If I type into a terminal,
export DISPLAY=:0.0
... where is the shell storing that environment variable?
I'm using Ubuntu 8.10. I've looked in the files ~/.profile and /etc/profile and can find no trace of DISPLAY.
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4Where in memory: unix.stackexchange.com/questions/75939/…Ciro Santilli OurBigBook.com– Ciro Santilli OurBigBook.com2020-03-18 14:53:13 +00:00Commented Mar 18, 2020 at 14:53
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5 Answers
The environment variables of a process exist at runtime, and are not stored in some file or so. They are stored in the process's own memory (that's where they are found to pass on to children). But there is a virtual file in
/proc/pid/environ
This file shows all the environment variables that were passed when calling the process (unless the process overwrote that part of its memory — most programs don't). The kernel makes them visible through that virtual file. One can list them. For example to view the variables of process 3940, one can do
cat /proc/3940/environ | tr '\0' '\n'
Each variable is delimited by a binary zero from the next one. tr replaces the zero into a newline.
Gilles 'SO- stop being evil'
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answered Feb 10, 2009 at 13:17
Johannes Schaub - litb
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10 Comments
atomice
The kernel does not store the environment variables, they are stored in user mode. /proc/<pid>/environ may return the wrong values if the environment block has been re-allocated.
Johannes Schaub - litb
@atomice I was in the impression that the kernel stores them and makes them visible to user programs, writable to user programs. Doesthe
environ file represent the wrong values if an environment variable was changed in a defined manner? Can you perhaps show a program that exploits that?atomice
The kernel stores them at a particular location in user space and that is what /proc/<pid>/environ exposes. However if you modify the environment in a program using putenv or setenv the initial environment block is liable to be re-allocated (to accomodate the new variable). The new variables won't show up in the output from /proc/<pid>/environ.
Antonio Rizzo
@atomice So, where the new updated environment will stay? Which command to show it?
StndFish
For user2284570's question. As to the location of environment variables in RAM, they are stored in the top of the stack of main() function. any dynamic modification by setenv() et al. are then allocated elsewhere.
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Type set and you will get a list of all the current variables.
If you want something to persist put it in ~/.bashrc or ~/.bash_profile (if you're using bash)
Comments
If you want to put the environment for system-wide use you can do so with /etc/environment file.
3 Comments
user2284570
The question is about where in ʀᴀᴍ.
JohnnyQ
@user2284570 where in the question specified that? Can you please elaborate?
StndFish
For user2284570's question. As to the location of environment variables in RAM, they are stored in the top of the stack of main() function. any dynamic modification by setenv() et al. are then allocated elsewhere
It's stored in the process (shell) and since you've exported it, any processes that process spawns.
Doing the above doesn't store it anywhere in the filesystem like /etc/profile. You have to put it there explicitly for that to happen.
Comments
As to the location of environment variables in RAM, they are stored in the top of the stack of main() function. any dynamic modification by setenv() et al. are then allocated elsewhere
