Test if bash script was initiated from systemd

Question

As a Unix sysadmin, we experience issues with others (E.g. DBAs) starting/stopping services by interactively running scripts. This means systemd has no knowledge of the state of a service. To prevent this, I insert a conditional in the scripts like this:

# The systemd unit creates a ORACLE_SYSTEMD_UNIT environment variable.  If this
# isn't set (i.e. Not running through systemd) then abort.
if [ "${ORACLE_SYSTEMD_UNIT:+set}" == "" ]; then
    logmsg "Not initiated through systemd.  Aborting"
    exit 2
fi

Can anyone suggest a cleaner method, or perhaps an Environment variable that is automatically set by the systemd unit activation?

Answer 1

As pointed out in the comments, you can check the command name of the parent process. In bash the process id of the parent process is stored in $PPID. The command name for a given pid can be obtained by ps. This only works if your script is running directly under systemd.

if [ "$(ps -o comm= $PPID)" != systemd ]; then
    echo "Not running under systemd"
fi

If you also want to check the parents of the parents and so on, iterate/recurse. The parent of any pid can be obtained by ps too.

runsUnderSystemd() {
    pid="${1-$$}"
    [ "$pid" = 0 ] && return 1
    [ "$(ps -o comm= "$pid")" = systemd ] ||
    runsUnderSystemd "$(ps -o ppid= "$pid" | tr -d ' ')"
}
if ! runsUnderSystemd; then
    echo "Not running under systemd"
fi

With pstree this can be shortend to

if ! pstree -s $PPID | grep -Fwq systemd; then
    echo "Not running under systemd"
fi

Alternatively, you can set an environment variable in your .service file for systemd ...

[Service]
Environment="STARTED_BY_SYSTEMD=yes"

... which can then be checked in all children of the started service

if [ "$STARTED_BY_SYSTEMD" != yes ]; then
   echo "Not running under systemd"
fi

Please note that both approaches are just safeguards against mistakes. An attacker could fake or hide systemd in either way.