List all environment variable names in busybox

Question

Environment variables with multiline values may confuse env's output:

# export A="B
> C=D
> E=F"
# env
A=B
C=D
E=F
TERM=xterm
SHELL=/bin/bash
USER=root
MAIL=/var/mail/root
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
PWD=/root
LANG=en_US.UTF-8
PS1=\h:\w\$
SHLVL=1
HOME=/root
LOGNAME=root
_=/usr/bin/env

In this case I can not just use awk -F= to extract all names because it shows wrong names C and E:

# env | awk -F= '{print $1}'
A
C
E
TERM
SHELL
USER
MAIL
PATH
PWD
LANG
PS1
SHLVL
HOME
LOGNAME
_

Then I figured out that env supports flag -0 to end each output line with 0 byte rather than newline, so using sed I could cut off the values in bash:

# env -0 | sed -e ':a;N;$!ba;s/\([^=]\+\)=\([^\x00]*\)\x00/\1\n/g'
A
TERM
SHELL
USER
MAIL
PATH
PWD
LANG
PS1
SHLVL
HOME
LOGNAME
_

But BusyBox's version of env does not support flag -0. Is there another way to do it?

Answer 1

If you are using linux (I thought busybox ran only on linux, but I may be wrong), /proc/self/environ contains a NUL separated environment in the same form as env -0 gives you. You can replace env -0 | with < /proc/self/environ.

sed -e ':a;N;$!ba;s/\([^=]\+\)=\([^\x00]*\)\x00/\1\n/g' < /proc/self/environ