diff --git a/JSON.md b/JSON.md
index bb946f8a..0b61ff1d 100644
--- a/JSON.md
+++ b/JSON.md
@@ -264,6 +264,9 @@ In `cfbs.json`'s `"steps"`, the build step name must be separated from the rest
   - Converts the input data for a module into the augments format and merges it with the target augments file.
   - Source is relative to module directory and target is relative to `out/masterfiles`.
     - In most cases, the build step should be: `input ./input.json def.json`
+- `replace_version <to_replace> <filename>`
+  - Replace the string inside the file with the version number of that module.
+  - The module must have a version and the string must occur exactly once in the file.
 
 When `def.json` is modified during a `json`, `input`, `directory`, `bundles`, or `policy_files` build step, the values of some lists of strings are deduplicated, when this does not make any difference in behavior.
 These cases are:
diff --git a/cfbs/build.py b/cfbs/build.py
index f81771ee..7c8edca7 100644
--- a/cfbs/build.py
+++ b/cfbs/build.py
@@ -30,6 +30,7 @@
     "input": 2,
     "policy_files": "1+",
     "bundles": "1+",
+    "replace_version": 2,  # string to replace and filename
 }
 
 
@@ -259,6 +260,41 @@ def _perform_build_step(module, step, max_length):
             merged = augment
         log.debug("Merged def.json: %s", pretty(merged))
         write_json(path, merged)
+    elif operation == "replace_version":
+        assert len(args) == 2
+        print("%s replace_version '%s'" % (prefix, "' '".join(args)))
+        file = os.path.join(destination, args[1])
+        if not os.path.isfile(file):
+            user_error(
+                "No such file '%s' in replace_version for module '%s"
+                % (file, module["name"])
+            )
+        try:
+            with open(file, "r") as f:
+                content = f.read()
+        except:
+            user_error(
+                "Could not open/read '%s' in replace_version for module '%s"
+                % (file, module["name"])
+            )
+        to_replace = args[0]
+        version = module["version"]
+        new_content = content.replace(to_replace, version, 1)
+        if new_content == content:
+            user_error(
+                "replace_version requires that '%s' has exactly 1 occurence of '%s' - 0 found"
+                % (file, to_replace)
+            )
+        if to_replace in new_content:
+            user_error(
+                "replace_version requires that '%s' has exactly 1 occurence of '%s' - more than 1 found"
+                % (file, to_replace)
+            )
+        try:
+            with open(file, "w") as f:
+                f.write(new_content)
+        except:
+            user_error("Failed to write to '%s'" % (file,))
 
 
 def perform_build(config) -> int:
diff --git a/tests/shell/043_replace_version.sh b/tests/shell/043_replace_version.sh
new file mode 100644
index 00000000..b5dfe377
--- /dev/null
+++ b/tests/shell/043_replace_version.sh
@@ -0,0 +1,27 @@
+set -e
+set -x
+cd tests/
+mkdir -p ./tmp/
+cd ./tmp/
+
+# Set up the project we will build:
+cp ../shell/043_replace_version/example-cfbs.json ./cfbs.json
+mkdir -p subdir
+cp ../shell/043_replace_version/subdir/example.py ./subdir/example.py
+
+# Before building, version number is 0.0.0:
+grep 'print("Version: 0.0.0")' ./subdir/example.py
+! grep 'print("Version: 1.2.3")' ./subdir/example.py
+
+cfbs build
+
+# After building, input and output should be different:
+! diff ./subdir/example.py ./out/masterfiles/services/cfbs/subdir/example.py
+
+# Check that version number is correct in output:
+grep 'print("Version: 1.2.3")' ./out/masterfiles/services/cfbs/subdir/example.py
+! grep 'print("Version: 0.0.0")' ./out/masterfiles/services/cfbs/subdir/example.py
+
+# Also check that the input was not modified:
+grep 'print("Version: 0.0.0")' ./subdir/example.py
+! grep 'print("Version: 1.2.3")' ./subdir/example.py
diff --git a/tests/shell/043_replace_version/example-cfbs.json b/tests/shell/043_replace_version/example-cfbs.json
new file mode 100644
index 00000000..54f1e458
--- /dev/null
+++ b/tests/shell/043_replace_version/example-cfbs.json
@@ -0,0 +1,18 @@
+{
+  "name": "Example project",
+  "description": "Example description",
+  "type": "policy-set",
+  "git": true,
+  "build": [
+    {
+      "name": "./subdir/",
+      "description": "Local subdirectory added using cfbs command line",
+      "added_by": "cfbs add",
+      "version": "1.2.3",
+      "steps": [
+        "copy example.py services/cfbs/subdir/example.py",
+        "replace_version 0.0.0 services/cfbs/subdir/example.py"
+      ]
+    }
+  ]
+}
diff --git a/tests/shell/043_replace_version/subdir/example.py b/tests/shell/043_replace_version/subdir/example.py
new file mode 100644
index 00000000..48761a74
--- /dev/null
+++ b/tests/shell/043_replace_version/subdir/example.py
@@ -0,0 +1,2 @@
+if __name__ == "__main__":
+    print("Version: 0.0.0")
diff --git a/tests/shell/all.sh b/tests/shell/all.sh
index 259f2963..ed6c5272 100644
--- a/tests/shell/all.sh
+++ b/tests/shell/all.sh
@@ -46,5 +46,6 @@ bash tests/shell/039_add_added_by_field_update_1.sh
 bash tests/shell/040_add_added_by_field_update_2.sh
 bash tests/shell/041_add_multidep.sh
 bash tests/shell/042_update_from_url.sh
+bash tests/shell/043_replace_version.sh
 
 echo "All cfbs shell tests completed successfully!"