How to set environment variable in script?

[haxung]

Thanks for your videos and examples, they help me much in jenkins study.

I got trouble when I wanted to set environment variable in script.
I write a shell script gen_tag.sh, like this:

#!/bin/bash

if [[ "$GIT_BRANCH" =~ ^v ]]; then
  export TAG="${GIT_BRANCH}"
else
  export TAG="${GIT_COMMIT:0:12}"
fi

echo "$TAG"

and in Jenkinsfile, this is a example:

pipeline {
  agent any
  stages {

    stage('PreTest') {
      steps {
        catchError(buildResult:'UNSTABLE',stageResult:'UNSTABLE') {
          sh 'chmod +x ./gen_tag.sh'
          sh './gen_tag.sh'
        }
      echo currentBuild.result
      }
    }

    stage('Prepare') {
      steps {
        catchError {
          echo "$TAG" // it cause wrong, cannot get TAG from env
        }
        echo currentBuild.result
      }
    }
  }
}

In fact, I cannot get TAG value from environment. Does there have some ideas to set environment variable in script?