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Nov 03: Command Line Scripts

Learning Objectives

After today's class, you should be able to:

  • Name and describe three commands used in the terminal.
  • Explain sys.argv and what command-line arguments are.
  • Summarize what you can do with the os and sys modules.

Reminders

Hints on PA2, Part A

[10 min]

  • See Clarifications posted this morning

    • In particular, update test_read_file()

  • Turn on type hint checking

    • Press F1 to open the command pallette.
    • Type work json and press Enter.
      • This will open a file named settings.json.
    • Add/update the following settings between the curly braces: 
      "editor.rulers": [100],
"python.analysis.typeCheckingMode": "basic",
    • And after ".", under "python.testing.pytestArgs" add: 
      "-v",
      • This will make the pytest output more verbose (helpful)

  • Do not "hard code" file paths

    • For example, don't do this: 
      # WRONG, the CS149/PA2 folder won't exist on Gradescope!
with open("CS149/PA2/log.txt") as file:
    ...
    • Instead, use this pattern to get the "current" path: 
      path = os.path.join(os.path.dirname(__file__), "log.txt")
with open(path) as file:
    ...

The Command Line

[5 min]

  • Also known as the "command line" or "terminal"
  • Take time after class to learn a few commands
  • Important symbols
    • ~ (tilde) means home directory
    • . (dot) means current directory
    • .. (dot dot) means parent directory

Program Arguments

[5 min]

  • Optional arguments can be given to a program on the command line.
  • sys.argv is the list of command-line arguments passed to program.

Example 1: Printing Arguments

print_args.py
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import sys

def main():
    for i, arg in enumerate(sys.argv):
        print(f"argv[{i}] == '{arg}'")

if __name__ == "__main__":
    main()

Walking the File System

[10 min]

  • Demo of the os module and the sys module
  • Step through this program using the debugger

Example 2: Finding Python Files

search.py
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"""Find all Python files in a given directory."""

import os
import sys

def main(path):

    # for each directory starting from path
    for root, dirs, files in os.walk(path):
        print()
        print(f"{root = }")
        print(f"{dirs = }")
        print(f"{files = }")
        print()
        # for each Python file in current directory
        for filename in sorted(files):
            if filename.endswith(".py"):
                filepath = os.path.join(root, filename)
                print(filepath)

if __name__ == "__main__":
    if len(sys.argv) == 1:
        # no arguments; use current directory
        main(".")
    else:
        # use the "first" command-line argument
        main(sys.argv[1])

In-Class Practice

[20 min]

Exercise

Implement the wc command on Unix systems:

  • Write a command line script named word_count.py.
  • If no arguments are given, print the following message: 
    print("Usage: python word_count.py FILE [FILE ...]")
print("Count lines, words, and chars in each file.")
  • For each command line argument:
    • Check if os.path.exists() before opening the file.
    • Count how many lines, words, and chars are in the file.
    • Print the results in this format: 
      print(f"{line_count:5d} {word_count:5d} {char_count:5d} {path}")