A fast partial replacement for the codemod tool
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swolchok and facebook-github-bot Add -F/--fixed-strings
Summary: Makes life easier if you just want to do a fixed string replacement.

Reviewed By: adamjernst

Differential Revision: D9174509

fbshipit-source-id: e5eaa7303ebccc370258ecdf86b4be72573c67ab
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fastmod is a fast partial replacement for codemod. Like codemod, it is a tool to assist you with large-scale codebase refactors, and it supports most of codemod's options. fastmod's major philosophical difference from codemod is that it is focused on improving the use case "I want to use interactive mode to make sure my regex is correct, and then I want to apply the regex everywhere". For this use case, it offers much better performance than codemod. Accordingly, fastmod does not support codemod's --start, --end, or --count options, nor does it support anything like codemod's Python API.


Let's say you're deprecating your use of the <font> tag. From the command line, you might make progress by running:

fastmod -m -d /home/jrosenstein/www --extensions php,html \
    '<font *color="?(.*?)"?>(.*?)</font>' \
    '<span style="color: ${1};">${2}</span>'

For each match of the regex, you'll be shown a colored diff and asked if you want to accept the change (the replacement of the <font> tag with a <span> tag), reject it, or edit the line in question in your $EDITOR of choice.

NOTE: Whereas codemod uses Python regexes, fastmod uses the Rust regex crate, which supports a slightly different regex syntax and does not support look around or backreferences. In particular, use ${1} instead of \1 to get the contents of the first capture group. See the regex crate's documentation for details.

A consequence of this syntax is that the use of single quotes instead of double quotes around the replacement text is important, because the bash shell itself cares about the $ character in double-quoted strings. If you must double-quote your input text, be careful to escape $ characters properly!

fastmod also offers a usability improvement over codemod: it accepts files or directories to process as extra positional arguments after the regex and substitution. For instance, the example above could have been rewritten as

fastmod -m --extensions php,html \
    '<font *color="?(.*?)"?>(.*?)</font>' \
    '<span style="color: ${1};">${2}</span>' \

This makes it possible to use fastmod to process a list of files from somewhere else if needed. Note, however, that fastmod does its own parallel directory traversal internally, so doing find ... | xargs fastmod ... may be much slower than using fastmod by itself.


fastmod is primarily supported on macOS and Linux.

fastmod has also been reported to work reasonably well on Windows. The major portability concerns are 1) the use of $EDITOR with a fallback and 2) the console UI, which is descended from codemod's ncurses-based text coloring & screen clearing code. Windows-specific issues and PRs will be considered as long as they aren't too invasive. For example, if something doesn't work on Windows because a Linux/Mac-specific API was used instead of equivalent POSIX or Rust standard library calls, we would be happy to fix that. On the other hand, we would like to avoid taking a direct winapi dependency or substantially increasing the size of our dependency graph for Windows-only enhancements.

Building fastmod

fastmod is written in (stable) Rust and compiles with Rust's cargo build system. To build:

$ git clone https://github.com/facebookincubator/fastmod.git
$ cd fastmod
$ cargo build --release
$ ./target/release/fastmod --help

Installing fastmod

If you have built fastmod following the directions above, you can install it with cargo install. You can also have cargo build it from https://crates.io/ via cargo install fastmod.

How fastmod works

fastmod uses the ignore crate to walk the given directory hierarchy while respecting .gitignore. It reads each matching file into memory, applies the given regex substitution one match at a time, and uses the diff crate to present the resulting change as a patch for human review. In --accept-all mode, it walks the directory hierarchy using multiple threads in parallel and avoids calculating patches for efficiency.

Full documentation

See fastmod --help.


fastmod is Apache-2.0-licensed.