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()
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" "$command"
done
In the end, I sent a message to the team and told them:
Hi all,
Please run this on each of your VMs and send me the output via email
echo "$(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