When writing shell scripts invoked by udev rules, I find the following template particularly useful:
#!/usr/bin/env bash
set -euo pipefail
CMD="`basename "${0:-udevscript}"`"
exec 1> >(tee >(logger -t "${CMD}"))
exec 2> >(ts '[stderr]' | tee >(logger -t "${CMD}"))
DEBUG="${1:-false}"
if [ "${DEBUG}" = true ]; then set -x; fi
echo "running: ${0}"
env
## Actual script starts here
We can then use such a script in a udev rule like we always would:
SUBSYSTEM=="usb", \
ACTION=="remove", \
ENV{ID_VENDOR_ID}=="17e9", \
ENV{ID_MODEL_ID}=="6015", \
RUN+="/usr/lib/udev/scripts/undock.sh"
And when the rule runs the script, everything printed is sent to the system log. Stderr messages will be prefixed with [stderr]. Which makes checking what the script does as simple as:
$ journalctl -e -t undock.sh
Oct 25 22:58:35 puddle undock.sh[32366]: running: /usr/lib/udev/scripts/undock.sh
Oct 25 22:58:35 puddle undock.sh[32366]: ID_SERIAL=DisplayLink_ThinkPad_Hybrid_USB-C_with_USB-A_Dock_10391787
Oct 25 22:58:35 puddle undock.sh[32366]: ID_MODEL_ID=6015
Oct 25 22:58:35 puddle undock.sh[32366]: ACTION=remove
...
The template also supports debug output. Setting the first argument to true will print commands and their arguments as they are executed to stderr.
Bonus: A Nix Expression
The script above is derived from the following nix expression.
{ pkgs ? import <nixpkgs> {}, ... }:
let
logger = "${pkgs.utillinux}/bin/logger";
ts = "${pkgs.moreutils}/bin/ts";
tee = "${pkgs.coreutils}/bin/tee";
in name : script: {
strict ? true,
log ? true,
debug ? false,
printenv ? true,
}: let
strictCmd = if strict then
''
set -euo pipefail
''
else "";
logCmd = if log then
''
exec 1> >(${tee} >(${logger} -t "${name}"))
exec 2> >(${ts} '[stderr]' | ${tee} >(${logger} -t "${name}"))
''
else "";
debugCmd = if debug then
''
set -x
''
else "";
printenvCmd = if printenv then
''
echo "running: ${name}"
env
''
else "";
in pkgs.writeShellScript name
''
${strictCmd}
${logCmd}
${debugCmd}
${printenvCmd}
${script}
''
If invoked, the expression writes such a script to the nix store.
nix-repl> writeLoggedScript = import ./writeLoggedScript.nix {}
nix-repl> :b writeLoggedScript "test.sh" "echo this is just a test" {}
this derivation produced the following outputs:
out -> /nix/store/bzjllwyhy0zwm2f352a0gya8fp13qc7q-test.sh