CentOS Systemd – How to Set Environment Variable for All Services

The processes are organized as a tree: every process has a unique parent, apart from init which PID is always 1 and has no parent.

The creation of a new process goes generally through a pair of fork/execv system calls, where the environment of the child process is a copy of the parent process.

To put a variable in the environment from the shell you have to export that variable, so that it is visible recursively to all children. But be aware that if a child change the value of a variable, the changed value is only visible to it and all processes created after that change (being a copy, as previously said).

Take also into account that a child process could change its environment, for example could reset it to default values, as is probably done from login for example.

systemd is not a shell, so it does not support shell-specific substitutions in environment files. Their format is simply KEY=VALUE, with possible empty lines and comments in between.

To quote systemd.exec(5):

Settings from these files override settings made with Environment=. If the same variable is set twice from these files, the files will be read in the order they are specified and the later setting will override the earlier setting.

So, you can accomplish your task by either:

• using multiple EnvironmentFile= directives (they override each other in the order of specification)

• using Environment= in the service file to specify default values (any EnvironmentFile= will override it)