pkgs.runCommand "tmux.conf" {} ''echo
  bind r
> $out''

/tmp/nix-build-tmux.conf.drv-0/.attr-0: line 2: bind: command not found
builder for ‘/nix/store/mfyrydvq3d6dd11azk91m9zdmqifg8fd-tmux.conf.drv’ failed with exit code 127
error: while evaluating the attribute ‘pkgs’ of the derivation ‘baseEnv’ at /nix/store/l75yqrcfbq78xqqwc39ds6zzcar