# Storing command output in a variable?
**URL:** https://discourse.nixos.org/t/storing-command-output-in-a-variable/80948
**Category:** Help
**Created:** [November 4, 2026, 7:24pm UTC](https://discourse.nixos.org/t/storing-command-output-in-a-variable/80948 "2026-11-04T19:24:31Z")
**Posts on this page:** 2
**Page:** 1
### Author:  [@rbd03](https://discourse.nixos.org/u/rbd03)
#### Post date: [November 4, 2026, 7:24pm UTC](https://discourse.nixos.org/t/storing-command-output-in-a-variable/80948/1 "2026-11-04T19:24:31Z")
Hi, I am trying to wrap an AppImage, without success. The `bambu.nix` file I have now is:
```nix
{ pkgs, lib, fetchurl, appimageTools, runCommandWith ? import {} }:
let
pname = "bambu";
version = "2.09.01.70";
fullname = lib.readFile "${runCommandWith { name="flnm"; derivationArgs.nativebuildInputs = [pkgs.curl pkgs.gnugrep]; } "${pkgs.curl}/bin/curl -Ls https://github.com/bambulab/BambuStudio/releases/expanded_assets/v${version} | ${pkgs.gnugrep}/bin/grep -Eom1 'v${version}-[0-9]+' > $out"}";
src = fetchurl {
url = "https://github.com/bambulab/BambuStudio/releases/download/v${version}/BambuStudio_ubu24-${fullname}.AppImage";
sha256 = "";
};
in
appimageTools.wrapType2 rec {
inherit pname version src;
extraPkgs = pkgs: [pkgs.webkitgtk_4_1];
}
```
and I call `bambu` in `configuration.nix` with `(callPackage ./bambu.nix {})`.
I suspect the main problem is storing the output of the command in `fullname` to be used in `src`, since `nixos-rebuild` does not show the file being downloaded at all (it must have failed before that). I have also had a whole host of problems relating to `readFile` and `runCommand` and `runCommandWith`. The command in fullname was modified from [How to create a timestamp in a Nix expression? - #2 by AtnNn](https://discourse.nixos.org/t/how-to-create-a-timestamp-in-a-nix-expression/30329/2) .
The current error is:
```bash
$ sudo nixos-rebuild switch
building the system configuration...
building '/nix/store/a3b1h3daf94rrw8qgv70hglsy638h74f-flnm.drv'...
error:
… while calling the 'derivationStrict' builtin
at «nix-internal»/derivation-internal.nix:37:12:
36|
37| strict = derivationStrict drvAttrs;
| ^
38|
… while evaluating derivation 'nixos-system-pyramid-26.11pre1105037.1c89083e451e'
whose name attribute is located at /nix/store/rk6gj8gw2ibi5by79wlciv0wklkkfw5n-nixos/nixos/pkgs/stdenv/generic/make-derivation.nix:651:11
… while evaluating attribute 'buildCommand' of derivation 'nixos-system-pyramid-26.11pre1105037.1c89083e451e'
at /nix/store/rk6gj8gw2ibi5by79wlciv0wklkkfw5n-nixos/nixos/nixos/modules/system/activation/top-level.nix:63:7:
62| allowSubstitutes = false;
63| buildCommand = systemBuilder;
| ^
64|
(stack trace truncated; use '--show-trace' to show the full, detailed trace)
error: Cannot build '/nix/store/a3b1h3daf94rrw8qgv70hglsy638h74f-flnm.drv'.
Reason: builder failed with exit code 1.
Output paths:
/nix/store/pzsq8rhkcfg4wd275dlrk4qdsl5mvx22-flnm
Command 'nix-build '' --attr config.system.build.toplevel --no-out-link' returned non-zero exit status 100.
```
I have no idea what to do from here, since `nix-store -l /nix/store/a3b1h3daf94rrw8qgv70hglsy638h74f-flnm.drv` does not output anything at all, while `ls -la /nix/store/pzsq8rhkcfg4wd275dlrk4qdsl5mvx22-flnm` outputs `-rw-r--r-- 1 nixbld1 nixbld 0 Nov 5 03:14 /nix/store/pzsq8rhkcfg4wd275dlrk4qdsl5mvx22-flnm` - an empty folder.
---
### Author:  [@tejing](https://discourse.nixos.org/u/tejing)
#### Post date: [November 4, 2026, 11:49pm UTC](https://discourse.nixos.org/t/storing-command-output-in-a-variable/80948/2 "2026-11-04T23:49:06Z")
You cannot access the internet inside a derivation. (Aside from a fixed-output derivation, but then you have to supply the hash of the results in advance.) So the way you’re getting `fullname` here is never going to work.
You’re also going the route of IFD (Import From Derivation) here, which has a number of problems and is best avoided if it’s feasible to do so.
I’d just hardcode the `fullname` value and live with the fact that you may need to change it when updating, honestly.
You could also put the relevant information in an json file that sits alongside your nix code, and have nix read the json for the data, so that you can cleanly update the information with an updater shell script that runs outside the nix framework.