These unary operators take one argument, either a filename or a filehandle, and test the associated file to see if something is true about it. If the argument is omitted, they test $_ (except for -t, which tests STDIN). If the special argument _ (underscore) is passed, they use the information from the preceding test or stat call.
File is readable/writable/executable by effective uid/gid.
File is readable/writable/executable by real uid/gid.
File is owned by effective/real uid.
File exists/has zero size.
File exists and has non-zero size. Returns the size.
File is a plain file/a directory.
File is a symbolic link/a socket/a named pipe (FIFO).
File is a block/character special file.
File has setuid/setgid/sticky bit set.
Tests if filehandle (STDIN by default) is opened to a tty.
File is a text/non-text (binary) file. -T and -B return true on a null file, or a file at EOF when testing a filehandle.
File modification/access/inode-change time. Measured in days. Value returned reflects the file age at the time the script started. See also $^T in Special Variables.