Description

In pkgs.writeShellApplication, when the runtimeEnv attrset includes a string attribute that could be interpreted as a command, it fails to build because of SC2209.

{ pkgs ? import <nixpkgs> {} }: pkgs.writeShellApplication {
  name = "test";
  runtimeEnv = {
    TEST_VAR = "ls";
  };
  text = ''echo "$TEST_VAR"'';
}

SC2209 (warning): Use var=$(command) to assign output (or quote to assign string).

Steps To Reproduce

1. Copy the following into a terminal with Nix installed.

nix-build - << EOF
{ pkgs ? import <nixpkgs> {} }: pkgs.writeShellApplication {
  name = "test";
  runtimeEnv = {
    TEST_VAR = "ls";
  };
  text = ''echo "$TEST_VAR"'';
}
EOF

2. Press enter to run the command.
3. Observe SC2209.

Expected behavior

The above script should successfully build. When run, the script would print "ls" to the console.

Screenshots

N/A

Additional context

pkgs.writeShellApplication uses lib.toShellVar to generate the variable in the shell script.

• See pkgs.writeShellApplication at pkgs/build-support/trivial-builders/default.nix.
• See lib.toShellVar at lib/strings.nix.
• See SC2209 at shellcheck.net/wiki/SC2209.

For sure, the solution is to modify the generated shell script such that:

TEST_VAR=ls -> TEST_VAR="ls"

This change must occur within either pkgs.writeShellApplication or lib.toShellVar.

I believe it would be best for the solution to be within the lib.toShellVar definition.
Perhaps the solution is as easy as adding quotes around the variable.
I believe Nix requires the input of toShellVar to be a string anyways.
If not, additional edge cases must be considered for each datatype that could be passed.

Metadata

 - system: `"x86_64-linux"`
 - host os: `Linux 6.6.66, NixOS, 24.11 (Vicuna), 24.11.20241216.3945713`
 - multi-user?: `yes`
 - sandbox: `yes`
 - version: `nix-env (Nix) 2.24.10`
 - nixpkgs: `/nix/store/wj2qla569hnxwqfc26imv5hqbxc1rc27-source`

Notify maintainers

• lib.toShellVar @ncfavier @roberth
• pkgs.writeShellApplication @lovesegfault @9999years

Note for maintainers: Please tag this issue in your PR.

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