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

This is maybe not an elegant but working solution. It first extracts all possible names from env's output, then verifies each of them using shell's expansion ${parameter:+word}. And finally it removes duplicates, since the same variable name could be printed on several lines in env's output (as a real variable name and as a part of some other variable's multiline value):

env | awk -F= '/[a-zA-Z_][a-zA-Z_0-9]*=/ {
        if (!system("[ -n \"${" $1 "+y}\" ]")) print $1 }' | sort | uniq

PS: The | sort | uniq part can be also implemented in awk.

Answer 2

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

Answer 3

This will break if your environment variable values contain nulls. But that would also break from POSIX compatibility.

So it should work.

...unless you expect to encounter environment variable names which contain newlines. In that case the newlines will be truncated when they're displayed. However I can't seem to fathom how to create an environment variable with a newline in it in a busybox shell. My local shells balk at it at any rate. So I don't think that would be a big problem. As far as POSIX says, Other characters may be permitted by an implementation; applications shall tolerate the presence of such names. so I think stripping them and not erroring-out is tolerable.

# Read our environment; it's delimited by null bytes.
# Remove newlines
# Replace null bytes with newlines
# On each line, grab everything before the first '='
cat /proc/self/environ | tr -d '\n' | tr '\0' '\n' | cut -d '=' -f 1