/dev/stderr

wikipedia:stderr

• 2>&1
• &>

• sudo -S or sudo --stdin

• 2>/dev/null

your-command-with-stdout-and-stderr 2>&1 | tee -a your file

• Google style guide: https://google.github.io/styleguide/shellguide.html#stdout-vs-stderr

err() {
  echo "[$(date +'%Y-%m-%dT%H:%M:%S%z')]: $*" >&2
}

if ! do_something; then
  err "Unable to do_something"
  exit 1
fi

Related

• CKA: Manage container stdout & stderr logs
• gcloud

See also

• Standard streams: /dev/stdin, /dev/stdout, /dev/stderr, /dev/null, File descriptor, set -x, 2>&1, stdbuf
• Linux logging, Cisco IOS logging