Inspect units:

systemctl [opts] [cmd]
    --type=TYPE             List only given types eg, service, timer, socket (use --type=help for a list)
    --state=STATE           List only given states eg running, enabled (use --state=help for a list)
    --failed                List only failed services

    list-units <pattern>    List units in memory

    status <unit>           Show runtime status of unit

    start <unit>            Start a unit
    stop <unit>             Stop a unit
    restart <unit>          Restart a unit
    reload <unit>           Reload a unit

    enable <unit>           Enable a unit (persistent)
    disable <unit>          Disable a unit

    cat <unit>      Print unit file
    show <unit>     Show properties of unit

Example: List failed units

# List all system failed units.
systemctl --failed

# List all user failed units.
systemctl --user --failed

Example: Trivial user unit

# Generate unit
mkdir -p ~/.config/systemd/user
echo '[Unit]
Description=Test logger

ExecStart=logger "Hello from test unit"' > ~/.config/systemd/user/test.service

# Run unit
systemctl --user start test

# See log message
journalctl --user -u test -n 5


Inspect journal logs:

journalctl [opts] [matches]
    --user          Current user journal (system by default)
    -u <unit>       Show logs for specified <unit>
    -n <lines>      Show only last <lines>
    -f              Follow journal
    -g <pattern>    Grep for <pattern>


journalctl [opts]
    --disk-usage            Show current disk usage
    --vacuum-size=<size>    Reduce journal log to <size> (K/M/G)