How to get the PID of a process by giving the process name in Mac OS X ?

This solution matches the process name more strictly:

ps -Ac -o pid,comm | awk '/^ *[0-9]+ Dropbox$/ {print $1}'

This solution has the following advantages:

• it ignores command line arguments like tail -f ~/Dropbox
• it ignores processes inside a directory like ~/Dropbox/foo.sh
• it ignores processes with names like ~/DropboxUID.sh

The answer above was mostly correct, just needed some tweaking for the different parameters in Mac OSX.

ps -A | grep [f]irefox | awk '{print $1}'

You can install pidof with Homebrew:

brew install pidof
pidof <process_name>

You can use the pgrep command like in the following example

$ pgrep Keychain\ Access
44186