pdb2sequence < input_pdb_file > output_sequence_file
pdb2sequence reads a PDB-file from the standard in, converts the 3-letter residue codes in the PDB-file into 1-letter codes and sends the resulting sequence to the standard out.
The translation between 3 letter codes and one letter codes can be customized by supplying a "res_code_dict.txt" file. (See the MinRMS documentation for a description of how to do this.)