How do I split a "/proc/*/environ" file in separate lines?

The entries are separated by the null character, see man 5 proc:

/proc/[pid]/environ
      This file contains the environment for the process.  The entries
      are separated by null bytes ('\0'), and there may be a null byte
      at  the  end.

So a simple way is to apply xargs -0 -L1 on it:

$ xargs -0 -L1 -a /proc/self/environ
LC_CTYPE=UTF-8
USER=muru
LOGNAME=muru
HOME=/home/muru
MAIL=/var/mail/muru
SHELL=/bin/zsh
...

• -0 - read null-delimited lines,
• -L1 - read one line per execution of command
• -a file read lines from file
• and if no command is specified, xargs simply prints the line.

Various GNU commands have options to work with null-delimited data: -z for sed, sort, uniq, grep etc, and for filenames, -print0 with find and -Z with grep.

Alternatively, you can use plain old bash:

while IFS= read -d '' -r line
do
    printf "%q\n" "$line"
done < /proc/.../environ

-d '' tells read to read until a null byte, IFS= and -r prevent field-splitting and backslash-escaping, so that the data is read as-is, %q will quote special characters in output.

Since you did use sed, you could have done:

sed -z 's/$/\n/' /proc/.../environ

which just tacks on a newline at the end of each null-delimited line.

• You could use cut:

  cut -d '' -f1- --output-delimiter=$'\n' /proc/$pid/environ
  

  using null as a delimiter, and outputting a newline delimiter, optionally picking only certain fields/lines.

• Or filter through tr, translating nulls to newlines:

  tr '\0' '\n' </proc/$pid/environ
  

• Or just stick with your sed version...

You can use strings as follows:

strings /proc/$(pgrep gnome-session -n -U $UID)/environ

Sample of the output:

$ strings /proc/$(pgrep gnome-session -n -U $UID)/environ
GNOME_KEYRING_PID=
LANGUAGE=en_US
J2SDKDIR=/usr/lib/jvm/java-8-oracle
LC_TIME=en_US.UTF-8
XDG_SEAT=seat0
XDG_SESSION_TYPE=x11
COMPIZ_CONFIG_PROFILE=ubuntu
SESSION=ubuntu

man strings

NAME

   strings - print the strings of printable characters in files.

DESCRIPTION

   For each file given, GNU strings prints the printable character
   sequences that are at least 4 characters long (or the number given with
   the options below) and are followed by an unprintable character.  By
   default, it only prints the strings from the initialized and loaded
   sections of object files; for other types of files, it prints the
   strings from the whole file.

   strings is mainly useful for determining the contents of non-text
   files.