which - locate a command

which  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.

0      if all specified commands are found and executable

       1      if one or more specified commands is nonexistent or not executable

       2      if an invalid option is specified

Debian                                             29 Jun 2016                                          WHICH(1)

       which - locate a command

-a     print all matching pathnames of each argument

       -s     silently return 0 if all of the executables were found or 1 otherwise

       which [-as] filename ...