systemctl cat — verify an env override took effect

Source: Cairn deploy — verifying CAIRN_DATA_DIR drop-in Category: Snippet — systemd

systemctl cat — prints the effective unit configuration, merging the base unit with every drop-in. Use it after any edit/override to confirm your change landed. Far more reliable than trusting the editor closed cleanly.

The command

sudo systemctl cat <unit-name>

Shows every file contributing to the unit, with comments pointing to the file path:

[Unit]
Description=Portfolio Web Server
After=network.target

[Service]
ExecStart=/usr/bin/node /opt/portfolio/ts/src/web-server.ts
Restart=on-failure

# /etc/systemd/system/portfolio-web.service.d/cairn-data-dir.conf
[Service]
Environment="CAIRN_DATA_DIR=/var/lib/cairn"

# /etc/systemd/system/portfolio-web.service.d/port.conf
[Service]
Environment=WEB_PORT=3000

Two drop-ins visible; each in its own file. If either is missing, cat would skip it — immediate confirmation that “did my override take”.

When to use it

After every one of:

systemctl edit <unit>
tee /etc/systemd/system/<unit>.d/override.conf
Editing the base unit file directly
Package upgrade that might have regenerated a unit

Combined with grep for specific values

sudo systemctl cat portfolio-web.service | grep CAIRN_DATA_DIR

Empty output means the env var isn’t set anywhere. Exactly what you need to confirm when troubleshooting “why isn’t my service picking up this variable”.

Command	What it shows
`systemctl cat <unit>`	merged unit + all drop-ins (this snippet)
`systemctl show <unit>`	computed state (every key the system has for this unit, expanded)
`systemctl status <unit>`	current status, recent log lines
`systemctl is-enabled <unit>`	enabled / disabled / masked / static
`journalctl -u <unit> -f`	live log tail

systemctl show is verbose and often overwhelming; systemctl cat is the pragmatic daily check.

The common failure mode

systemctl edit <unit> opens an editor for override.conf. If you save without adding content (just wrote nothing and closed), the override file is created but empty. systemctl cat shows the empty drop-in — that’s how you notice.

(empty)

Fix: write the drop-in directly with tee, skipping the editor:

sudo tee /etc/systemd/system/portfolio-web.service.d/cairn-data-dir.conf <<'EOF'
[Service]
Environment="CAIRN_DATA_DIR=/var/lib/cairn"
EOF
sudo systemctl daemon-reload
sudo systemctl cat portfolio-web.service | grep CAIRN_DATA_DIR

Gotchas

daemon-reload is required after changes. systemctl cat shows the file content — if you forgot daemon-reload, the running service still uses the old config even though cat shows the new one. Always: edit → daemon-reload → cat (sanity) → restart.
cat and show disagree on purpose. cat is textual (unit files). show is runtime state (what systemd parsed and kept). Runtime can differ from files when daemon-reload is skipped.
Permissions. sudo systemctl cat usually works without sudo because unit files are world-readable. systemctl edit requires sudo because it writes to /etc/systemd/.
User units. For --user services, the path is ~/.config/systemd/user/ and you don’t use sudo.
No instance matching. For template units like [email protected], systemctl cat [email protected] works; systemctl cat foo@ doesn’t.
Masked units. If a unit is masked (ln -s /dev/null ...), cat returns essentially nothing. is-enabled surfaces “masked”.