How to pass variables from Jenkinsfile to shell command

Your code is using a literal string and therefore your Jenkins variable will not be interpolated inside the shell command. You need to use " to interpolate your variable inside your strings inside the sh. ' will just pass a literal string. So we need to make a few changes here.

The first is to change the ' to ":

for (i in [ 'a', 'b', 'c' ]) {
  echo i
  sh "echo "from shell i=$i""
}

However, now we need to escape the " on the inside:

for (i in [ 'a', 'b', 'c' ]) {
  echo i
  sh "echo \"from shell i=$i\""
}

Additionally, if a variable is being appended directly to a string like you are doing above ($i onto i=), we need to close it off with some curly braces:

for (i in [ 'a', 'b', 'c' ]) {
  echo i
  sh "echo \"from shell i=${i}\""
}

That will get you the behavior you desire.

Extension to Matts answer: For multi-line sh scripts use

sh """
  echo ${paramName}
"""

instead of

sh '''
  echo ${paramName}
'''