diff --git a/activate.fish b/activate.fish
new file mode 120000
index 000000000..07bfcce50
--- /dev/null
+++ b/activate.fish
@@ -0,0 +1 @@
+bootstrap.fish
\ No newline at end of file
diff --git a/bootstrap.fish b/bootstrap.fish
new file mode 100644
index 000000000..97eb75796
--- /dev/null
+++ b/bootstrap.fish
@@ -0,0 +1,90 @@
+# Copyright 2024 The Pigweed Authors
+#
+# Licensed under the Apache License, Version 2.0 (the "License"); you may not
+# use this file except in compliance with the License. You may obtain a copy of
+# the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+# WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+# License for the specific language governing permissions and limitations under
+# the License.
+
+# This script must be tested on fish 3.6.0
+
+# Get the absolute path of the bootstrap script.
+set _PW_BOOTSTRAP_PATH (path resolve (status current-filename))
+
+# Check if this file is being executed or sourced.
+set _pw_sourced 0
+if string match --quiet '*from sourcing file*' (status)
+    set _pw_sourced 1
+end
+
+# Downstream projects need to set something other than PW_ROOT here, like
+# YOUR_PROJECT_ROOT. Please also set PW_PROJECT_ROOT and PW_ROOT before
+# invoking pw_bootstrap or pw_activate.
+######### BEGIN PROJECT-SPECIFIC CODE #########
+set --export PW_ROOT (path resolve (dirname $_PW_BOOTSTRAP_PATH))
+
+# Please also set PW_PROJECT_ROOT to YOUR_PROJECT_ROOT.
+set --export PW_PROJECT_ROOT $PW_ROOT
+
+# Downstream projects may wish to set PW_BANNER_FUNC to a function that prints
+# an ASCII art banner here. For example:
+#
+#   set --export PW_BANNER_FUNC banner_function_name
+#
+########## END PROJECT-SPECIFIC CODE ##########
+
+source $PW_ROOT/pw_env_setup/util.fish
+
+# Check environment properties
+pw_deactivate
+pw_eval_sourced $_pw_sourced $_PW_BOOTSTRAP_PATH
+if not pw_check_root $PW_ROOT
+    return
+end
+
+set --export _PW_ACTUAL_ENVIRONMENT_ROOT (pw_get_env_root)
+
+set SETUP_SH "$_PW_ACTUAL_ENVIRONMENT_ROOT/activate.fish"
+
+# Run full bootstrap when invoked as bootstrap, or env file is missing/empty.
+if test (status basename) = "bootstrap.fish"
+    or not test -e $SETUP_SH
+    or not test -s $SETUP_SH
+    ######### BEGIN PROJECT-SPECIFIC CODE #########
+    pw_bootstrap --shell-file "$SETUP_SH" --install-dir "$_PW_ACTUAL_ENVIRONMENT_ROOT"
+    ########## END PROJECT-SPECIFIC CODE ##########
+    set finalize_mode bootstrap
+else
+    pw_activate_message
+    set finalize_mode activate
+end
+# NOTE: Sourced scripts in fish cannot be sourced within a fuction if
+# variables should be exported to the calling shell. So activate.fish
+# must be sourced here instead of within pw_finalize or another
+# function.
+pw_finalize_pre_check $finalize_mode "$SETUP_SH"
+source $SETUP_SH
+pw_finalize_post_check $finalize_mode "$SETUP_SH"
+
+if set --query _PW_ENV_SETUP_STATUS; and test "$_PW_ENV_SETUP_STATUS" -eq 0
+    # This is where any additional checks about the environment should go.
+    ######### BEGIN PROJECT-SPECIFIC CODE #########
+    echo -n
+    ########## END PROJECT-SPECIFIC CODE ##########
+end
+
+set -e _pw_sourced
+set -e _PW_BOOTSTRAP_PATH
+set -e SETUP_SH
+
+# TODO(tonymd): Source fish pw_cli shell completion.
+
+pw_cleanup
+
+git -C "$PW_ROOT" config blame.ignoreRevsFile .git-blame-ignore-revs
diff --git a/docs/get_started/upstream.rst b/docs/get_started/upstream.rst
index 2a6327f9d..9437a1789 100644
--- a/docs/get_started/upstream.rst
+++ b/docs/get_started/upstream.rst
@@ -45,6 +45,11 @@ To get setup:
       $ bootstrap.bat         (On Windows)
       ...
 
+   .. tip::
+
+      If you use the `Fish shell <https://fishshell.com/>`_ run `source
+      ./bootstrap.fish` instead.
+
 #. Configure the GN build.
 
    .. code-block:: bash
diff --git a/pw_env_setup/docs.rst b/pw_env_setup/docs.rst
index 2aa540262..366a8a6c8 100644
--- a/pw_env_setup/docs.rst
+++ b/pw_env_setup/docs.rst
@@ -40,8 +40,13 @@ runs bootstrap.
 .. note::
 
    On Windows the scripts used to set up the environment are ``bootstrap.bat``
-   and ``activate.bat``. For simplicity they will be referred to with the
-   ``.sh`` endings unless the distinction is relevant.
+   and ``activate.bat``.
+
+   ``bootstrap.fish`` and ``activate.fish`` are also available for `Fish shell
+   <https://fishshell.com/>`_ users.
+
+   For simplicity they will be referred to with the ``.sh`` endings unless the
+   distinction is relevant.
 
 On POSIX systems, the environment can be deactivated by running ``deactivate``.
 
diff --git a/pw_env_setup/py/environment_test.py b/pw_env_setup/py/environment_test.py
index 54627a4cd..b51bc1eca 100644
--- a/pw_env_setup/py/environment_test.py
+++ b/pw_env_setup/py/environment_test.py
@@ -60,8 +60,8 @@ def _evaluate_env_in_shell(env):
         delete=False,
         mode='w+',
     ) as temp:
-        env.write(temp)
         temp_name = temp.name
+        env.write(temp, shell_file=temp_name)
 
     # Evaluate env sourcing script and capture output of 'env'.
     if os.name == 'nt':
@@ -164,7 +164,7 @@ class EnvironmentTest(unittest.TestCase):
         self.env.set(self.var_not_set, '/foo/bar/baz')
         self.env.add_replacement('FOOBAR', '/foo/bar')
         buf = six.StringIO()
-        self.env.write(buf)
+        self.env.write(buf, shell_file='test.sh')
         assert '/foo/bar' not in buf.getvalue()
 
     def test_variable_replacement(self):
@@ -172,7 +172,7 @@ class EnvironmentTest(unittest.TestCase):
         self.env.set(self.var_not_set, '/foo/bar/baz')
         self.env.add_replacement('FOOBAR')
         buf = six.StringIO()
-        self.env.write(buf)
+        self.env.write(buf, shell_file='test.sh')
         print(buf.getvalue())
         assert '/foo/bar/baz' not in buf.getvalue()
 
diff --git a/pw_env_setup/py/pw_env_setup/env_setup.py b/pw_env_setup/py/pw_env_setup/env_setup.py
index 3daff581f..7df2c1a8a 100755
--- a/pw_env_setup/py/pw_env_setup/env_setup.py
+++ b/pw_env_setup/py/pw_env_setup/env_setup.py
@@ -200,6 +200,7 @@ class EnvSetup(object):
         )
         self._cipd_cache_dir = cipd_cache_dir
         self._shell_file = shell_file
+        self._env._shell_file = shell_file
         self._is_windows = os.name == 'nt'
         self._quiet = quiet
         self._install_dir = install_dir
@@ -628,14 +629,14 @@ Then use `set +x` to go back to normal.
             return 0
 
         with open(self._shell_file, 'w') as outs:
-            self._env.write(outs)
+            self._env.write(outs, shell_file=self._shell_file)
 
         deactivate = os.path.join(
             self._install_dir,
             'deactivate{}'.format(os.path.splitext(self._shell_file)[1]),
         )
         with open(deactivate, 'w') as outs:
-            self._env.write_deactivate(outs)
+            self._env.write_deactivate(outs, shell_file=deactivate)
 
         config = {
             # Skipping sysname and nodename in os.uname(). nodename could change
diff --git a/pw_env_setup/py/pw_env_setup/environment.py b/pw_env_setup/py/pw_env_setup/environment.py
index a624d1550..4c379089f 100644
--- a/pw_env_setup/py/pw_env_setup/environment.py
+++ b/pw_env_setup/py/pw_env_setup/environment.py
@@ -333,6 +333,7 @@ class Environment(object):
         self.replacements = []
         self._join = Join(pathsep)
         self._finalized = False
+        self._shell_file = ''
 
     def add_replacement(self, variable, value=None):
         self.replacements.append((variable, value))
@@ -440,7 +441,7 @@ class Environment(object):
 
         if not self._windows:
             buf = StringIO()
-            self.write_deactivate(buf)
+            self.write_deactivate(buf, shell_file=self._shell_file)
             self._actions.append(Function('_pw_deactivate', buf.getvalue()))
             self._blankline()
 
@@ -457,17 +458,27 @@ class Environment(object):
     def json(self, outs):
         json_visitor.JSONVisitor().serialize(self, outs)
 
-    def write(self, outs):
+    def write(self, outs, shell_file):
         if self._windows:
             visitor = batch_visitor.BatchVisitor(pathsep=self._pathsep)
         else:
-            visitor = shell_visitor.ShellVisitor(pathsep=self._pathsep)
+            if shell_file.endswith('.fish'):
+                visitor = shell_visitor.FishShellVisitor()
+            else:
+                visitor = shell_visitor.ShellVisitor(pathsep=self._pathsep)
         visitor.serialize(self, outs)
 
-    def write_deactivate(self, outs):
+    def write_deactivate(self, outs, shell_file):
         if self._windows:
             return
-        visitor = shell_visitor.DeactivateShellVisitor(pathsep=self._pathsep)
+        if shell_file.endswith('.fish'):
+            visitor = shell_visitor.DeactivateFishShellVisitor(
+                pathsep=self._pathsep
+            )
+        else:
+            visitor = shell_visitor.DeactivateShellVisitor(
+                pathsep=self._pathsep
+            )
         visitor.serialize(self, outs)
 
     @contextlib.contextmanager
diff --git a/pw_env_setup/py/pw_env_setup/shell_visitor.py b/pw_env_setup/py/pw_env_setup/shell_visitor.py
index c76653c6b..341f81cc9 100644
--- a/pw_env_setup/py/pw_env_setup/shell_visitor.py
+++ b/pw_env_setup/py/pw_env_setup/shell_visitor.py
@@ -54,7 +54,7 @@ class _BaseShellVisitor(object):  # pylint: disable=useless-object-inheritance
 
 
 class ShellVisitor(_BaseShellVisitor):
-    """Serializes an Environment into a shell file."""
+    """Serializes an Environment into a bash-like shell file."""
 
     def __init__(self, *args, **kwargs):
         super(ShellVisitor, self).__init__(*args, **kwargs)
@@ -174,7 +174,7 @@ class ShellVisitor(_BaseShellVisitor):
 
 
 class DeactivateShellVisitor(_BaseShellVisitor):
-    """Removes values from an Environment."""
+    """Removes values from a bash-like shell environment."""
 
     def __init__(self, *args, **kwargs):
         pathsep = kwargs.pop('pathsep', ':')
@@ -227,3 +227,132 @@ class DeactivateShellVisitor(_BaseShellVisitor):
 
     def visit_function(self, function):
         pass  # Not relevant.
+
+
+class FishShellVisitor(ShellVisitor):
+    """Serializes an Environment into a fish shell file."""
+
+    def __init__(self, *args, **kwargs):
+        super(FishShellVisitor, self).__init__(*args, **kwargs)
+        self._pathsep = ' '
+
+    def _remove_value_from_path(self, variable, value):
+        return 'set PATH (string match -v {value} ${variable})\n'.format(
+            variable=variable, value=value
+        )
+
+    def visit_set(self, set):  # pylint: disable=redefined-builtin
+        value = self._apply_replacements(set)
+        self._outs.write(
+            'set -x {name} {value}\n'.format(name=set.name, value=value)
+        )
+
+    def visit_clear(self, clear):
+        self._outs.write('set -e {name}\n'.format(**vars(clear)))
+
+    def visit_remove(self, remove):
+        value = self._apply_replacements(remove)
+        self._remove_value_from_path(remove.name, value)
+
+    def visit_prepend(self, prepend):
+        value = self._apply_replacements(prepend)
+        self._outs.write(
+            'set -x --prepend {name} {value}\n'.format(
+                name=prepend.name, value=value
+            )
+        )
+
+    def visit_append(self, append):
+        value = self._apply_replacements(append)
+        self._outs.write(
+            'set -x --append {name} {value}\n'.format(
+                name=append.name, value=value
+            )
+        )
+
+    def visit_echo(self, echo):
+        self._outs.write('if not set -q PW_ENVSETUP_QUIET\n')
+        if echo.newline:
+            self._outs.write('  echo "{}"\n'.format(echo.value))
+        else:
+            self._outs.write('  echo -n "{}"\n'.format(echo.value))
+        self._outs.write('end\n')
+
+    def visit_hash(self, hash):  # pylint: disable=redefined-builtin
+        del hash
+
+    def visit_function(self, function):
+        self._outs.write(
+            'function {name}\n{body}\nend\n'.format(
+                name=function.name, body=function.body
+            )
+        )
+
+    def visit_command(self, command):
+        self._outs.write('{}\n'.format(' '.join(command.command)))
+        if not command.exit_on_error:
+            return
+
+        # Assume failing command produced relevant output.
+        self._outs.write('if test $status -ne 0\n  return 1\nend\n')
+
+    def visit_doctor(self, doctor):
+        self._outs.write('if not set -q PW_ACTIVATE_SKIP_CHECKS\n')
+        self.visit_command(doctor)
+        self._outs.write('else\n')
+        self._outs.write(
+            'echo Skipping environment check because '
+            'PW_ACTIVATE_SKIP_CHECKS is set\n'
+        )
+        self._outs.write('end\n')
+
+
+class DeactivateFishShellVisitor(FishShellVisitor):
+    """Removes values from a fish shell environment."""
+
+    def serialize(self, env, outs):
+        try:
+            self._outs = outs
+
+            env.accept(self)
+
+        finally:
+            self._outs = None
+
+    def visit_set(self, set):  # pylint: disable=redefined-builtin
+        if set.deactivate:
+            self._outs.write('set -e {name}\n'.format(name=set.name))
+
+    def visit_clear(self, clear):
+        pass  # Not relevant.
+
+    def visit_remove(self, remove):
+        pass  # Not relevant.
+
+    def visit_prepend(self, prepend):
+        self._outs.write(
+            self._remove_value_from_path(prepend.name, prepend.value)
+        )
+
+    def visit_append(self, append):
+        self._outs.write(
+            self._remove_value_from_path(append.name, append.value)
+        )
+
+    def visit_echo(self, echo):
+        pass  # Not relevant.
+
+    def visit_comment(self, comment):
+        pass  # Not relevant.
+
+    def visit_command(self, command):
+        pass  # Not relevant.
+
+    def visit_doctor(self, doctor):
+        pass  # Not relevant.
+
+    def visit_blank_line(self, blank_line):
+        pass  # Not relevant.
+
+    def visit_function(self, function):
+        pass  # Not relevant.
diff --git a/pw_env_setup/util.fish b/pw_env_setup/util.fish
new file mode 100644
index 000000000..20ee6da42
--- /dev/null
+++ b/pw_env_setup/util.fish
@@ -0,0 +1,410 @@
+# Copyright 2024 The Pigweed Authors
+#
+# Licensed under the Apache License, Version 2.0 (the "License"); you may not
+# use this file except in compliance with the License. You may obtain a copy of
+# the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+# WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+# License for the specific language governing permissions and limitations under
+# the License.
+
+# This script must be tested on fish 3.6.0
+
+# Note: Colors are unfortunately duplicated in several places; and removing the
+# duplication is not easy. Their locations are:
+#
+#   - bootstrap.sh
+#   - pw_cli/color.py
+#   - pw_env_setup/py/pw_env_setup/colors.py
+#   - pw_env_setup/util.fish
+#   - pw_env_setup/util.sh
+#
+# So please keep them matching then modifying them.
+function pw_none
+    set_color normal
+    echo -e $argv
+end
+
+function pw_red
+    set_color red
+    echo -e $argv
+    set_color normal
+end
+
+function pw_bold_red
+    set_color --bold red
+    echo -e $argv
+    set_color normal
+end
+
+function pw_yellow
+    set_color yellow
+    echo -e $argv
+    set_color normal
+end
+
+function pw_bold_yellow
+    set_color --bold yellow
+    echo -e $argv
+    set_color normal
+end
+
+function pw_green
+    set_color green
+    echo -e $argv
+    set_color normal
+end
+
+function pw_bold_green
+    set_color --bold green
+    echo -e $argv
+    set_color normal
+end
+
+function pw_blue
+    set_color blue
+    echo -e $argv
+    set_color normal
+end
+
+function pw_cyan
+    set_color cyan
+    echo -e $argv
+    set_color normal
+end
+
+function pw_magenta
+    set_color magenta
+    echo -e $argv
+    set_color normal
+end
+
+function pw_bold_white
+    set_color --bold white
+    echo -e $argv
+    set_color normal
+end
+
+function pw_error
+    set_color --bold brred
+    echo -e $argv 1>&2
+    set_color normal
+end
+
+function pw_error_info
+    set_color red
+    echo -e $argv 1>&2
+    set_color normal
+end
+
+# Print an error and exit if $is_sourced == 0 (the first argument).
+function pw_eval_sourced --argument-names is_sourced bootstrap_path
+    if test $is_sourced -eq 0
+        set _PW_NAME (basename $bootstrap_path)
+        pw_error "Error: Attempting to $_PW_NAME in a subshell"
+        pw_error_info "  Since $_PW_NAME modifies your shell's environment"
+        pw_error_info "  variables, it must be sourced rather than executed. In"
+        pw_error_info "  particular, 'fish $_PW_NAME' will not work since the "
+        pw_error_info "  modified environment will get destroyed at the end of the"
+        pw_error_info "  script. Instead, source the script's contents in your"
+        pw_error_info "  shell:"
+        pw_error_info ""
+        pw_error_info "    \$ source $_PW_NAME"
+        exit 1
+    end
+end
+
+# Check for spaces in $_PW_ROOT (the first argument).
+function pw_check_root --argument-names _PW_ROOT
+    if string match --quiet '* *' $_PW_ROOT
+        pw_error "Error: The Pigweed path contains spaces"
+        pw_error_info "  The path '$_PW_ROOT' contains spaces. "
+        pw_error_info "  Pigweed's Python environment currently requires Pigweed to"
+        pw_error_info "  be at a path without spaces. Please checkout Pigweed in a"
+        pw_error_info "  directory without spaces and retry running bootstrap."
+        return -1
+    end
+end
+
+# Check for and return the environment directory location.
+function pw_get_env_root
+    # PW_ENVIRONMENT_ROOT allows callers to specify where the environment should
+    # be installed. bootstrap.sh scripts should not use that variable to store the
+    # result of this function. This separation allows scripts to assume
+    # PW_ENVIRONMENT_ROOT came from the caller and not from a previous bootstrap
+    # possibly from another workspace. PW_ENVIRONMENT_ROOT will be cleared after
+    # environment setup completes.
+    if set --query PW_ENVIRONMENT_ROOT
+        echo $PW_ENVIRONMENT_ROOT
+        return
+    end
+
+    # Determine project-level root directory.
+    if set --query PW_PROJECT_ROOT
+        set _PW_ENV_PREFIX $PW_PROJECT_ROOT
+    else
+        set _PW_ENV_PREFIX $PW_ROOT
+    end
+
+    # If <root>/environment exists, use it. Otherwise, if <root>/.environment
+    # exists, use it. Finally, use <root>/environment.
+    set _PW_DOTENV $_PW_ENV_PREFIX/.environment
+    set _PW_ENV $_PW_ENV_PREFIX/environment
+
+    if test -d $_PW_DOTENV
+        if test -d $_PW_ENV
+            pw_error "Error: both possible environment directories exist."
+            pw_error_info "  $_PW_DOTENV"
+            pw_error_info "  $_PW_ENV"
+            pw_error_info "  If only one of these folders exists it will be used for"
+            pw_error_info "  the Pigweed environment. If neither exists"
+            pw_error_info "  '<...>/environment' will be used. Since both exist,"
+            pw_error_info "  bootstrap doesn't know which to use. Please delete one"
+            pw_error_info "  or both and rerun bootstrap."
+            exit 1
+        end
+    end
+
+    if test -d $_PW_ENV
+        echo $_PW_ENV
+    else if test -d $_PW_DOTENV
+        echo $_PW_DOTENV
+    else
+        echo $_PW_ENV
+    end
+end
+
+
+set --export _PW_BANNER "\
+ ▒█████▄   █▓  ▄███▒  ▒█    ▒█ ░▓████▒ ░▓████▒ ▒▓████▄
+  ▒█░  █░ ░█▒ ██▒ ▀█▒ ▒█░ █ ▒█  ▒█   ▀  ▒█   ▀  ▒█  ▀█▌
+  ▒█▄▄▄█░ ░█▒ █▓░ ▄▄░ ▒█░ █ ▒█  ▒███    ▒███    ░█   █▌
+  ▒█▀     ░█░ ▓█   █▓ ░█░ █ ▒█  ▒█   ▄  ▒█   ▄  ░█  ▄█▌
+  ▒█      ░█░ ░▓███▀   ▒█▓▀▓█░ ░▓████▒ ░▓████▒ ▒▓████▀
+"
+
+function _default_pw_banner_func
+    pw_magenta $_PW_BANNER
+end
+
+set --export _PW_BANNER_FUNC _default_pw_banner_func
+
+function _pw_hello --argument-names _PW_TEXT
+    # If $PW_BANNER_FUNC is defined, use that as the banner function
+    # instead of the default.
+    if set --query PW_BANNER_FUNC
+        set _PW_BANNER_FUNC $PW_BANNER_FUNC
+    end
+    # Display the banner unless PW_ENVSETUP_QUIET or
+    # PW_ENVSETUP_NO_BANNER is set.
+    if test -z "$PW_ENVSETUP_QUIET" && test -z "$PW_ENVSETUP_NO_BANNER"
+        pw_green "\n  WELCOME TO...\n"
+        set_color normal
+        $_PW_BANNER_FUNC
+        set_color normal
+        pw_green $_PW_TEXT
+    end
+end
+
+function pw_deactivate
+    # Assume PW_ROOT and PW_PROJECT_ROOT have already been set and we need to
+    # preserve their values.
+    set _NEW_PW_ROOT $PW_ROOT
+    set _NEW_PW_PROJECT_ROOT $PW_PROJECT_ROOT
+
+    # Find deactivate script, run it, and then delete it. This way if the
+    # deactivate script is doing something wrong subsequent bootstraps still
+    # have a chance to pass.
+    set _PW_DEACTIVATE_SH $_PW_ACTUAL_ENVIRONMENT_ROOT/deactivate.fish
+    if test -f $_PW_DEACTIVATE_SH
+        . $_PW_DEACTIVATE_SH
+        rm -f $_PW_DEACTIVATE_SH &>/dev/null
+    end
+
+    # If there's a _pw_deactivate function run it. Redirect output to /dev/null
+    # in case _pw_deactivate doesn't exist. Remove _pw_deactivate when complete.
+    if functions --query _pw_deactivate
+        _pw_deactivate >/dev/null 2>/dev/null
+        functions -e _pw_deactivate
+    end
+
+    # Restore.
+    set --export PW_ROOT $_NEW_PW_ROOT
+    set --export PW_PROJECT_ROOT $_NEW_PW_PROJECT_ROOT
+end
+
+function deactivate
+    pw_deactivate
+    functions -e pw_deactivate
+    functions -e deactivate
+    set -e PW_ROOT
+    set -e PW_PROJECT_ROOT
+    set -e PW_BRANDING_BANNER
+    set -e PW_BRANDING_BANNER_COLOR
+end
+
+# The next three functions use the following variables.
+# * PW_BANNER_FUNC: function to print banner
+# * PW_BOOTSTRAP_PYTHON: specific Python interpreter to use for bootstrap
+# * PW_ROOT: path to Pigweed root
+# * PW_ENVSETUP_QUIET: limit output if "true"
+#
+# All arguments passed in are passed on to env_setup.py in pw_bootstrap,
+# pw_activate takes no arguments, and pw_finalize takes the name of the script
+# "bootstrap" or "activate" and the path to the setup script written by
+# bootstrap.fish.
+function pw_bootstrap
+    _pw_hello "  BOOTSTRAP! Bootstrap may take a few minutes; please be patient.\n"
+
+    if functions --query python
+        pw_error "Error: 'python' is an alias"
+        pw_error_info "The shell has a 'python' alias set. This causes many obscure"
+        pw_error_info "Python-related issues both in and out of Pigweed. Please"
+        pw_error_info "remove the Python alias from your shell init file or at"
+        pw_error_info "least run the following command before bootstrapping"
+        pw_error_info "Pigweed."
+        pw_error_info
+        pw_error_info "  functions --erase python"
+        pw_error_info
+        return
+    end
+
+    # Allow forcing a specific version of Python for testing pursposes.
+    if test -n "$PW_BOOTSTRAP_PYTHON"
+        set _PW_PYTHON "$PW_BOOTSTRAP_PYTHON"
+    else if command -v python3 >/dev/null 2>/dev/null
+        set _PW_PYTHON python3
+    else if command -v python2 >/dev/null 2>/dev/null
+        set _PW_PYTHON python2
+    else if command -v python >/dev/null 2>/dev/null
+        set _PW_PYTHON python
+    else
+        pw_error "Error: No system Python present\n"
+        pw_error_info "  Pigweed's bootstrap process requires a local system"
+        pw_error_info "  Python. Please install Python on your system, add it to "
+        pw_error_info "  your PATH and re-try running bootstrap."
+        return
+    end
+
+    if test -n "$_PW_ENV_SETUP"
+        $_PW_ENV_SETUP $argv
+        set _PW_ENV_SETUP_STATUS $status
+    else
+        $_PW_PYTHON $PW_ROOT/pw_env_setup/py/pw_env_setup/env_setup.py $argv
+        set _PW_ENV_SETUP_STATUS $status
+    end
+
+    # Write the directory path at bootstrap time into the directory. This helps
+    # us double-check things are still in the same space when calling activate.
+    set _PW_ENV_ROOT_TXT $_PW_ACTUAL_ENVIRONMENT_ROOT/env_root.txt
+    echo $_PW_ACTUAL_ENVIRONMENT_ROOT >$_PW_ENV_ROOT_TXT 2>/dev/null
+
+    # Create the environment README file. Use quotes to prevent alias expansion.
+    cp $PW_ROOT/pw_env_setup/destination.md $_PW_ACTUAL_ENVIRONMENT_ROOT/README.md &>/dev/null
+end
+
+function pw_activate_message
+    _pw_hello "  ACTIVATOR! This sets your shell environment variables.\n"
+    set _PW_ENV_SETUP_STATUS 0
+end
+
+function pw_finalize_pre_check --argument-names _PW_NAME _PW_SETUP_SH
+    # Check that the environment directory agrees that the path it's at matches
+    # where it thinks it should be. If not, bail.
+    set _PW_ENV_ROOT_TXT "$_PW_ACTUAL_ENVIRONMENT_ROOT/env_root.txt"
+    if test -f $_PW_ENV_ROOT_TXT
+        set _PW_PREV_ENV_ROOT (cat $_PW_ENV_ROOT_TXT)
+        if test "$_PW_ACTUAL_ENVIRONMENT_ROOT" != "$_PW_PREV_ENV_ROOT"
+            pw_error "Error: Environment directory moved"
+            pw_error_info "This Pigweed environment was created at"
+            pw_error_info
+            pw_error_info "    $_PW_PREV_ENV_ROOT"
+            pw_error_info
+            pw_error_info "But it is now being activated from"
+            pw_error_info
+            pw_error_info "    $_PW_ACTUAL_ENVIRONMENT_ROOT"
+            pw_error_info
+            pw_error_info "This is likely because the checkout moved. After moving "
+            pw_error_info "the checkout a full '. ./bootstrap.fish' is required."
+            pw_error_info
+            set _PW_ENV_SETUP_STATUS 1
+        end
+    end
+
+    if test -n "$_PW_ENV_SETUP_STATUS" && test "$_PW_ENV_SETUP_STATUS" -ne 0
+        return
+    end
+
+    if not test -f "$_PW_SETUP_SH"
+        pw_error "Error during $_PW_NAME--see messages above."
+    end
+end
+
+function pw_finalize_post_check --argument-names _PW_NAME _PW_SETUP_SH
+    if test "$status" -eq 0
+        if test "$_PW_NAME" = bootstrap && test -z "$PW_ENVSETUP_QUIET"
+            echo "To reactivate this environment in the future, run this in your "
+            echo "terminal:"
+            echo
+            pw_green "  source ./activate.fish"
+            echo
+            echo "To deactivate this environment, run this:"
+            echo
+            pw_green "  deactivate"
+            echo
+        end
+    else
+        pw_error "Error during $_PW_NAME--see messages above."
+    end
+end
+
+function pw_install_post_checkout_hook
+    cp $PW_ROOT/pw_env_setup/post-checkout-hook.sh $PW_PROJECT_ROOT/.git/hooks/post-checkout
+end
+
+function pw_cleanup
+    set -e _PW_BANNER
+    set -e _PW_BANNER_FUNC
+    set -e PW_BANNER_FUNC
+    set -e _PW_ENV_SETUP
+    set -e _PW_NAME
+    set -e PW_ENVIRONMENT_ROOT
+    set -e _PW_PYTHON
+    set -e _PW_ENV_ROOT_TXT
+    set -e _PW_PREV_ENV_ROOT
+    set -e _PW_SETUP_SH
+    set -e _PW_DEACTIVATE_SH
+    set -e _NEW_PW_ROOT
+    set -e _PW_ENV_SETUP_STATUS
+    set -e _PW_ENV_PREFIX
+    set -e _PW_ENV
+    set -e _PW_DOTENV
+
+    functions -e pw_none
+    functions -e pw_red
+    functions -e pw_bold_red
+    functions -e pw_yellow
+    functions -e pw_bold_yellow
+    functions -e pw_green
+    functions -e pw_bold_green
+    functions -e pw_blue
+    functions -e pw_cyan
+    functions -e pw_magenta
+    functions -e pw_bold_white
+    functions -e pw_eval_sourced
+    functions -e pw_check_root
+    functions -e pw_get_env_root
+    functions -e _pw_banner
+    functions -e pw_bootstrap
+    functions -e pw_activate
+    functions -e pw_finalize
+    functions -e pw_install_post_checkout_hook
+    functions -e pw_cleanup
+    functions -e _pw_hello
+    functions -e pw_error
+    functions -e pw_error_info
+end
diff --git a/pw_env_setup/util.sh b/pw_env_setup/util.sh
index c8e1bf2f8..0fb667566 100644
--- a/pw_env_setup/util.sh
+++ b/pw_env_setup/util.sh
@@ -21,6 +21,8 @@ export PATH
 #   - bootstrap.sh
 #   - pw_cli/color.py
 #   - pw_env_setup/py/pw_env_setup/colors.py
+#   - pw_env_setup/util.fish
+#   - pw_env_setup/util.sh
 #
 # So please keep them matching then modifying them.
 pw_none() {
@@ -160,6 +162,7 @@ pw_get_env_root() {
 # the contortions that would be needed to share this snippet across shell,
 # batch, and Python. Locations:
 #
+#   - pw_env_setup/util.fish
 #   - pw_env_setup/util.sh
 #   - pw_cli/branding.py
 #   - pw_env_setup/py/pw_env_setup/windows_env_start.py
@@ -181,9 +184,13 @@ _PW_BANNER_FUNC="_pw_banner"
 
 _pw_hello() {
   _PW_TEXT="$1"
+  # If $PW_BANNER_FUNC is defined, use that as the banner function
+  # instead of the default.
   if [ -n "$PW_BANNER_FUNC" ]; then
     _PW_BANNER_FUNC="$PW_BANNER_FUNC"
   fi
+  # Display the banner unless PW_ENVSETUP_QUIET or
+  # PW_ENVSETUP_NO_BANNER is set.
   if [ -z "$PW_ENVSETUP_QUIET" ] && [ -z "$PW_ENVSETUP_NO_BANNER" ]; then
     pw_green "\n  WELCOME TO...\n"
     "$_PW_BANNER_FUNC"