CODEWITHSUNDEEP
linuxshellunixgdbenvironment-variables
raldi
raldi

Reputation: 22170

Is there a way to change the environment variables of another process in Unix?

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?

Upvotes: 126

Views: 55978

Answers (13)

837468220
837468220

Reputation: 1

Change /proc/?/environ via gdb

$ gdb -p pid

(gdb) p *environ@20
$1 = {[0] = 0x7fff41fb0928 "CONSOLE=/dev/console", [1] = 0x7fff41fb093d "TERM=vt100",
  [2] = 0x7fff41fb0948 "SHELL=/bin/csh", [3] = 0x7fff41fb0957 "rootmnt=/root",
  [4] = 0x7fff41fb0965 "INIT_VERSION=sysvinit-2.88", [5] = 0x7fff41fb0980 "init=/sbin/init",
  [6] = 0x7fff41fb0990 "COLUMNS=80",
(gdb) x/s 0x7fff41fb0990
0x7fff41fb0990:  "COLUMNS=80"
(gdb) x/s 0x7fff41fb0990+8
0x7fff41fb0998:  "80"
(gdb) x/s 0x7fff41fb0990+9
0x7fff41fb0999:  "0"
(gdb) set {char}(0x7fff41fb0990+9)=0x31
(gdb) x/s 0x7fff41fb0990+8
0x7fff41fb0998:  "81"


$ strings /proc/4351/environ | grep COL
COLUMNS=81

Upvotes: -1

DenisKolodin
DenisKolodin

Reputation: 15131

Yes, you can, but it doesn't make sense. To do that you should invade into a running process and change its memory.

Environment variables are the part of the running process, that also means env vars are copied and stored to the memory of a process when it started, and only the process itself owns it.

If you change evn vars when the process is trying to read them it could be corrupted, but you still can change them if you attach a debugger to the process, because it has access to the memory of the attached process.

Upvotes: 1

Kakash1hatake
Kakash1hatake

Reputation: 148

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

Upvotes: 5

Ruben Bartelink
Ruben Bartelink

Reputation: 61875

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, there’s nothing that is supported for programmatic access to another process’s command line, at least nothing provided by the kernel. ...

That there isn’t is a consequence of the principle of not keeping track of information which you don’t 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 kernel’s responsibilities are done.

Since the command line is copied into the process’s 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.

Upvotes: 0

Rob Wells
Rob Wells

Reputation: 37149

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.

Upvotes: 3

dvd
dvd

Reputation: 1820

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.

Upvotes: 1

sleske
sleske

Reputation: 83635

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 :-).

Upvotes: 25

An̲̳̳drew
An̲̳̳drew

Reputation: 13892

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.

Upvotes: 164

Jonathan Leffler
Jonathan Leffler

Reputation: 754760

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.

Upvotes: 16

ADEpt
ADEpt

Reputation: 5542

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).

Upvotes: 8

Davide
Davide

Reputation: 17828

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.

Upvotes: 7

Mike G.
Mike G.

Reputation: 1690

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)

Upvotes: 1

Stephen Darlington
Stephen Darlington

Reputation: 52565

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.

Upvotes: 2

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