시스템 관리자 작업을 위한 Bash 스크립트 vs. Python 스크립트?

Which do you prefer to sling around, a bash command or a python script?

In my goal to write more python and less shell, for the benefit of my colleagues’ sanity, I have been trying to favour Python – specifically, readable and maintainable python.

I have moaned before about how sysadmin scripts often look very messy and unmaintainable, and I have found a perfect example of how that arises.

I needed my team to send me the hostnames, IPs and MAC addresses of their VMs to send to IT for reserving in DHCP, and I started cooking up a python script. Just as I was finishing, I had a flash of inspiration and thought… can’t I do just a one-liner in shell?

It’s just a matter of getting the default IP and looking for the interface that uses it… I wrote the scirpt so that it could give result for a local host, or a series of specified remote hosts.

Which would you prefer to commit to a tooling repo? 🙂

Here’s the python script, idiomatic, and hopefully easy to read and maintain. I was going to tell my colleagues “Please clone (script repo) and run the script, supplying the hosts as arguments, like ‘gather-ips.py jack@machine1 jack@machine2 …’ (yeah, I would need to send instructions too)

#!/usr/bin/env python

import re
import subprocess
import sys

class SubprocessError(Exception):
    pass


def run(command:list[str]) -> str:
    proc = subprocess.Popen(command, stdout=subprocess.PIPE, text=True)
    stdout, _ = proc.communicate()
    if proc.returncode > 0:
        raise SubprocessError(f"Failed {command}, see stderr.")

    return stdout


def read_ipa(ipa_lines):
    ipa_entries = {}

    current = None

    for line in ipa_lines:
        m = re.match(r"^[0-9]+:\s+(\S+?):.*", line)
        if m:
            current = m.group(1)
            ipa_entries[current] = {}
            continue
        if current is None:
            continue

        current_entry = ipa_entries[current]

        eth_m = re.match(r".+link/ether ([a-fA-F0-9:]+)", line)
        if eth_m:
            current_entry["mac"] = eth_m.group(1)

        inet_m = re.match(r"\s+inet ([0-9.]+)/[0-9]+", line)
        if inet_m:
            current_entry["ipv4"] = inet_m.group(1)

    return ipa_entries


def get_default_ip(ipr_lines):
    default_line = [line for line in ipr_lines if 'default via' in line][0]
    return re.findall(r"src [0-9.]+", default_line)[0][4:]


def get_ipra_lines(host):
    if host:
        lines = run(["ssh", host, "ip r; echo ===; ip a"]).split("\n")
        idx = 0
        for n in range(len(lines)):
            if lines[n] == "===":
                idx = n
                break
        ipr_lines = lines[:idx]
        ipa_lines = lines[idx+1:]
    else:
        ipr_lines = run(["ip", "r"]).split("\n")
        ipa_lines = run(["ip", "a"]).split("\n")

    return ipr_lines,ipa_lines


def ublat(host_string:str):
    try:
        i = host_string.index("@")
        return host_string[i+1:]
    except ValueError:
        return host_string


def get_target(host=None):
    ipr_lines, ipa_lines = get_ipra_lines(host)
    entries = read_ipa(ipa_lines)
    def_ip = get_default_ip(ipr_lines)
    hostname = ublat(host) if host else run(["hostname"]).strip()

    for _, data in entries.items():
        if def_ip in data.get("ipv4", ""):
            print(f"{hostname}\t{data.get('ipv4')}\t{data.get('mac')}")
            return
    print(f"# Nothing in {entries}")


def main():
    if not sys.argv[1:] or "get" in sys.argv[1:]:
        get_target()
        return

    for host in sys.argv[1:]:
        get_target(host)


main()

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For sake of comparison, this is its bash equivalent – it does EXACTLY the same thing:

command="echo \"\$(hostname)  \$(ip a|grep \"\$(ip r|grep 'default via'|grep -Po '(?<=src )[0-9.]+')\" -B 1|grep -Po '(?<=ether |inet )(\S+)')\"|xargs echo"

if [[ -z "$*" ]]; then
    bash -c "$command"
    exit
fi

for node in "$@"; do 
    ssh "$node" "$command"
done

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In the end, I sent a message to the team and told them:

Hi all,

Please run this on each of your VMs and send me the output via email

echo "$(hostname) $(ip a|grep "$(ip r|grep 'default via'|grep -Po '(?<=src )[0-9.]+')\" -B 1|grep -Po '(?<=ether |inet )(\S+)')"|xargs echo

A one-liner in a single message. No sending scripts around. No instructing to clone and execute. Just “run this on each machine.”

Which approach would you have taken