awk study(4)

1.1.5 comments in awk programs
      A comment is some text that is included in a program for the sake of human readers;it is not really an executable part of the program,comments can explain what's the program does and how it works.Nearly all programming languages have provisions for comments, as programs are typically hard to understand without comments.
    In the awk programs, a comment starts with the sharp sign character("#") and continus to the end of the line, The "#" does't have to be the first character on the line,the awk language ignores the rest of the line following a sharp sign. For example,we could have put the following into advice file:

     # This program prints a nice friendly message.  It helps
     # keep novice users from being afraid of the computer.
     BEGIN    { print "Don't Panic!" }

You can put comment lines into keyboard-composed throwaway awk programs, but this usually is not very useful; the purpose of a comment is help you or another person understand the program when reading it at a later time.
Caution:  As mentioned in One-shot, you can enclose small to medium programs in single quotes, in order to keep your shell scripts self-contained.When doing so, don't put a apostrophe(i.e.,sigle quote)into a comment(or anywhere else at your program). The shell interprets the quotes as the closing quotes  for the entire program. As a result, usually the shell prints a message about mismatched quotes,and if awk actually runs, it will probably print strange messages about syntax errors.For example, look at the following:

     $ awk '{ print "hello" } # let's be cute'
     >

The shell sees that the first two quotes match, and that a new quoted object begins at the end of command line,it therefore prompts with the secondary, waiting for more input. With unix awk, closing the quoted string produce this result: 

     $ awk '{ print "hello" } # let's be cute'
     > '
     error--> awk: can't open file be
     error-->  source line number 1

       Putting a backslash before the single quote in "let's "wouldn't help, since backslashs are not spcial inside sigle quotes. The next subsection describes the shell's quoting rules.