How do I retrieve an artifact checksum from Nexus using their rest api via curl?

You can either fetch the file directly or use the Nexus API to retrieve it programmatically.

The following URL:

http://localhost:8081/nexus/service/local/artifact/maven/resolve?g=log4j&a=log4j&v=1.2.9&r=central

Returns the following result:

<artifact-resolution>
  <data>
    <presentLocally>true</presentLocally>
    <groupId>log4j</groupId>
    <artifactId>log4j</artifactId>
    <version>1.2.9</version>
    <extension>jar</extension>
    <snapshot>false</snapshot>
    <snapshotBuildNumber>0</snapshotBuildNumber>
    <snapshotTimeStamp>0</snapshotTimeStamp>
    <sha1>55856d711ab8b88f8c7b04fd85ff1643ffbfde7c</sha1>
    <repositoryPath>/log4j/log4j/1.2.9/log4j-1.2.9.jar</repositoryPath>
  </data>
</artifact-resolution>

The xmllint command can be used to parse out the sha1 checksum value as follows:

$ curl -s "http://localhost:8081/nexus/service/local/artifact/maven/resolve?g=log4j&a=log4j&v=1.2.9&r=central" | xmllint --xpath "///sha1/text()" -
55856d711ab8b88f8c7b04fd85ff1643ffbfde7c

Nexus 2

Use the artifact content API to directly get the MD5/SHA1 checksum file by specifying the p (packaging) or e (extension) parameter as jar.md5 or jar.sha1 (or other relevant for your actual packaging).

Example:

$ curl -s 'http://mynexus/service/local/artifact/maven/content?g=com.example&a=example&v=1.2.3&r=my-repo&e=jar.sha1'
55856d711ab8b88f8c7b04fd85ff1643ffbfde7c

Nexus 3

Use the Search API to find and download the asset.

$ curl -sL 'https://mynexus/service/rest/v1/search/assets/download?repository=my-repo&sort=version&maven.groupId=com.example&maven.artifactId=example&maven.baseVersion=LATEST-SNAPSHOT&maven.extension=jar.sha1&maven.classifier='
7ff1ca9fb889c73d095b69a52d5c8609482b63ab