Man page 1 : which

which - locate a command

which [-a] filename ...

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 follow symbolic links.

-a

    print all matching pathnames of each argument

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