locate a command
which [-a] filename ...
returns the pathnames of the files (or links) which would be executed
in the current environment, had its arguments been given as commands
in a strictly POSIX-conformant shell. It does this by searching the
PATH for executable files matching the names of the arguments. It does
not canonicalize path names.
print all matching pathnames of each argument
if all specified commands are found and executable
if one or more specified commands is nonexistent or not executable
if an invalid option is specified