From the man bash pages (especially the CONDITIONAL EXPRESSIONS section):

   -a file

          True if file exists.

   -b file

          True if file exists and is a block special file.

   -c file

          True if file exists and is a character special file.

   -d file

          True if file exists and is a directory.

   -e file

          True if file exists.

   -f file

          True if file exists and is a regular file.

   -g file

          True if file exists and is set-group-id.

   -h file

          True if file exists and is a symbolic link.

   -k file

          True if file exists and its ``sticky'' bit is set.

   -p file

          True if file exists and is a named pipe (FIFO).

   -r file

          True if file exists and is readable.

   -s file

          True if file exists and has a size greater than zero.

   -t fd  True if file descriptor fd is open and refers to a terminal.

   -u file

          True if file exists and its set-user-id bit is set.

   -w file

          True if file exists and is writable.

   -x file

          True if file exists and is executable.



   [...]