On Unix, is there any way that one process can change another's environment variables (assuming they're all being run by the same user)? A general solution would be best, but if not, what about the specific case where one is a child of the other?

Edit: How about via gdb?

Solution 1

Via gdb:

(gdb) attach process_id

(gdb) call putenv ("env_var_name=env_var_value")

(gdb) detach

This is quite a nasty hack and should only be done in the context of a debugging scenario, of course.

Solution 2

You probably can do it technically (see other answers), but it might not help you.

Most programs will expect that env vars cannot be changed from the outside after startup, hence most will probably just read the vars they are interested in at startup and initialize based on that. So changing them afterwards will not make a difference, since the program will never re-read them.

If you posted this as a concrete problem, you should probably take a different approach. If it was just out of curiosity: Nice question :-).

Solution 3

Substantially, no. If you had sufficient privileges (root, or thereabouts) and poked around /dev/kmem (kernel memory), and you made changes to the process's environment, and if the process actually re-referenced the environment variable afterwards (that is, the process had not already taken a copy of the env var and was not using just that copy), then maybe, if you were lucky and clever, and the wind was blowing in the right direction, and the phase of the moon was correct, perhaps, you might achieve something.

Solution 4

Quoting Jerry Peek:

You can't teach an old dog new tricks.

The only thing you can do is to change the environment variable of the child process before starting it: it gets the copy of the parent environment, sorry.

See http://www.unix.com.ua/orelly/unix/upt/ch06_02.htm for details.

Just a comment on the answer about using /proc. Under linux /proc is supported but, it does not work, you cannot change the /proc/${pid}/environ file, even if you are root: it is absolutely read-only.

Solution 5

I could think of the rather contrived way to do that, and it will not work for arbitrary processes.

Suppose that you write your own shared library which implements 'char *getenv'. Then, you set up 'LD_PRELOAD' or 'LD_LIBRARY_PATH' env. vars so that both your processes are run with your shared library preloaded.

This way, you will essentially have a control over the code of the 'getenv' function. Then, you could do all sorts of nasty tricks. Your 'getenv' could consult external config file or SHM segment for alternate values of env vars. Or you could do regexp search/replace on the requested values. Or ...

I can't think of an easy way to do that for arbitrary running processes (even if you are root), short of rewriting dynamic linker (ld-linux.so).

Solution 6

Or get your process to update a config file for the new process and then either:

  • perform a kill -HUP on the new process to reread the updated config file, or
  • have the process check the config file for updates every now and then. If changes are found, then reread the config file.

Solution 7

It seems that putenv doesn't work now, but setenv does. I was testing the accepted answer while trying to set the variable in the current shell with no success

$] sudo gdb -p $$
(gdb) call putenv("TEST=1234")
$1 = 0
(gdb) call (char*) getenv("TEST")
$2 = 0x0
(gdb) detach
(gdb) quit
$] echo "TEST=$TEST"
TEST=

and the variant how it works:

$] sudo gdb -p $$
(gdb) call (int) setenv("TEST", "1234", 1)
$1 = 0
(gdb) call (char*) getenv("TEST")
$2 = 0x55f19ff5edc0 "1234"
(gdb) detach
(gdb) quit
$] echo "TEST=$TEST"
TEST=1234

Solution 8

Not as far as I know. Really you're trying to communicate from one process to another which calls for one of the IPC methods (shared memory, semaphores, sockets, etc.). Having received data by one of these methods you could then set environment variables or perform other actions more directly.

Solution 9

If your unix supports the /proc filesystem, then it's trivial to READ the env - you can read the environment, commandline, and many other attributes of any process you own that way. Changing it... Well, I can think of a way, but it's a BAD idea.

The more general case... I don't know, but I doubt there's a portable answer.

(Edited: my original answer assumed the OP wanted to READ the env, not change it)

Solution 10

UNIX is full of Inter-process communication. Check if your target instance has some. Dbus is becoming a standard in "desktop" IPC.

I change environment variables inside of Awesome window manager using awesome-client with is a Dbus "sender" of lua code.

Solution 11

Not a direct answer but... Raymond Chen had a [Windows-based] rationale around this only the other day :-

... Although there are certainly unsupported ways of doing it or ways that work with the assistance of a debugger, theres nothing that is supported for programmatic access to another processs command line, at least nothing provided by the kernel. ...

That there isnt is a consequence of the principle of not keeping track of information which you dont need. The kernel has no need to obtain the command line of another process. It takes the command line passed to the CreateProcess function and copies it into the address space of the process being launched, in a location where the GetCommandLine function can retrieve it. Once the process can access its own command line, the kernels responsibilities are done.

Since the command line is copied into the processs address space, the process might even write to the memory that holds the command line and modify it. If that happens, then the original command line is lost forever; the only known copy got overwritten.

In other words, any such kernel facilities would be

  • difficult to implement
  • potentially a security concern

However the most likely reason is simply that there's limited use cases for such a facility.