How do I use arguments together with bash aliases

Today, I learned the very handy trick:

man [some command] -t | open -f -a Preview

I'd like to create an alias in my .bash_profile file as a shortcut. In order for this to work, I need to pass an argument into the aliased command. A little poking around and I came up with this:

alias manp="man $1 -t | open -f -a Preview"

This almost works, but if I type in, say "manp ls", it seems to be expecting the ls manpage to be in whatever my current directory is.

You can't pass arguments to aliases. Aliases are just text substitutions without further logic.

But you can use shell functions to achieve the same result:

function pman() {
    if [ -x /usr/bin/open ]; then
        man -t "$1" | open -f -a Preview
    else
        man "$1"
    fi
}

The if-then-else part is there just to make sure it also works on non-OSX systems.

To define a function, just include the definition from above into your ~/.bash_profile. Usage is identical to aliases or any other command: pman ls.

As @stuffe pointed out in a comment keeping aliases/functions in a separate file has its advantages (especially it allows you to re-read your aliases/functions into the current shell without executing any setup stuff from .bashrc or .bash_profile). To do this, create a separate file called .alias, .functions or similar and add

test -e ~/.alias && . ~/.alias

to .bashrc or .bash_profile.