Lab #4: SSH and Git

Other labs: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12.

Lab #4 (Mar 9 – Mar 13)

Before class

• What is asymmetric cryptography? What is a public and a private key?
• What is SSH?
• On which port is SSH daemon typically listening?
• What does git config --global user.name and git config --global user.email set?
• Get yourself familiar with the following program. It uses argparse (including custom Action class) and csv.DictReader.

  #!/usr/bin/env python3

  import argparse
  import csv
  from pathlib import Path
  import sys


  def parse_filter_description(filter_description):
        """
        Filter has the general form of COLUMN OPERATOR ARGS where COLUMN
        refers to column name as given in CSV and OPERATOR with ARGS denotes
        which condition to evaluate.

        Currently, OPERATOR can be only 'greaterthan' where ARGS is single
        integer to compare value with.
        """

        parts = filter_description.split()
        (column_name, operator, args) = (parts[0], parts[1], parts[2:])
        if operator == 'greaterthan':
            limit = int(args[0])
            return lambda row: int(row[column_name]) > limit
        raise ValueError('Unknown operator')


  class RowFilterAction(argparse.Action):
        def __init__(self, *args, **kwargs):
            argparse.Action.__init__(self, *args, **kwargs)

        def __call__(self, parser, namespace, values, option_string=None):
            try:
                result = parse_filter_description(values)
                setattr(namespace, self.dest, result)
            except Exception as ex:
                parser.error('Invalid filter specification ({})'.format(ex))


  def main(argv):
        args = argparse.ArgumentParser(description='Simple CSV templater')
        args.add_argument('-t', '--template',
                          required=True,
                          dest='template_filename',
                          metavar='PATH',
                          help='Template file (with Python formatting).')
        args.add_argument('-o', '--output',
                          default=None,
                          dest='output_filename',
                          metavar='TEMPLATED_PATH',
                          help='Output filename (with Python formatting).')
        args.add_argument('-F', '--filter',
                          required=False,
                          default=lambda _: True,
                          dest='record_filter',
                          metavar='CONDITION',
                          action=RowFilterAction,
                          help='Filter for each row.')
        config = args.parse_args(argv)

        template = Path(config.template_filename).read_text()
        records = csv.DictReader(sys.stdin)

        for rec in records:
            if not config.record_filter(rec):
                continue
            output = template.format(**rec)
            if config.output_filename:
                with open(config.output_filename.format(**rec), 'w') as out:
                     print(output, file=out)
            else:
                print(output)

  if __name__ == '__main__':
        main(sys.argv[1:])

Topic

• Remote shell with SSH.
• Public/private key generation, its use with SSH (and Git/GitLab).
• Git: basic concepts (fork, clone, commit, patch, pull & push).

Exercises

git commit -m "Typo fix"
# or interactively in your $EDITOR
git commit

ssh LOGIN@u-pl1.ms.mff.cuni.cz

Host linux-intro-machine
   HostName unixadmin.ms.mff.cuni.cz
   User LOGIN

git COMMAND --help
man git-COMMAND

git config --global user.name "First-name Second-name"
git config --global user.email "your-email@provider.domain.name"