fix: handle undefined env.SHELL (#657)

* fix: test _shell function

* fix: allow for undefined env.SHELL for nix

src/config/config.ts

@@ -18,6 +18,8 @@ import {CompletableOptionFlag, Arg} from '../interfaces/parser'
 import {stdout} from '../cli-ux/stream'
 import {Performance} from '../performance'
 import {settings} from '../settings'
+import {userInfo as osUserInfo} from 'node:os'
+import {sep} from 'node:path'
 
 // eslint-disable-next-line new-cap
 const debug = Debug()
@@ -550,7 +552,8 @@ export class Config implements IConfig {
 
   protected _shell(): string {
     let shellPath
-    const {SHELL, COMSPEC} = process.env
+    const COMSPEC = process.env.COMSPEC
+    const SHELL = process.env.SHELL ?? osUserInfo().shell?.split(sep)?.pop()
     if (SHELL) {
       shellPath = SHELL.split('/')
     } else if (this.windows && COMSPEC) {

test/config/config.shell.test.ts

@@ -0,0 +1,14 @@
+import {expect} from 'chai'
+import {Config} from '../../src'
+import {userInfo as osUserInfo} from 'node:os'
+import {sep} from 'node:path'
+
+const getShell = () => osUserInfo().shell?.split(sep)?.pop() || 'unknown'
+
+describe('config shell', () => {
+  it('has a default shell', () => {
+    const config = new Config({root: '/tmp'})
+    // @ts-ignore
+    expect(config._shell()).to.equal(getShell(), `SHELL: ${process.env.SHELL} COMSPEC: ${process.env.COMSPEC}`)
+  })
+})