In Jenkins Pipeline script can be add external programs, in this post walk through you how to use BASH commands on Jenkins Pipeline script. We can telling the script to run the shell commands with option /bin/bash  instead of /bin/sh which nees to be on the first line or can igonred it. 
Use "sh" with single quotes, we have a sample script lines below,
 

pipeline {
....
....
stage('Execute Bash Command') {
    steps {
         sh 'free -m'
    }
}

If you would like to add multiple shell commands in a single sh,
 
pipeline {
....
....
stage('Execute Bash Command') {
    steps {
         sh '''
             #!/bin/bash
             echo "hello world"
             free -m
             who
         '''
    }
}