A script to generate a valid semver version in any git repo

gistfile1.sh

#!/bin/bash
#
# If run inside a git repository will return a valid semver based on
# the semver formatted tags. For example if the current HEAD is tagged
# at 0.0.1, then the version echoed will simply be 0.0.1. However, if
# the tag is say, 3 patches behind, the tag will be in the form
# `0.0.1+build.3.0ace960`. This is basically, the current tag a
# monotonically increasing commit (the number of commits since the
# tag, and then a git short ref to identify the commit.
#
# You may also pass this script the a `release` argument. If that is
# the case it will exit with a non-zero value if the current head is
# not tagged.
#
set -e

commit_count=
version_candidate=
version_tag=
vsn=

get-version-candidate()
{
    local ver_regex='tag: (v([^,\)]+)|([0-9]+(\.[0-9]+)*))'
    local tag_lines=`git log --oneline --decorate  |  fgrep "tag: "`

    if [[ $tag_lines =~ $ver_regex ]]; then
        if [[ ${BASH_REMATCH[1]:0:1} = "v" ]]; then
            version_tag=${BASH_REMATCH[1]}
            version_candidate=${BASH_REMATCH[2]}
        else
            version_tag=${BASH_REMATCH[3]}
            version_candidate=${BASH_REMATCH[3]}
        fi
    else
        version_tag=""
        version_candidate="0.0.0"
    fi
}

get-commit-count()
{
    if [[ $version_tag = "" ]]; then
        commit_count=`git rev-list HEAD | wc -l`
    else
        commit_count=`git rev-list ${version_tag}..HEAD | wc -l`
    fi
    commit_count=`echo $commit_count | tr -d ' 't`
}

build-version()
{
    if [[ $commit_count = 0 ]]; then
        vsn=$version_candidate
    else
        local ref=`git log -n 1 --pretty=format:'%h'`
        vsn="${version_candidate}+build.${commit_count}.${ref}"
    fi
}

check-tag()
{
    if [[ "$1" = "release" ]]; then
        if [[ $commit_count != 0 ]]; then
            echo "The current head *must* be tagged"
            exit 127
        fi
    fi
}

get-version-candidate
get-commit-count
build-version
check-tag "$1"

echo $vsn

How would you go about adding pre-release identifier support to this script?

https://semver.org/#spec-item-9

Great stuff by the way!