14

At my organization we have a number of simple-to-use base AMIs for different services such as ECS and Docker. Since many of our projects involve CloudFormation, we're using cfn-bootstrap, which consists of a couple of scripts and a service which run on boot to install certain packages and do certain configuration management tasks for us.

On startup of a system, an equivalent of the following script must be executed:

#!/bin/bash

# capture stderr only
output="$(cfn-init -s $STACK_NAME -r $RESOURCE_NAME --region $REGION >/dev/null)"

# if it failed, signal to CloudFormation that it failed and include a reason
returncode=$?
if [[ $returncode == 0]]; then
    cfn-signal -e $returncode -r "$output"
    exit $returncode
fi

# otherwise, signal success
cfn-signal -s

I was thinking of running this as a systemd oneshot service which runs After=network.target and WantedBy=multi-user.target.

The only problem is that I'd like my AMI to be flexible and only execute this if a certain file exists. Rather than embedding the above script into the EC2 user data, I can have the user data just define an environment file which defines the variables I need and only run my one-shot service if that environment file exists:

#cloud-init
write_files:
    - path: /etc/sysconfig/cloudformation
      # ...
      content: |
          CFN_STACK_NAME="stack-name"
          CFN_RESOURCE="resource-name"
          CFN_REGION="region"

Is there a way to make systemd only run a service if a given condition is met?

16

systemd provides a wide variety of conditions you can test. For instance, you can use ConditionPathExists= to test for the existence of a file.

[Unit]
ConditionPathExists=/etc/sysconfig/cloudformation
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  • 3
    It's worth noting that this isn't a while condition, but an if, meaning that if the path specified in ConditionPathExists doesn't exist by the time the service starts the rest of the service will just not run. I.e., it does not wait for the path to exist. – Mahn Jul 12 '16 at 20:59
  • @Mahn using a systemd timer, it should be possible to repeatedly trigger the service on an interval to overcome that limitation. – Naftuli Kay Feb 15 '17 at 23:23
  • @Mahn Take a look at freedesktop.org/software/systemd/man/systemd.path.html#. It can monitor a path and provide activation based on, for example, when a path comes into existence. – benf Sep 13 '19 at 22:28
2

I stumbled upon this question looking for ways to start a systemd service using a condition. There are many ways:

ConditionArchitecture=, ConditionVirtualization=, ConditionHost=, ConditionKernelCommandLine=, ConditionSecurity=, ConditionCapability=, ConditionACPower=, ConditionNeedsUpdate=, ConditionFirstBoot=, ConditionPathExists=, ConditionPathExistsGlob=, ConditionPathIsDirectory=, ConditionPathIsSymbolicLink=, ConditionPathIsMountPoint=, ConditionPathIsReadWrite=, ConditionDirectoryNotEmpty=, ConditionFileNotEmpty=, ConditionFileIsExecutable=

I wanted to start the service based on a specific host name.

ConditionHost= may be used to match against the hostname or machine ID of the host. This either takes a hostname string (optionally with shell style globs) which is tested against the locally set hostname as returned by gethostname(2), or a machine ID formatted as string (see machine-id(5)). The test may be negated by prepending an exclamation mark.

More on that here.

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