Extracting substring in bash by matching

Introduction

In a Bitbucket Pipelines configuration I am building a release pipeline that matches a release branch by name release/*. In this build step I am building a docker image, then in turn tagging it. I want to tag it using the version in the branch name, just without the release/ part.

The branch name is available in $BITBUCKET_BRANCH, and I can use bash to extract a substring.

Extract substring

Remove matching prefix pattern ${parameter##word}

$ branch=release/branch/2.2.1; version=${branch##*/}; echo $version;
2.2.1

Using ##(as above) will return the longest match. Using a single # (as below) will return the shortest match.

$ branch=release/branch/2.2.1; version=${branch#*/}; echo $version;
branch/2.2.1

This means I can use the following to get the image tag I want

$ docker build -t my-image:${BITBUCKET_BRANCH#release/}

To be shorter, less explicit, alas less readable for the untrained basher

$ docker build -t my-image:${BITBUCKET_BRANCH#*/}

Readability matters the most!

References

• http://www.tldp.org/LDP/abs/html/string-manipulation.html
• man bash