I\'m using a EC2 server instance. Used the following to install Jenkins:
wget -q -O - http://pkg.jenkins-ci.org/debian/jenkins-ci.org.key | sudo apt-key add
I was running Jenkins executing java -jar jenkins.war.
java -jar jenkins.war
In my case Jenkins wrote webroot in an stdout: webroot: $user.home/.jenkins. So admin secret key was placed in a ~/.jenkins/secrets/initialAdminPassword.
webroot: $user.home/.jenkins
~/.jenkins/secrets/initialAdminPassword