diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index 01085ef..4d34bf6 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -8,16 +8,15 @@ jobs:
       fail-fast: false
       matrix:
         os: [ubuntu-latest, macos-latest, windows-latest]
-        config_file: [./test/setup.cfg, ./test/pyproject.toml]
     steps:
     - uses: actions/checkout@v4
     - name: run local action code
       uses: ./
       with:
-        file: ${{ matrix.config_file }}
+        files: ./test/pyproject.toml[doc] ./test/setup.cfg[pip_only] ./test/environment.yaml ./test/requirements.txt
         output: environment_test.yml
         channels: conda-forge defaults
-        extras: test pip_only
+        extras: test
         setup_requires: include
         pip: bidict
     - uses: mamba-org/setup-micromamba@v1.8.0
@@ -34,8 +33,6 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       fail-fast: false
-      matrix:
-        config_file: [./test/setup.cfg, ./test/pyproject.toml]
     steps:
     - uses: actions/checkout@v4
     - uses: actions/setup-python@v5
@@ -44,4 +41,4 @@ jobs:
     - name: pip install package
       run: pip install .
     - name: test cmd script
-      run: pydeps2env ${{ matrix.config_file }} output.yaml -c defaults --extras test -b include --pip pandas
+      run: pydeps2env ./test/setup.cfg ./test/pyproject.toml[doc] ./test/environment.yaml ./test/requirements.txt -o output.yaml -c defaults --extras test -b include --pip pandas
diff --git a/.pre-commit-config.yaml b/.pre-commit-config.yaml
index f9e3c92..befa9ea 100644
--- a/.pre-commit-config.yaml
+++ b/.pre-commit-config.yaml
@@ -1,6 +1,6 @@
 repos:
 - repo: https://github.com/pre-commit/pre-commit-hooks
-  rev: v4.1.0
+  rev: v4.5.0
   hooks:
   - id: trailing-whitespace
     args: [--markdown-linebreak-ext=md]
@@ -8,7 +8,7 @@ repos:
   - id: check-yaml
   # ----- Python formatting -----
 - repo: https://github.com/charliermarsh/ruff-pre-commit
-  rev: v0.1.13
+  rev: v0.3.2
   hooks:
       # Run ruff linter.
   - id: ruff
@@ -24,10 +24,10 @@ repos:
   - id: pretty-format-yaml
     args: [--autofix, --indent, '2']
 - repo: https://github.com/tox-dev/pyproject-fmt
-  rev: 1.6.0
+  rev: 1.7.0
   hooks:
   - id: pyproject-fmt
 - repo: https://github.com/abravalheri/validate-pyproject
-  rev: v0.15
+  rev: v0.16
   hooks:
   - id: validate-pyproject
diff --git a/README.md b/README.md
index 1cb130b..0448512 100644
--- a/README.md
+++ b/README.md
@@ -3,14 +3,29 @@
 An easy way to create conda environment files from you python project dependencies.  
 Creates a conda `environment.yml` file from python package dependencies listed in a `pyproject.toml` or `setup.cfg` file.
 
-## basic usage
+The project contains
+- GitHub action
+- python package
+- command line script
+
+```mermaid
+flowchart LR
+    pyproject.toml --> pydeps2env
+    setup.cfg --> pydeps2env
+    environment.yaml --> pydeps2env
+    requirements.txt --> pydeps2env
+    pydeps2env --> E2[environment.yaml]
+    pydeps2env --> R2[requirements.txt]
+```
+
+## basic usage (GitHub action)
 
 By default, the action will parse a `pyproject.toml` file in your root directory into `environment.yml`. Here is an example
 of a simple setup:
 
 ```yaml
 steps:
-  - uses: CagtayFabry/pydeps2env@v0.3.0
+  - uses: CagtayFabry/pydeps2env@v1.0.0
 ```
 
 ```toml
@@ -40,7 +55,7 @@ dependencies:
   - pandas>=1.0
 ```
 
-A full output with options `--setup_requires include --extras test pip_only --pip bidict`
+A full output with options `--build_system include --extras test pip_only --pip bidict`
 
 ```yaml
 channels:
@@ -63,9 +78,10 @@ dependencies:
 
 To customize the output the input options are available to the action:
 
-### file
+### files
 
-Specify the location of the `'setup.cfg'` or `'pyproject.toml'` file to parse. (defaults to `'pyproject.toml'`)
+Specify the location of the `'setup.cfg'` or `'pyproject.toml'` files to parse. (defaults to `'pyproject.toml'`)
+Multiple files can be listed. This will result in a combined environment file.
 
 ### output:
 
@@ -78,8 +94,9 @@ Separate a list of multiple channels by spaces (e.g. `'conda-forge defaults'`).
 
 ### extras:
 
-Specify one or more optional `[extras_require]` sections to add to the environment (e.g. `'test'` to include package that
-you would normally install with `pip install pkg[test]`)
+Specify one or more optional `[extras_require]` sections to add to all the environments (e.g. `'test'` to include package that
+you would normally install with `pip install pkg[test]`).
+Note that for individual packages, the [extra]` syntax is also possible.
 
 ### setup_requires:
 
@@ -94,9 +111,9 @@ The dependencies will be listet under the `pip:` section in the environment file
 
 ```yaml
 steps:
-  - uses: CagtayFabry/pydeps2env@main
+  - uses: CagtayFabry/pydeps2env@v1.0.0
     with:
-      file: './test/setup.cfg' # or ./test/pyproject.toml
+      files: ./test/pyproject.toml[doc] ./test/setup.cfg # comine both files, add [doc] only for pyproject.toml
       output: 'environment_test.yml'
       channels: 'conda-forge defaults'
       extras: 'test'
@@ -131,6 +148,7 @@ dependencies:
   - pytest
   - setuptools>=40.9.0
   - setuptools_scm
+  - sphinx
   - wheel
   - pip:
     - bidict
diff --git a/action.yml b/action.yml
index 0b9c2b3..03c5cf1 100644
--- a/action.yml
+++ b/action.yml
@@ -1,11 +1,11 @@
 name: pydeps2env
 description: create conda environment file from python project dependencies
 inputs:
-  file:
+  files:
     description: >-
-      Specify the location of the 'setup.cfg' file to parse. (defaults to 'setup.cfg')
+      Specify the location of the dependencies files to parse. (defaults to 'pyproject.toml')
     required: true
-    default: setup.cfg
+    default: pyproject.toml
   output:
     description: >-
       Specify the location and name of the conda environment file to generate. (defaults to 'environment.yml')
@@ -39,7 +39,8 @@ runs:
   - name: create environment file
     run: >
       pip3 install tomli packaging pyyaml &&
-      python3 $GITHUB_ACTION_PATH/pydeps2env/generate_environment.py ${{ inputs.file }} ${{ inputs.output }}
+      python3 $GITHUB_ACTION_PATH/pydeps2env/generate_environment.py ${{ inputs.files }}
+      --output ${{ inputs.output }}
       --channels ${{ inputs.channels }}
       --extras ${{ inputs.extras }}
       --setup_requires ${{ inputs.setup_requires }}
diff --git a/pydeps2env/environment.py b/pydeps2env/environment.py
index 216a3b3..2d3ae71 100644
--- a/pydeps2env/environment.py
+++ b/pydeps2env/environment.py
@@ -10,6 +10,21 @@
 import warnings
 
 
+def clean_list(item: list, sort: bool = True) -> list:
+    """Remove duplicate entries from a list."""
+    pass
+
+
+def split_extras(filename: str) -> tuple[str, set]:
+    """Split extras requirements indicated in []."""
+    if "[" in filename:
+        filename, extras = filename.split("[", 1)
+        extras = set(extras.split("]", 1)[0].split(","))
+    else:
+        extras = {}
+    return filename, extras
+
+
 def add_requirement(
     req: Requirement | str,
     requirements: dict[str, Requirement],
@@ -45,16 +60,29 @@ def combine_requirements(
 class Environment:
     filename: str | Path
     channels: list[str] = field(default_factory=lambda: ["conda-forge"])
-    extras: list[str] = field(default_factory=list)
+    extras: set[str] | list[str] = field(default_factory=set)
     pip_packages: set[str] = field(default_factory=set)  # install via pip
     requirements: dict[str, Requirement] = field(default_factory=dict, init=False)
     build_system: dict[str, Requirement] = field(default_factory=dict, init=False)
 
     def __post_init__(self):
+        # cleanup duplicates etc.
+        self.extras = set(self.extras)
+        self.channels = list(dict.fromkeys(self.channels))
+        self.pip_packages = set(self.pip_packages)
+
+        if isinstance(self.filename, str):
+            self.filename, extras = split_extras(self.filename)
+            self.extras |= set(extras)
+
         if Path(self.filename).suffix == ".toml":
             self.load_pyproject()
         elif Path(self.filename).suffix == ".cfg":
             self.load_config()
+        elif Path(self.filename).suffix in [".yaml", ".yml"]:
+            self.load_yaml()
+        elif Path(self.filename).suffix in [".txt"]:
+            self.load_txt()
         else:
             raise ValueError(f"Unsupported input {self.filename}")
 
@@ -104,7 +132,35 @@ def load_config(self):
             for dep in extra_deps:
                 add_requirement(dep, self.requirements)
 
-    def _get_dependencies(self, include_build_system: bool = True):
+    def load_yaml(self):
+        """Load a conda-style environment.yaml file."""
+        with open(self.filename, "r") as f:
+            env = yaml.load(f.read(), yaml.SafeLoader)
+
+        self.channels += env.get("channels", [])
+        self.channels = list(dict.fromkeys(self.channels))
+
+        for dep in env.get("dependencies"):
+            if isinstance(dep, str):
+                add_requirement(dep, self.requirements)
+            elif isinstance(dep, dict) and "pip" in dep:
+                add_requirement("pip", self.requirements)
+                for pip_dep in dep["pip"]:
+                    req = Requirement(pip_dep)
+                    self.pip_packages |= {req.name}
+                    add_requirement(req, self.requirements)
+
+    def load_txt(self):
+        """Load simple list of requirements from txt file."""
+        with open(self.filename, "r") as f:
+            deps = f.readlines()
+
+        for dep in deps:
+            add_requirement(dep, self.requirements)
+
+    def _get_dependencies(
+        self, include_build_system: bool = True
+    ) -> tuple[list[str], list[str]]:
         """Get the default conda environment entries."""
 
         reqs = self.requirements.copy()
@@ -132,7 +188,9 @@ def export(
 
         conda_env = {"channels": self.channels, "dependencies": deps.copy()}
         if pip:
-            conda_env["dependencies"] += ["pip", {"pip": pip}]
+            if "pip" not in self.requirements:
+                conda_env["dependencies"] += ["pip"]
+            conda_env["dependencies"] += [{"pip": pip}]
 
         if outfile is None:
             return conda_env
@@ -150,3 +208,9 @@ def export(
                 )
             with open(p, "w") as outfile:
                 yaml.dump(conda_env, outfile, default_flow_style=False)
+
+    def combine(self, other: Environment):
+        """Merge other Environment requirements into this Environment."""
+        self.requirements = combine_requirements(self.requirements, other.requirements)
+        self.build_system = combine_requirements(self.build_system, other.build_system)
+        self.pip_packages = self.pip_packages | other.pip_packages
diff --git a/pydeps2env/generate_environment.py b/pydeps2env/generate_environment.py
index 7d6ea40..07a6154 100644
--- a/pydeps2env/generate_environment.py
+++ b/pydeps2env/generate_environment.py
@@ -2,21 +2,24 @@
 
 from pathlib import Path
 
+try:
+    from pydeps2env.environment import Environment, split_extras
+except ModuleNotFoundError:  # try local file if not installed
+    from environment import Environment, split_extras
+
 
 def create_environment_file(
-    filename: str,
+    filename: list[str],
     output_file: str,
     channels: list[str],
     extras: list[str],
-    pip: list[str],
+    pip: set[str],
     include_build_system: bool = False,
 ):
-    try:
-        from pydeps2env.environment import Environment
-    except ModuleNotFoundError:  # try local file if not installed
-        from environment import Environment
-
-    env = Environment(filename, pip_packages=pip, extras=extras, channels=channels)
+    pip = set(pip)
+    env = Environment(filename[0], pip_packages=pip, extras=extras, channels=channels)
+    for f in filename[1:]:
+        env.combine(Environment(f, pip_packages=pip, extras=extras, channels=channels))
 
     _include = include_build_system == "include"
     env.export(output_file, include_build_system=_include)
@@ -27,9 +30,11 @@ def main():
 
     parser = argparse.ArgumentParser()
     parser.add_argument(
-        "setup", type=str, default="pyproject.toml", help="dependency file"
+        "setup", type=str, nargs="*", default="pyproject.toml", help="dependency file"
+    )
+    parser.add_argument(
+        "-o", "--output", type=str, default="environment.yml", help="output file"
     )
-    parser.add_argument("env", type=str, default="environment.yml", help="output file")
     parser.add_argument("-c", "--channels", type=str, nargs="*", default=["defaults"])
     parser.add_argument("-e", "--extras", type=str, nargs="*", default=[])
     parser.add_argument(
@@ -42,12 +47,14 @@ def main():
     parser.add_argument("-p", "--pip", type=str, nargs="*", default=[])
     args = parser.parse_args()
 
-    if not Path(args.setup).is_file():
-        raise FileNotFoundError(f"Could not find file {args.setup}")
+    for file in args.setup:
+        filename, _ = split_extras(file)
+        if not Path(filename).is_file():
+            raise FileNotFoundError(f"Could not find file {filename}")
 
     create_environment_file(
         filename=args.setup,
-        output_file=args.env,
+        output_file=args.output,
         channels=args.channels,
         extras=args.extras,
         pip=args.pip,
diff --git a/test/environment.yaml b/test/environment.yaml
new file mode 100644
index 0000000..2601a87
--- /dev/null
+++ b/test/environment.yaml
@@ -0,0 +1,8 @@
+channels:
+- conda-forge
+dependencies:
+- python>3.7
+- yaml
+- pip
+- pip:
+  - requests
diff --git a/test/pyproject.toml b/test/pyproject.toml
index a27d8b8..d54f4fa 100644
--- a/test/pyproject.toml
+++ b/test/pyproject.toml
@@ -13,6 +13,9 @@ dependencies = [
   "pandas>=1",
 ]
 [project.optional-dependencies]
+doc = [
+  "sphinx",
+]
 pip_only = [
   "bidict",
 ]
diff --git a/test/requirements.txt b/test/requirements.txt
new file mode 100644
index 0000000..8a60482
--- /dev/null
+++ b/test/requirements.txt
@@ -0,0 +1,2 @@
+setuptools
+urllib3
diff --git a/test/setup.cfg b/test/setup.cfg
index ed4bbb1..a3112ac 100644
--- a/test/setup.cfg
+++ b/test/setup.cfg
@@ -11,10 +11,12 @@ setup_requires =
     setuptools_scm
 install_requires =
     numpy >=1.20
-    pandas >=1.0
+    pandas <2.0
+    boltons >=1.0
 
 [options.extras_require]
 test =
     pytest
+    pytest-cov
 pip_only =
     bidict