Functions

Most (all?) shells have the means of creating new "internal" commands. This is done by creating shell functions. Shell functions are just like those in a programming language. Sets of commands are grouped together and jointly called by a single name.

The format for functions is:

function_name()
{
first thing to do
second thing to do
third thing to do
}

Functions can be defined anywhere, including from the command line. All you need to do is simply type in the lines one at a time, similar to the way shown above. The thing to bear in mind is that if you type a function from a command line, once you exit that shell, the function is gone.

Shell functions have the ability to accept arguments, just like commands. A simple example is a script that looks like this:

display()
{
echo $1
}

display Hello

The output would be

Hello

Here we need to be careful. The variable $1 is the positional parameter from the call to the display function and not to the script. We can see this when we change the script to look like this:

display()
{
echo $1
}

echo $1

display Hello

Lets call the script display.sh and start it like this:

display.sh Hi

The output would then look like this:

Hi

Hello

The first echo shows us the parameter from the command line and the second one shows us the parameter from the function.

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