Run another command before running the command the user wants to run

Question

Let's say I want to modify the original behavior of the ls tool this way:

$ ls
Hello World
file1 file2 ...

How can I do this?

When running ls I would like to run another command let's say echo "Hello World!".

The quick solution I see is using alias:

alias orig_ls="ls"
alias ls='echo "Hello World!"'

However, this is not a real solution since when I will run orig_ls it will output "Hello World!".

Answer 1

Sometimes an alias isn't powerful enough to easily do what you want, so here's a way without using them.

In some file that is sourced when your shell starts (e.g. .bashrc), add the following function:

ls () {
    echo "Hello world!"
    command ls "$@"
}

Unlike an alias, a function can recurse. That's why command ls is used instead of ls; it tells your shell to use the actual ls instead of the function you've just defined.

Answer 2

You must not forget to call ls:

alias ls='echo "Hello World!"; ls'