Jira keeps it's logs inside atlassian-jira.log file. When this file reaches a certain size it backed up as atlassian-jira.log.1 and Jira continues to write new logs to atlassian-jira.log. For new administrators location of this file may be confusing because they search this file in JIRA_INSTALL_DIR instead of JIRA_DATA_DIR. It is located in JIRA_DATA_DIR/log folder. To see exact location of this file you can check "System Info" page of Jira. Login as administrator and navigate to "Administration/System/System Info" section. This page shows a lot of information about Jira such as how much memory it is using, listeners in the system or paths of some system files. By default Jira and add-ons only log error conditions in this file, for logging more detailed information you need to turn on logging of related package.