Linux – How to get number of opened file descriptors for user

nproc was the problem:

[root@localhost ~]# ps -eLf | grep pascal | wc -l
4068
[root@localhost ~]# cat /etc/security/limits.d/20-nproc.conf
# Default limit for number of user's processes to prevent
# accidental fork bombs.
# See rhbz #432903 for reasoning.

*          soft    nproc     4096
root       soft    nproc     unlimited
[root@localhost ~]#

man limits.conf states:

   Also, please note that all limit settings are set per login. They are
   not global, nor are they permanent; existing only for the duration of
   the session. One exception is the maxlogin option, this one is system
   wide. But there is a race, concurrent logins at the same time will not
   always be detected as such but only counted as one.

It appears to me that nproc is only enforced per login but counts globally. So a login with nproc 8192 and 5000 threads would have no problems, but a simultaneous login of the same UID with nproc 4096 and 50 threads would not be able to create more because the global count (5050) is above its nproc setting.

[root@localhost ~]# ps -eLf | grep pascal | grep google/chrome | wc -l
3792

Just use a shell where mv is or can be made builtin, and you won't have the problem (which is a limitation of the execve() system call, so only with external commands). It will also not matter as much how many times you call mv.

zsh, busybox sh, ksh93 (depending on how it was built) are some of those shells. With zsh:

#! /bin/zsh -

zmodload zsh/files # makes mv and a few other file manipulation commands builtin
batch=1000
files=(files/*(N))

for ((start = 1; start <= $#files; start += batch)) {
  (( end = start + batch - 1))
  mkdir -p ${start}_${end} || exit
  mv -- $files[start,end] ${start}_${end}/ || exit
}

The execve() E2BIG limit applies differently depending on the system (and version thereof), can depend on things like stacksize limit. It generally takes into account the size of each argv[] and envp[] strings (including the terminating NUL character), often the size of those arrays of pointers (and terminating NULL pointer) as well (so it depends both on the size and number of arguments). Beware that the shell can set some env vars at the last minute as well (like the _ one that some shells set to the path of the commands being executed).

It could also depend on the type of executable (ELF, script, binfmt_misc). For instance, for scripts, execve() ends up doing a second execve() with a generally longer arg list (["myscrip", "arg", NULL] becomes ["/path/to/interpreter" or "myscript" depending on system, "-<option>" if any on the shebang, "myscript", "arg"]).

Also beware that some commands end up executing other commands with the same list of args and possibly some extra env vars. For instance, sudo cmd arg runs cmd arg with SUDO_COMMAND=/path/to/cmd arg in its environment (doubling the space required to hold the list of arguments).

You may be able to come up with the right algorithm for your current Linux kernel version, with the current version of your shell and the specific command you want to execute, to maximise the number of arguments you can pass to execve(), but that may no longer be valid of the next version of the kernel/shell/command. Better would be to take xargs approach and give enough slack to account for all those extra variations or use xargs.

GNU xargs has a --show-limits option that details how it handles it:

$ getconf ARG_MAX
2097152
$ uname -rs
Linux 5.7.0-3-amd64
$ xargs --show-limits < /dev/null
Your environment variables take up 3456 bytes
POSIX upper limit on argument length (this system): 2091648
POSIX smallest allowable upper limit on argument length (all systems): 4096
Maximum length of command we could actually use: 2088192
Size of command buffer we are actually using: 131072
Maximum parallelism (--max-procs must be no greater): 2147483647

You can see ARG_MAX is 2MiB in my case, xargs thinks it could use up to 2088192, but chooses to limit itself to 128KiB.

Just as well as:

$ yes '""' | xargs -s 230000 | head -1 | wc -c
229995
$ yes '""' | strace -fe execve xargs -s 240000 | head -1 | wc -c
[...]
[pid 25598] execve("/bin/echo", ["echo", "", "", "", ...], 0x7ffe2e742bf8 /* 47 vars */) = -1 E2BIG (Argument list too long)
[pid 25599] execve("/bin/echo", ["echo", "", "", "", ...], 0x7ffe2e742bf8 /* 47 vars */) = 0
[...]
119997

It could not pass 239,995 empty arguments (with total string size of 239,995 bytes for the NUL delimiters, so fitting in that 240,000 buffer) so tried again with half as many. That's a small amount of data, but you have to consider that the pointer list for those strings is 8 times as big, and if we add up those, we get over 2MiB.

When I did this same kind of tests over 6 years ago in that Q&A here with Linux 3.11, I was getting a different behaviour which had already changed recently at the time, showing that the exercise of coming up with the right algorithm to maximise the number of arguments to pass is a bit pointless.

Here, with an average file path size of 32 bytes, with a 128KiB buffer, that's still 4096 filenames passed to mv and the cost of starting mv is alreadly becoming negligible compared to the cost of renaming/moving all those files.

For a less conservative buffer size (to pass to xargs -s) but that should still work for any arg list with past versions of Linux at least, you could do:

$ (env | wc; getconf ARG_MAX) | awk '
  {env = $1 * 8 + $3; getline; printf "%d\n", ($0 - env) / 9 - 4096}'
228499

Where we compute a high estimate of the space used by the environment (number of lines in env output should be at least as big as the number of envp[] pointers we passed to env, and we count 8 bytes for each of those, plus their size (including NULs which env replaced with NL)), substract that from ARG_MAX and divide by 9 to cover for the worst case scenario of a list of empty args and add 4KiB of slack.

Note that if you limit the stack size to 4MiB or below (with limit stacksize 4M in zsh for instance), that becomes more conservative than GNU xargs's default buffer size (which remains 128K in my case and fails to pass a list of empty vars properly).

$ limit stacksize 4M
$ (env | wc; getconf ARG_MAX) | awk '
  {env = $1 * 8 + $3; getline; printf "%d\n", ($0 - env) / 9 - 4096}'
111991
$ xargs --show-limits < /dev/null |& grep actually
Maximum length of command we could actually use: 1039698
Size of command buffer we are actually using: 131072
$ yes '""' | xargs  | head -1 | wc -c
65193
$ yes '""' | xargs -s 111991 | head -1 | wc -c
111986