[nix-shell:~/nixos/nixpkgs]$ echo $SHELL
/nix/store/m7v6p7yxqyhnc4sxp49zp857cjamp996-bash-interactive-4.4-p23/bin/bash

[nix-shell:~/nixos/nixpkgs]$ pstree $$
strace: Process 2637 attached
[pid  2637] execve("/home/dema/.nix-profile/bin/pstree", ["pstree", "2598"], 0xcaf008 /* 152 vars */) = 0
-+= 02598 dema bash --rcfile /tmp/nix-shell-2598-0/rc
 \--= 02637 dema pstree 2598
[pid  2637] +++ exited with 0 +++
--- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=2637, si_uid=1000, si_status=0, si_utime=0, si_stime=2} ---

[nix-shell:~/nixos/nixpkgs]$