fixes tag if there are no existing tags

Dockerfile

@@ -3,8 +3,8 @@ LABEL "repository"="https://github.com/anothrNick/github-tag-action"
 LABEL "homepage"="https://github.com/anothrNick/github-tag-action"
 LABEL "maintainer"="Nick Sjostrom"
 
-COPY entrypoint.sh /entrypoint.sh
-
 RUN apk update && apk add bash git curl jq && npm install -g semver
 
+COPY entrypoint.sh /entrypoint.sh
+
 ENTRYPOINT ["/entrypoint.sh"]

entrypoint.sh

@@ -50,18 +50,26 @@ git fetch --tags
 tagFmt="^v?[0-9]+\.[0-9]+\.[0-9]+$" 
 preTagFmt="^v?[0-9]+\.[0-9]+\.[0-9]+(-$suffix\.[0-9]+)?$" 
 
+tag=""
+
 # get latest tag that looks like a semver (with or without v)
 case "$tag_context" in
  *repo*) 
  taglist="$(git for-each-ref --sort=-v:refname --format '%(refname:lstrip=2)' | grep -E "$tagFmt")"
- tag="$(semver $taglist | tail -n 1)"
+ if [ ! -z "$taglist" ]
+ then
+ tag="$(semver $taglist | tail -n 1)"
+ fi
 
  pre_taglist="$(git for-each-ref --sort=-v:refname --format '%(refname:lstrip=2)' | grep -E "$preTagFmt")"
  pre_tag="$(semver "$pre_taglist" | tail -n 1)"
  ;;
  *branch*) 
  taglist="$(git tag --list --merged HEAD --sort=-v:refname | grep -E "$tagFmt")"
- tag="$(semver $taglist | tail -n 1)"
+ if [ ! -z "$taglist" ]
+ then
+ tag="$(semver $taglist | tail -n 1)"
+ fi
 
  pre_taglist="$(git tag --list --merged HEAD --sort=-v:refname | grep -E "$preTagFmt")"
  pre_tag=$(semver "$pre_taglist" | tail -n 1)