Shell – Updating environment variable in a shell script

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)

The $_ value is visible to the child process. How you read it in the child is up to the language you use.

e.g. in C

#include <stdlib.h>
#include <stdio.h>

main()
{
  char *x=getenv("_");
  printf("%s\n",x);
}

However a program can not rely on $_ being set because other shells may do different things.

e.g.

$ env -i /bin/bash -c ./a.out
./a.out

$ env -i /bin/ksh -c ./a.out 
*31801*./a.out

$ env -i /bin/sh -c ./a.out 
Segmentation fault (core dumped)

/bin/sh (which is "dash" on this Ubuntu system) doesn't set $_ at all and so this simple program crashes. Oops :-)

Your calling program can't see this value of $_ at all. Indeed, in the calling shell $_ is set to the last parameter of the previous line

$ ls /tmp > /dev/null
$ echo $_
/tmp

$ echo hello
hello
$ echo $_
hello