title: How to understand `if [ ! -f $R_C_S_L/j_r_d* ] ...` in unix?
body: <p>Please tell me how to debug like the below statements in Unix.
  How to understand what the below code is doing, step by step.</p>
  
  <pre><code>if [ ! -f $R_C_S_L/j_r_d* ]
      then
  echo 0&gt;$R_C_S_L/j_r_d
  fi
  </code></pre>