home/ics_vr/anaconda3/bin:$PATH"
  export PATH="$PATH:/home/user/anaconda3/bin"
  </code></pre>
  
  <p>Regardless the content of path in each export lines, my first question is how do I distinguish thoes lines starting with <strong>export PATH=</strong>? e.g. grammer and its functions, regardless the variable I used in thoes lines.</p>
  
  <p>Secondly, I see many people use <strong># to comment on/off</strong> to switch those path,but this is not convenient. Is there any union way to realize all, without commenting the export line every time? </p>
  
  <p>This is convenient because people want to use system python for example as default, but if the path is settled not properly, anaconda python interpreter will be settled by default. We need a way that default is the system python interpreter, and when I need anaconda, I will use </p>
  
  <pre><code>source activate ENV_I_BUILD
  </code></pre>
  
  <p>Thank you for your time and help. I am very appreciate on that.</p>