diff --git a/lib/index.js b/lib/index.js
index 3453276..8cabbe0 100644
--- a/lib/index.js
+++ b/lib/index.js
@@ -1,7 +1,7 @@
 const { SUPPORTED_SHELLS, SHELL_LOCATIONS } = require('./constants');
 const prompt = require('./prompt');
 const installer = require('./installer');
-const { tabtabDebug, systemShell } = require('./utils');
+const { tabtabDebug } = require('./utils');
 
 /**
  * @typedef {import('./constants').SupportedShell} SupportedShell
@@ -18,6 +18,33 @@ const debug = tabtabDebug('tabtab');
  */
 const isShellSupported = shell => (/** @type {ReadonlyArray.<String>} */ (SUPPORTED_SHELLS)).includes(shell);
 
+/**
+ * This function is to be used inside a completer.
+ *
+ * An environment variable named `SHELL` shall be explicitly set
+ * by the completion script when it invokes the completer.
+ *
+ * The value of `SHELL` is expected to be one of the supported shells.
+ * If this expectation isn't met, it will result in an error.
+ *
+ * @example
+ * const shell = getShellFromEnv(process.env)
+ *
+ * @param {Readonly.<Record.<String, String | undefined>>} env - Env objects that may contain `SHELL`, usually `process.env`.
+ * @returns {SupportedShell}
+ */
+const getShellFromEnv = env => {
+  const shell = env.SHELL;
+  if (!shell) {
+    throw new TypeError('SHELL cannot be empty');
+  }
+  if (!isShellSupported(shell)) {
+    const supportedValues = SUPPORTED_SHELLS.map(x => `'${x}'`).join(', ');
+    throw new TypeError(`SHELL was set to an invalid value (${shell}). Supported values are: ${supportedValues}`);
+  }
+  return shell;
+}
+
 /**
  * Construct a completion script.
  * @param {Object} options - Options object.
@@ -253,7 +280,7 @@ const logFiles = () => {
 
 module.exports = {
   SUPPORTED_SHELLS,
-  shell: systemShell,
+  getShellFromEnv,
   isShellSupported,
   getCompletionScript,
   install,
diff --git a/lib/installer.js b/lib/installer.js
index 3a238af..1c9ae5e 100644
--- a/lib/installer.js
+++ b/lib/installer.js
@@ -2,7 +2,7 @@ const fs = require('fs');
 const path = require('path');
 const untildify = require('untildify');
 const { promisify } = require('util');
-const { tabtabDebug, systemShell, exists } = require('./utils');
+const { tabtabDebug, exists } = require('./utils');
 const { SUPPORTED_SHELLS } = require('./constants')
 
 const debug = tabtabDebug('tabtab:installer');
diff --git a/lib/templates/completion.bash b/lib/templates/completion.bash
index 090a953..8cf6fb9 100644
--- a/lib/templates/completion.bash
+++ b/lib/templates/completion.bash
@@ -13,6 +13,7 @@ if type complete &>/dev/null; then
     IFS=$'\n' COMPREPLY=($(COMP_CWORD="$cword" \
                            COMP_LINE="$COMP_LINE" \
                            COMP_POINT="$COMP_POINT" \
+                           SHELL=bash \
                            {completer} completion-server -- "${words[@]}" \
                            2>/dev/null)) || return $?
     IFS="$si"
diff --git a/lib/templates/completion.fish b/lib/templates/completion.fish
index 4fa81e0..71ca357 100644
--- a/lib/templates/completion.fish
+++ b/lib/templates/completion.fish
@@ -4,7 +4,7 @@ function _{pkgname}_completion
   set cursor (commandline -C)
   set words (count $cmd)
 
-  set completions (eval env DEBUG=\"" \"" COMP_CWORD=\""$words\"" COMP_LINE=\""$cmd \"" COMP_POINT=\""$cursor\"" {completer} completion-server -- $cmd)
+  set completions (eval env DEBUG=\"" \"" COMP_CWORD=\""$words\"" COMP_LINE=\""$cmd \"" COMP_POINT=\""$cursor\"" SHELL=fish {completer} completion-server -- $cmd)
 
   if [ "$completions" = "__tabtab_complete_files__" ]
     set -l matches (commandline -ct)*
diff --git a/lib/templates/completion.zsh b/lib/templates/completion.zsh
index a5744b4..5ca0f89 100644
--- a/lib/templates/completion.zsh
+++ b/lib/templates/completion.zsh
@@ -4,7 +4,7 @@ if type compdef &>/dev/null; then
     local reply
     local si=$IFS
 
-    IFS=$'\n' reply=($(COMP_CWORD="$((CURRENT-1))" COMP_LINE="$BUFFER" COMP_POINT="$CURSOR" {completer} completion-server -- "${words[@]}"))
+    IFS=$'\n' reply=($(COMP_CWORD="$((CURRENT-1))" COMP_LINE="$BUFFER" COMP_POINT="$CURSOR" SHELL=zsh {completer} completion-server -- "${words[@]}"))
     IFS=$si
 
     if [ "$reply" = "__tabtab_complete_files__" ]; then
diff --git a/lib/utils/index.js b/lib/utils/index.js
index 244849d..4d3da4c 100644
--- a/lib/utils/index.js
+++ b/lib/utils/index.js
@@ -1,9 +1,7 @@
 const tabtabDebug = require('./tabtabDebug');
-const systemShell = require('./systemShell');
 const exists = require('./exists');
 
 module.exports = {
-  systemShell,
   tabtabDebug,
   exists
 };
diff --git a/lib/utils/systemShell.js b/lib/utils/systemShell.js
deleted file mode 100644
index 3978a91..0000000
--- a/lib/utils/systemShell.js
+++ /dev/null
@@ -1,11 +0,0 @@
-/**
- * Utility to figure out the shell used on the system.
- *
- * Sadly, we can't use `echo $0` in node, maybe with more work. So we rely on
- * process.env.SHELL.
- *
- * TODO: More work on this, namely to detect Git bash on Windows (bash.exe)
- */
-const systemShell = () => (process.env.SHELL || '').split('/').slice(-1)[0];
-
-module.exports = systemShell;
diff --git a/test/basic.js b/test/basic.js
deleted file mode 100644
index f3f373c..0000000
--- a/test/basic.js
+++ /dev/null
@@ -1,21 +0,0 @@
-const assert = require('assert');
-const tabtab = require('..');
-
-describe('tabtab', () => {
-  it('tabtab.shell()', () => {
-    const previousShell = process.env.SHELL;
-    process.env.SHELL = '/bin/bash';
-    let shell = tabtab.shell();
-    assert.equal(shell, 'bash');
-
-    process.env.SHELL = '/usr/bin/zsh';
-    shell = tabtab.shell();
-    assert.equal(shell, 'zsh');
-
-    process.env.SHELL = '/usr/bin/fish';
-    shell = tabtab.shell();
-    assert.equal(shell, 'fish');
-
-    process.env.SHELL = previousShell;
-  });
-});
diff --git a/test/getShellFromEnv.js b/test/getShellFromEnv.js
new file mode 100644
index 0000000..876c2b3
--- /dev/null
+++ b/test/getShellFromEnv.js
@@ -0,0 +1,29 @@
+const assert = require('assert');
+const { SUPPORTED_SHELLS, getShellFromEnv } = require('..');
+
+describe('getShellFromEnv', () => {
+  it('errors when env lacks SHELL', () => {
+    assert.throws(
+      () => getShellFromEnv({}),
+      {
+        message: 'SHELL cannot be empty',
+      },
+    )
+  });
+
+  it('errors on unsupported shells', () => {
+    assert.throws(
+      () => getShellFromEnv({ SHELL: 'unknown' }),
+      {
+        message: "SHELL was set to an invalid value (unknown). Supported values are: 'bash', 'fish', 'pwsh', 'zsh'",
+      },
+    );
+  })
+
+  it('returns supported shells', () => {
+    assert.deepStrictEqual(
+      SUPPORTED_SHELLS.map(SHELL => getShellFromEnv({ SHELL })),
+      SUPPORTED_SHELLS,
+    );
+  });
+});