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07. Functions (Part 1)

Overview

  • Just like other programming languages, the shell supports the use of functions, which leads to more structured and easier to maintain scripts
  • The here document is used to output multiple lines of text from the script. The here document is not a function, but it is used to group lines of text into a mini text file within a script, making it easier to work with multiple lines of text.
Topics:

    • Functions

    • Here Document


Function Introduction

  • Often in programming, a particular logical block of code needs to run at several places in the script
  • For example, a logical block of code keeps looping to get a filename and until it gets a valid file
    • This block of code could need to run at several places in the script, whenever a file is needed from the user
  • Rather than copy and paste the block of code over and over again in the script, we create a function from the block of code and put it at the beginning of the script
  • Then, every time a file name is needed, we only need to call the function, we don’t need to repeat the block of code in the script
  • Functions are useful because they:
    • Reduce the size of the script (less typing by the programmer)
    • Can be re-used by many scripts (no need to reinvent the wheel)
    • Produce logically structured (organized) scripts that are easy to read, debug, and maintain

Functions in a Script

  • The shell is an interpreter, not a compiler, so it interprets the script line by line from top to bottom of the file
  • This means that the shell needs to “see” the function block before the function is called. Otherwise, the shell will give a syntax error if it sees the function call and has not seen the function block
  • In a script, all function blocks are put at the beginning of the script, then the main block of code appears last in the script
  • The order in the script file will be:
    • 1. all function blocks
    • 2. the main script commands


Syntax for a Function
  • The function block is made of 2 parts: function header and function body
        functionA ()
        {
            commands 
            ...
            commands
            optional return value
        }

  • The header has the function name, followed by empty parentheses
  • It is recommended that the function name be descriptive so it’s easy to recognize what task the function does
  • The empty parentheses are required, and they are always empty. Without the parentheses the shell will interpret the function name as a variable name
  • The function body starts and ends with {  } , and in between are commands and programming constructs for the function to do its task
  • It is recommended that the commands in a function body be indented from the function header so it’s easy to read
  • The commands and programming constructs should be left justified (line up on the left) and further indented if necessary
  • If a function is short, the entire function can be coded on the same line. In this case, every command in the function body must end with semicolon
    • Example:   goHome ( ) {  cd;  echo  At `pwd`;  }

Input to a Function

  • When a function is called, we can give input arguments to the function the same way that we give input arguments on the command line:
functionA file1 file2
    • functionA is called and gets 2 filenames as input
  • The function accesses its input through the positional parameters $1, $2, $3, etc.
  • Example of input arguments, given the function call above
functionA ( )
{
if [[ -e $1 && -e $2 ]]
then
  echo both files exist
else
  echo at least one filename is not valid
fi
}


Output from a Function

  • A function can return one and only one integer value, through the return statement:
return  $size
    • The value in variable size is returned to the caller
  • The integer return value is in the range of 0 - 255
  • The return value is stored in the ? variable and is accessed by the caller with $?
  • Example
functionB ( )
{
size=‘ls –l $1 | awk {print $5}`  # get file size
return $size
}

  • call functionB and pass in fileA, then access return value to print:
functionB  fileA
echo fileA has $? bytes


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