Useful Bash Commands

Viewing Log Files

Tailing the File

When the same file name is used when logs are rotated (i.e. app.log is renamed to app.yyyymmdd.log and a new app.log is created), use the -F flag to follow the name instead of the file descriptor

tail -F /var/log/app.log

Tailing with Filtering

When you are looking for something specific in the log file, it often helps to run the log output through grep. This example watches a sendmail log for communication with the host 10.5.5.5

tail -F /var/log/maillog | grep "10.5.5.5"

Handling Log Files with Date Specific Naming

I alias out commands for viewing commonly read log files. This is easy enough when the current log file is always /var/log/application/content.log, but some active log files have date components in the file name. As an example, our Postgresql servers have the short day-of-week string in the log. Use command substitution to get the date-specific elements from the date executable. Here, I tail a file named postgresql-Tue.log on Tuesday. Since logs rotate to a new name, tail -F doesn’t really do anything. You’ll still need to ctrl-c the tail and restart it for the next day.

tail -f /pgdata/log/postgresql-$(date +%a).log

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