• the most simple command is locate :

    locate Python.h
    

    ; it is based on a database updated regulalry (most often daily).

  • the most powerful is find :

    # To find all files modified in ~/Sites three days ago:
    find ~/Sites -mtime 3
    # and 10 minutes ago:
    find ~/Sites -mmin 10
    #A time specified by -n means less than, while +n means more than.
    
    #To find all files in you home directory modified within the last week use:
    find ~ -mtime -7
    find ~ -newer last-backup.log
    
    # will find all files changed (or created) since last-backup.log was files larger than 2 megabytes (4000 of these 512 byte blocks):
    find ~ -size +4000
    find empty files% find . -empty