Jenkinsfile ${steps.env.BUILD_NUMBER}: bad substitution - jenkins

Question

I am trying to print a variable in Jenkins. But I am getting an error saying "bad substitution". I am using Jenkinsfile to achieve that. This is what I am doing.
static def printbn() {
sh '''
#!/usr/bin/env bash
echo \"${env.BUILD_NUMBER}\"
'''
}
pipeline {
agent any
stages {
stage('Print Build Number') {
steps {
printbn()
}
}
}
}
Error that I am getting
/var/lib/jenkins/workspace/groovymethod#tmp/durable-7d9ef0b0/script.sh: line 4: ${steps.env.BUILD_NUMBER}: bad substitution
NOTE: I am using Jenkins version Jenkins ver. 2.163

Answer 1

In Shell, variable name is not allow use ., that's why you get following error: bad substitution
In Groovy, there are 4 ways to represent a string:
single quote: ' a string '
tripe single quote: ''' a string '''
double quote: " a string "
tripe double quote: """ a string """
And Groovy only execute string interpolation on double and triple double quote string.
For example:
def name = 'Tom'
print "Hello ${name}"
print """Hello ${name}"""
// do interpolation before print, thus get Hello Tom printed out
print 'Hello ${name}'
print '''Hello ${name}'''
//no interpolation thus, print Hello ${name} out directly.
BUILD_NUMBER is Jenkins job's build-in environment variable. You can directly access it in shell/bat.
static def printbn() {
sh '''
#!/usr/bin/env bash
echo ${BUILD_NUMBER}
// directly access any Jenkins build-in environment variable,
// no need to use pattern `env.xxxx` which only works in groovy not in shell/bat
'''
}
If you want use env.xxxx pattern, you can archive that via groovy string interpolation.
static def printbn() {
// use pipeline step: echo
echo "${env.BUILD_NUMBER}" // env.BUILD_NUMBER is groovy variable
// or use pipeline step: sh
sh """#!/usr/bin/env bash
echo ${env.BUILD_NUMBER}
"""
// will do interpolation firstly, to replace ${env.BUILD_NUMBER} with real value
// then execute the whole shell script.
}