Functions

• Most good programmers can't write a program without functions, so you should use them in your code.
• A function allows your program to execute the same code multiple times and pass it different arguments. First you must write the function, then you can call it.
• The basic format for writing a function in shell is:

  function_name()
  {
      command
      command
  }
  

• In shell, you do not specify how many arguments are expected or a return type for the function.
• Functions can take any number of arguments and always return an int (and it must be a positive number).
• 0 means the function was successful, any other number means it failed.
• Arguments are accessed positionally. $1 is the first argument, $2 is the second argument, $3 is the third argument, $4 is the fourth argument, ...
• Here is an example that takes an argument:

  check_assigned()
  {
      NAME="$1"
      grep "$NAME" $DIR/assigned 
      if [ $? -eq 0 ]
      then
          echo ""
          echo "*** $NAME has already been assigned."
          echo "    Contact us if you have forgotten what the value is."
          return 0
      fi
      return 1
  }
  
  # Here are 2 calls to the function
  
  check_assigned "Cathy Bishop"
  check_assigned "Denis Niwemugisha"
  

• The if statement is checking the exit code from the command that was last executed. The special variable $? means the "value returned by the last-executed command".
• In this case, the if is checking the value that was returned by grep. The grep command returns 0 if the string was found.
• To call the function, the shell script just uses the function name and then list the arguments (notice there are no parenthesis in a SHELL function call).
• The functions should all be at the top of your shell script.
• The functions are only accessible from within the file that has the function.
• You can have functions that can be used from your command-line prompt or by any shell script by putting the functions in a script that is "dotted" (more later).