systemd string formatting for instance parameter

ajwood

10/19/23, 1:40 PM

I'm trying to set up a systemd service that takes a parameter, and uses it in a couple places. I think I'm very close: with my config file name [email protected], I can run systemctl start myservice@foo myservice@bar, and use %i in the config to get foo and bar in the service instances.

The trick is that I need the parameter to be upper case in one section, but lowercase in another.

# foo instance
User=myuser-%i   # myuser-foo
ExecStart=my_command /data/%i  # needs to be /data/FOO



# bar instance
User=myuser-%i   # myuser-bar
ExecStart=my_command /data/%i  # needs to be /data/BAR

So far the only way I can think to do this is write a wrapper script, and change to ExecStart=my_wrapper.sh %i to let the wrapper do the necessary string formatting. That seems clunky though, is this possible directly in the service config?

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meuh

10/19/23, 3:24 PM

You could use bash's uppercase parameter syntax: ${v^^} converts the characters in variable $v to uppercase. So you would like to do something like v="%i"; cmd /data/${v^^}. You need to stop systemd from interpreting ${}, so use $$ for $.

ExecStart=bash -c 'v="%i"; exec my_command /data/"$${v^^}"'

+ 2

ajwood

10/19/23, 9:59 PM

Cool, I think this should work. My system doesn't seem to like that as-is though, it wants an absolute path to the executable (so `/bin/bash`)

meuh

10/20/23, 1:00 PM

Yes, older versions of systemd only take absolute pathnames. For newer ones see [man systemd.service](https://www.freedesktop.org/software/systemd/man/systemd.service.html#Command%20lines), and the section headed "Command Lines" for the allowed syntax, and the command `systemd-path search-binaries-default` to show the PATH used for `Exec...=`.

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