Lightweight license checker.
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Verifying Licenses

This is an automatic license checker aimed at ensuring compliance with the Apache Software Foundation processes. It does not ensure compliance, but it catches many common errors automatically.

To get started quickly, install weasel with go get:

go get

Then, cd into your project's directory and run:

weasel -q

That will list all the potentially problematic licenses in the project. You'll probably need to configure your .dependency_license file to document the dependencies that can't be detected automatically and suppress the ones that are detected wrongly.


This is the binary with all the logic in it. weasel will list every file in every subdirectory, starting from the target directory, and determine the most likely license for that file. It errs on the side of false positives, since the consequences of a false negative are considerably more serious.

weasel [-q] [--] <target_dir>:

  • -q Suppress the printing of non-problematic files.
  • -- Nothing after this is interpreted as an argument.
  • <target_dir> To run weasel against a different target. The target directory must be the root of the project. If it is omitted, weasel will search directories upward from the current directory, looking for a .git folder to indicate the root.


The LICENSE file is at the root of the project (from whence you ought to run license). This must comply with the requirements of the Apache Software Foundation and is intended for human consumption.

Nevertheless, with a bit of careful writing, it's possible to have weasel help verify that everything gets covered.

Lines that begin with an @-symbol are interpreted as a path specification that describes a set of files covered by the license. weasel does not validate that @ files are actually licensed correctly, merely that they are mentioned. This covers the most common case, which is adding a (even potentially correctly licensed!) file and forgetting to mention it in the LICENSE file.

Likewise, it's impermissible to use an @-line that describes no files. This usually happens when a dependency is removed and the LICENSE file does not get updated properly.

@-lines are interpreted by path.Match, the syntax for which is:

    { term }
    '*'         matches any sequence of non-/ characters
    '?'         matches any single non-/ character
    '[' [ '^' ] { character-range } ']'
                character class (must be non-empty)
    c           matches character c (c != '*', '?', '\\', '[')
    '\\' c      matches character c

    c           matches character c (c != '\\', '-', ']')
    '\\' c      matches character c
    lo '-' hi   matches character c for lo <= c <= hi


Sometimes, there's no reasonable way to automatically detect the appropriate license for a file, especially files that don't support comments or are binary. .dependency_license allows you to document the exceptions so that new files show up clearly.

The .dependency_license must appear in the root of the project.

Each line should either be empty, a comment (prepended by an octothorp), or a license exception line. A license exception line is a regular expression, a comma, then the name of a license, then optionally an octothorp followed by a comment (which may not contain a comma!).

    regex ',' license-name [ '#' { commentable-char } ]       Associates the license with the file.        
    regex ',' '!' license-name [ '#' { commentable-char } ]   Disassociates the license from the file.

regex: A regular expression accepted by golang regexps, described here:

    'Apache'    Apache License
    'BSD'       Berkeley Software Distribution License
    'MIT'       Massachusetts Institute of Technology License
    'GoBSD'     BSD-style license used by the GoLang team
    'ISC'       Internet Systems Consortium
    'X11'       MIT License, by an older name.
    'WTFPL'     Do What the Fuck You Want to Public License
    'GPL/LGPL'  Either the GNU General Public License or the GNU Lesser General Public License
    'Docs'      A documentation file
    'Empty'     An empty file
    'Ignored'   A file that ought not be analyzed for compliance

commentable-char: Any character other than a ','

Best Practices

License management can be tricky at the best of times. The goal of this tool is to automate as much of that as possible. Here are some best practices:

  • weasel -q before you commit. If it prints anything, you probably need to add license headers.
  • Do not Ignore files. If it's reasonable to quiet weasel about a false positive or negative in another way, do that instead.
  • If an unrecognized file has a header, update weasel, not .dependency_license. It's relatively straightforward to add license recognition to licenseList.go. Doing it that way benefits future files as well.
  • Run weasel as part of Continuous Integration. Issues are not usually difficult to fix, but automatic running allows them to be fixed promptly.