ln - link files
/bin/ln oldfile newfile
/bin/ln -s oldfile newfile
ln creates links to files. The first usage creates a hard link, that is, an additional name for the same file. The second usage, with the -s option, creates a symbolic link, a special filesystem entry that redirects accesses back to the first original file.
The symlink created is of the form newfile -> oldfile.
Note that ln does not support the Unix idiom ln file1 file2 ... destination-dir to link a number of files at once. In particular, ln foo bar/ will fail, probably with "Is a directory".
ln uses the following syscalls:
ln without the -s option should work once (or if) you implement hard links. ln with the -s option should work once (or if) you implement symbolic links. Check your assignments for when (or if) you need to implement these features.
ln is able to create symlinks even if hard links are not implemented, and vice versa.