Systemd.service: Use the output of a bash function to set up network

Question

I try to write a service to set up the network interface.
In Sysvinit I used a script:

IP=$(fw_printenv -n ipaddr)
ifconfig eth0 $IP netmask 255.255.254.0 up

Now I want to do this in a Service:

[Service]
Type=oneshot
Environment="IP=$(/sbin/fw_printenv -n ipaddr)"
ExecStart=/sbin/ifconfig eth0 $IP netmask 255.255.255.0 up

This does not work. In systemctl status -l start-network.service it says:

Oct 23 06:17:29 lcd5a-dsp ifconfig[261]: ifconfig: invalid number 'sbin/fw_printenv'

Does anybody know how to use the output of a function as a variable in a systemd.service?

Answer 1

Note that Environment="var=$(cmd arg)" does not actually evaluate the $() part. The variable is just set to the string "$(cmd arg)".

Also, ExecStart=... $var ... will replace $var by the string, but will also split the resulting line on spaces, so you will effectively get "$(cmd" and "arg)" in the result. You need to use ${var} to avoid this and keep the string as one arg.

However, this isn't going to work, as nothing is going to evaluate the $(), so you need to explicitly call bash -c and pass the whole command as a single arg by enclosing it in quotes. These quotes are not like bash quotes and do not stop the expansion of ${var} (though it does stop $var expanding if it is not on its own). So finally, this should work:

ExecStart=/bin/bash -c '/sbin/ifconfig eth0 ${IP} netmask 255.255.255.0 up'