Determine if a command is installed

[rib00]

How do I determine if a particular command is available (installed)? I think that in Linux, this is roughly how it could be done

dpkg -l | grep "commandname"

dpkg is a package manager
l flag lists the packages that are installed
| sends the output to the next command
grep processes the input by searching for the word in quotes
grep prints the full line when it finds the search term on a line

or maybe just
commandname --version
prints the version of the command that is installed.

Maybe something like this?
pkgutil --pkgs | grep "commandname"

 

[Bigwaff]

which <command>

 

[splifingate]

...and dpkg is from Debian.

 

[DankeBrutus]

which <command>

That's assuming the executable/bin file is in the user's path. But ya like 99/100 this will do the trick.