What is the maximum size of a Linux environment variable value?

I don't think there is a per-environment variable limit on Linux. The total size of all the environment variables put together is limited at execve() time. See "Limits on size of arguments and environment" here for more information.

A process may use setenv() or putenv() to grow the environment beyond the initial space allocated by exec.

Here's a quick and dirty program that creates a 256 MB environment variable.

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

int main(void)
{
  size_t size = 1 << 28; /* 256 MB */
  char *var;

  var = malloc(size);
  if (var == NULL) {
  perror("malloc");
  return 1;
}

  memset(var, 'X', size);
  var[size - 1] = '\0';
  var[0] = 'A';
  var[1] = '=';

  if (putenv(var) != 0) {
  perror("putenv");
  return 1;
}

  /*  Demonstrate E2BIG failure explained by paxdiablo */
  execl("/bin/true", "true", (char *)NULL);
  perror("execl");


  printf("A=%s\n", getenv("A"));

  return 0;
}

Well, it's at least 4M on my box. At that point, I got bored and wandered off. Hopefully the terminal output will be finished before I'm back at work on Monday :-)

export b1=A
export b2=$b1$b1
export b4=$b2$b2
export b8=$b4$b4
export b16=$b8$b8
export b32=$b16$b16
export b64=$b32$b32
export b128=$b64$b64
export b256=$b128$b128
export b512=$b256$b256
export b1k=$b512$b512
export b2k=$b1k$b1k
export b4k=$b2k$b2k
export b8k=$b4k$b4k
export b16k=$b8k$b8k
export b32k=$b16k$b16k
export b64k=$b32k$b32k
export b128k=$b64k$b64k
export b256k=$b128k$b128k
export b512k=$b256k$b256k
export b1m=$b512k$b512k
export b2m=$b1m$b1m
export b4m=$b2m$b2m
echo $b4m
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
:    :    :    :    :    :    :    :    :    :    :    :
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA

If you're worried that 4M may not be enough for your environment variable, you may want to rethink how you're doing things.

Perhaps it would be a better idea to put the information into a file and then use an environment variable to reference that file. I've seen cases where, if the variable is of the form @/path/to/any/fspec, it gets the actual information from the file path/to/any/fspec. If it doesn't begin with @, it uses the value of the environment variable itself.

Interestingly enough, with all those variables set, every single command starts complaining that the argument list is too long so, even though it lets you set them, it may not be able to start programs after you've done it (since it has to pass the environment to those programs).

I did a quick test on my Linux box with the following snippet:

a="1"
while true
do
    a=$a$a
    echo "$(date) $(numfmt --to=iec-i --suffix=B --padding=7 ${#a})" 
done

On my box (Gentoo 3.17.8-gentoo-r1) this results in (last lines of output):

Wed Jan  3 12:16:10 CET 2018   16MiB
Wed Jan  3 12:16:11 CET 2018   32MiB
Wed Jan  3 12:16:12 CET 2018   64MiB
Wed Jan  3 12:16:15 CET 2018  128MiB
Wed Jan  3 12:16:21 CET 2018  256MiB
Wed Jan  3 12:16:33 CET 2018  512MiB
xrealloc: cannot allocate 18446744071562068096 bytes

So: the limit is quite high!

Here are two helpful commands:

getconf -a | grep ARG_MAX
true | xargs --show-limits

Don't know exactly but a quick experiment shows that no error occurs e.g. with 64kB of value:

% perl -e 'print "#include <stdlib.h>\nint main() { return setenv(\"FOO\", \"", "x"x65536, "\", 1); }\n";'\
| gcc -x c -o envtest - && ./envtest && echo $?
0

What is the maximum size of a Linux environment variable value?

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