if

• An if statement evaluates a logical expression. If the expression is true, the code inside the if is executed.
• Shell uses test to evaluate expressions. You can look this up in the man page for sh. Some commonly used expressions are listed in the link for Logical Expressions.
• I recommend using the square brackets (shown below).
• Do NOT use parenthesis - this does not work efficiently in all versions of Linux, and should be avoided.
• The basic format for an if in shell is:

  if [ expression ]
  then
      command
      command
  fi
  

• In shell, you may put as many statements inside the if as you would like.
• The then and the fi are required, even if there is only one statement to execute. (And yes, fi is if spelled backwards.)
• Be careful! The square brackets are tricky - you must have the whitespace around them.
• Here is the format for an if-else:

  if [ expression ]
  then
      command
  else
      command
  fi
  

• Here is the format for an if-else-if (elif), which can also have an else at the end:

  if [ expression ]
  then
      command
  elif [ expression ]
  then
      command
  fi