program wanted for Posix

[snippsat]

Skaperen you should show some effort when posting these requests 💤

Here is a complete script (for a single file argument). Assumes utf8 encoding of the target file

Good,to make more robust as it fail sometime on files with Unicodes on Windows.
Windows dos bad guessing:

Error:
UnicodeDecodeError: 'charmap' codec can't decode byte .....

with open(arg.file, encoding='utf-8', errors='ignore') as infile:
    print(f'==> (head) {arg.file} <==') 

---

Here my take on this and i use Typer for CLI application.

import typer
from collections import deque

app = typer.Typer()

@app.command()
def headtail(filename: str, head: int = 5, tail: int = 5):
    tail_deque = deque(maxlen=tail)
    head_list = []
    total_lines = 0
    try:
        with open(filename, encoding='utf-8', errors='ignore') as file:
            for i, line in enumerate(file):
                if i < head:
                    head_list.append(line)
                tail_deque.append(line)
                total_lines += 1
        if total_lines <= head + tail:
            for line in head_list:
                print(line, end='')
        else:
            for line in head_list:
                print(line, end='')
            if tail > 0:
                print("...........")
            for line in tail_deque:
                print(line, end='')
    except FileNotFoundError:
        typer.echo(f"Error: The file '{filename}' does not exist.", err=True)
    except Exception as e:
        typer.echo(f"An error occurred: {e}", err=True)

if __name__ == "__main__":
    app()

Compare to see that it work the same.

Output:
G:\div_code\reader_env
λ python head_tail_grib.py -n 2 -m 7 contry.txt
==> (head) contry.txt <==
Tokyo;35.6897
Jakarta;-6.1750
==> (tail) contry.txt <==
Nanyang;32.9987
Hangzhou;30.2500
Foshan;23.0292
Nagoya;35.1833
Taipei;25.0375
Tongshan;34.2610
Dhanbād;23.7998
G:\div_code\reader_env

λ python head_tail.py contry.txt --head 2 --tail 7
Tokyo;35.6897
Jakarta;-6.1750
............
Nanyang;32.9987
Hangzhou;30.2500
Foshan;23.0292
Nagoya;35.1833
Taipei;25.0375
Tongshan;34.2610
Dhanbād;23.7998
G:\div_code\reader_env

Update: had two "with open" need only one.