CP367: Lab 09 - Winter 2026 - Shell Programming III

Functions

Script functions provide the ability to store and repeat chunks of code. Functions are declared as:

				
name () {
   ... code
   return exit status
}

Local Variables

Script functions have access to all variables in the same script file as the function - script variables are global. However, you can declare local variables within a function by using the local keyword. These local variables exist only so long as the function is executing.

Calling Functions

Functions are called just by giving their name. Parmeters are passed by just listing their names or values after the function name. Parameters are passed by value. (It is possible to pass parameters by reference, but that is beyond the scope of this lab.) Inside the function these arguments are numbered just like the arguments passed on the command line to a script. The first argument is $1 , the scond $2 , and so on. This includes $# to tell you how many arguments were passed. Be careful! It is easy to mix up the function arguments with the command line arguments.

The return statement is optional, but it allows a function to return an explicit exit code. This exit code is the functional equivalent of the script exit code, except that the script keeps executing after a function return . If no return statment is given, the function's exit code is determined by the exit code of the last command executed within the function. The calling script access the return value through the variable $? . The script functest and its execution illustrates some of these concepts:

#!/bin/bash

some_func() {
  echo "Calling some_func"
  local local_var="Hi There"
  echo "$local_var"

  if [ "$#" -ne "3" ]
  then
    echo "Expects three parameters"
    return 1
  fi

  echo "Parameters are:"
  for i in $@
  do
    echo $i
  done

  return 0
}

some_func a b
echo "Return value $?"
echo
some_func a b c
echo "Return value $?"
echo
echo "local variable local_var: $local_var"

exit 0

Executing functest :

/home/dbrown/CP367> ./functest
Calling some_func
Hi There
Expects three parameters
Return value 1

Calling some_func
Hi There
Parameters are:
a
b
c
Return value 0

local variable local_var:

The script funcwrap expands upon examples from previous labs. The script defines two local return values within the function wrap , and calls this function from within a for loop, passing it arguments from the command-line.

#!/bin/bash

wrap () {
  local RETURN_OK=0
  local RETURN_BAD=1

  echo "file $1"
  if [ ! -e "$1" ]
  then
    echo "$1 does not exist"
    local RETURN=$RETURN_BAD
  else
    newfile=${1}.htm
    `sed -f wraphtml.sed $1 > $newfile`
    echo "$1 was wrapped as $newfile"
    local RETURN=$RETURN_OK
  fi
  return $RETURN
}

# Pass each command-line argument to the function.
for arg in "$@"
do
  wrap $arg
  echo "wrap function returned $?"
  echo
done
exit 0

The result of calling the script:

/home/dbrown/CP367> ./funcwrap error.txt good.txt stuff.txt
file error.txt
error.txt was wrapped as error.txt.htm
wrap function returned 0

file good.txt
good.txt was wrapped as good.txt.htm
wrap function returned 0

file stuff.txt
stuff.txt does not exist
wrap function returned 1

Add the following numbers to a file nums.txt and use it for testing for the tasks.

5
3
7
31
1
9
4
12
27

1. Write a script function funcsort that takes a filename as an argument and outputs the contents of the file in sorted order.

   ---

2. Improve the above script to allow the use of a -n argument that sorts the contents of the file in numeric order.

   ---

3. Improve the script again to check the arguments. The script allows one or two arguments: if there is one argument only, it is a filename. If there are two arguments, the first argument must be -n and the second a filename. Inform the user if they use an invalid argument. Set appropriate exit codes. Your script must use a function to check the file validity.

   ---

4. Improve the script once again to check the file argument. Inform the user if the file does not exist, is not a normal file, or cannot be read. Allow the user to enter a third argument that is the output file to write to. Thus your script must be able to correctly handle the following tests:

   • ./funcsort nums.txt
   • ./funcsort -n nums.txt
   • ./funcsort nums.txt out.txt
   • ./funcsort -n nums.txt out.txt

   and any errors in the input file or the argument count.

   ---