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----
-title: Verifying "npm ci" reproducibility
-date: 2018-08-01
-layout: post
-lang: en
-ref: verifying-npm-ci-reproducibility
-updated_at: 2019-05-22
----
-When [npm@5](https://blog.npmjs.org/post/161081169345/v500) came bringing
-[package-locks](https://docs.npmjs.com/files/package-locks) with it, I was
-confused about the benefits it provided, since running `npm install` more than
-once could resolve all the dependencies again and yield yet another fresh
-`package-lock.json` file. The message saying "you should add this file to
-version control" left me hesitant on what to do[^package-lock-message].
-
-However the [addition of `npm ci`](https://blog.npmjs.org/post/171556855892/introducing-npm-ci-for-faster-more-reliable)
-filled this gap: it's a stricter variation of `npm install` which
-guarantees that "[subsequent installs are able to generate identical trees](https://docs.npmjs.com/files/package-lock.json)". But are they
-really identical? I could see that I didn't have the same problems of
-different installation outputs, but I didn't know for **sure** if it
-was really identical.
-
-## Computing the hash of a directory's content
-
-I quickly searched for a way to check for the hash signature of an
-entire directory tree, but I couldn't find one. I've made a poor
-man's [Merkle tree](https://en.wikipedia.org/wiki/Merkle_tree)
-implementation using `sha256sum` and a few piped commands at the
-terminal:
-
-```bash
-merkle-tree () {
-  dirname="${1-.}"
-  pushd "$dirname"
-  find . -type f              | \
-    sort                      | \
-    xargs -I{} sha256sum "{}" | \
-    sha256sum                 | \
-    awk '{print $1}'
-  popd
-}
-```
-
-Going through it line by line:
-
-- #1 we define a Bash function called `merkle-tree`;
-- #2 it accepts a single argument: the directory to compute the
-  merkle tree from. If nothing is given, it runs on the current
-  directory (`.`);
-- #3 we go to the directory, so we don't get different prefixes in
-  `find`'s output (like `../a/b`);
-- #4 we get all files from the directory tree. Since we're using
-  `sha256sum` to compute the hash of the file contents, we need to
-  filter out folders from it;
-- #5 we need to sort the output, since different file systems and
-  `find` implementations may return files in different orders;
-- #6 we use `xargs` to compute the hash of each file individually
-  through `sha256sum`. Since a file may contain spaces we need to
-  escape it with quotes;
-- #7 we compute the hash of the combined hashes. Since `sha256sum`
-  output is formatted like `<hash> <filename>`, it produces a
-  different final hash if a file ever changes name without changing
-  it's content;
-- #8 we get the final hash output, excluding the `<filename>` (which
-    is `-` in this case, aka `stdin`).
-
-### Positive points:
-
-1.  ignore timestamp: running more than once on different installation
-    yields the same hash;
-2.  the name of the file is included in the final hash computation.
-
-### Limitations:
-
-1.  it ignores empty folders from the hash computation;
-2.  the implementation's only goal is to represent using a digest
-    whether the content of a given directory is the same or not. Leaf
-    presence checking is obviously missing from it.
-
-### Testing locally with sample data
-
-```bash
-mkdir /tmp/merkle-tree-test/
-cd /tmp/merkle-tree-test/
-mkdir -p a/b/ a/c/ d/
-echo "one"   > a/b/one.txt
-echo "two"   > a/c/two.txt
-echo "three" > d/three.txt
-merkle-tree . # output is       be343bb01fe00aeb8fef14a3e16b1c3d1dccbf86d7e41b4753e6ccb7dc3a57c3
-merkle-tree . # output still is be343bb01fe00aeb8fef14a3e16b1c3d1dccbf86d7e41b4753e6ccb7dc3a57c3
-echo "four"  > d/four.txt
-merkle-tree . # output is now   b5464b958969ed81815641ace96b33f7fd52c20db71a7fccc45a36b3a2ae4d4c
-rm d/four.txt
-merkle-tree . # output back to  be343bb01fe00aeb8fef14a3e16b1c3d1dccbf86d7e41b4753e6ccb7dc3a57c3
-echo "hidden-five" > a/b/one.txt
-merkle-tree . # output changed  471fae0d074947e4955e9ac53e95b56e4bc08d263d89d82003fb58a0ffba66f5
-```
-
-It seems to work for this simple test case.
-
-You can try copying and pasting it to verify the hash signatures.
-
-## Using `merkle-tree` to check the output of `npm ci`
-
-*I've done all of the following using Node.js v8.11.3 and npm@6.1.0.*
-
-In this test case I'll take the main repo of
-[Lerna](https://lernajs.io/)[^lerna-package-lock]:
-
-```bash
-cd /tmp/
-git clone https://github.com/lerna/lerna.git
-cd lerna/
-git checkout 57ff865c0839df75dbe1974971d7310f235e1109
-npm ci
-merkle-tree node_modules/ # outputs 11e218c4ac32fac8a9607a8da644fe870a25c99821167d21b607af45699afafa
-rm -rf node_modules/
-npm ci
-merkle-tree node_modules/ # outputs 11e218c4ac32fac8a9607a8da644fe870a25c99821167d21b607af45699afafa
-npm ci      # test if it also works with an existing node_modules/ folder
-merkle-tree node_modules/ # outputs 11e218c4ac32fac8a9607a8da644fe870a25c99821167d21b607af45699afafa
-```
-
-Good job `npm ci` :)
-
-#6 and #9 take some time to run (21 seconds in my machine), but this
-specific use case isn't performance sensitive. The slowest step is
-computing the hash of each individual file.
-
-## Conclusion
-
-`npm ci` really "generates identical trees".
-
-I'm not aware of any other existing solution for verifying the hash
-signature of a directory. If you know any I'd
-[like to know](mailto:{{ site.author.email }}).
-
-## *Edit*
-
-2019-05-22: Fix spelling.
-
-[^package-lock-message]: The
-    [documentation](https://docs.npmjs.com/cli/install#description) claims `npm
-    install` is driven by the existing `package-lock.json`, but that's actually
-    [a little bit tricky](https://github.com/npm/npm/issues/17979#issuecomment-332701215).
-
-[^lerna-package-lock]: Finding a big known repo that actually committed the
-    `package-lock.json` file was harder than I expected.