Take a semantic version string and outputs potential docker tags

semver-to-docker-labels.sh

#!/bin/sh

# GPL-3.0-or-later

#
# Usage: semver-to-docker-labels.sh SEMANTIC_VERSION_STRING
#
# Print possible docker labels for a semantic version string.
# For semantic version with no pre-release or build number. It will output:
#
# major
# major.minor
# major.minor.patch
# 
# For other format, it will output the semantic version string as is.
#

main() {

  #
  # The regex is the official one from Semantic Versioning 2.0.0
  # https://semver.org/#is-there-a-suggested-regular-expression-regex-to-check-a-semver-string
  #
  semver=$(trim_string "$1")
  regex="^(?P<major>0|[1-9]\d*)\.(?P<minor>0|[1-9]\d*)\.(?P<patch>0|[1-9]\d*)(?:-(?P<prerelease>(?:0|[1-9]\d*|\d*[a-zA-Z-][0-9a-zA-Z-]*)(?:\.(?:0|[1-9]\d*|\d*[a-zA-Z-][0-9a-zA-Z-]*))*))?(?:\+(?P<buildmetadata>[0-9a-zA-Z-]+(?:\.[0-9a-zA-Z-]+)*))?$"
  if ! echo "$semver" | grep -P "$regex" > /dev/null;
  then
    >&2 echo "$semver is not a valid semantic versionning string"
    exit 1
  fi

  case $semver in
  *+*)
    >&2 echo "$semver is a valid semantic versionning but docker labels doesn't support buildmetadata."
    exit 1
    ;;
  *-*)
    echo "$semver"
    exit 0
    ;;
  esac

  i=0
  semver=$(split "$semver" "." | tr '\n' ' ')
  for part in $semver; do
    case $i in
    0) major=$part ;;
    1) minor=$part ;;
    2) patch=$part ;;
    esac
    i=$((i+1))
  done

  echo "$major"
  echo "$major.$minor"
  echo "$major.$minor.$patch"
}

split() {
  set -f
  old_ifs=$IFS
  IFS=$2
  # shellcheck disable=2086
  set -- $1
  printf '%s\n' "$@"
  IFS=$old_ifs
  set +f
}

trim_string() {
  trim=${1#${1%%[![:space:]]*}}
  trim=${trim%${trim##*[![:space:]]}}
  printf '%s\n' "$trim"
}

main "$1"