I have 2 jobs 'job1' and 'job2'. I will be triggering 'job2' from 'job1'. I need to get the console output of the 'job1' build which triggered 'job2' and want to process it in 'job2'.
The difficult part is to find out the build number of the upstream job (job1) that triggered the downstream job (job2). Once you have it, you can access the console log, e.g. via ${BUILD_URL}consoleOutput, as pointed out by ivoruJavaBoy.
How can a downstream build determine by what job and build number it has been triggered? This is surprisingly difficult in Jenkins:
Solution A ("explicit" approach): use the Parameterized Trigger Plugin to "manually" pass a build number parameter from job1 to job2. Apart from the administrational overhead, there is one bigger drawback with that approach: job1 and job2 will no longer run asynchronously; there will be exactly one run of job2 for each run of job1. If job2 is slower than job1, then you need to plan your resources to avoid builds of job2 piling up in the queue.
So, some solution "B" will be better, where you extract the implicit upstream information from Jenkins' internal data. You can use Groovy for that; a simpler approch may be to use the Job Exporter Plugin in job2. That plugin will create a "properties" (text) file in the workspace of a job2 build. That file contains two entries that contain exactly the information that you're looking for:
build.upstream.number Number of upstream job that triggered this job. Only filled if the build was triggered by an upstream project.
build.upstream.project Upstream project that triggered this job.
Read that file, then use the information to read the console log via the URL above.