This program scans within a list of folders passed in certain log (or any) files and tails them on current process' stdout. tail_folders listens to changes in the directory being monitored and when it finds a matching file, it will start tailing it. Files and folders (nested to one of the initial folders being tracked) can be created after tail_folders is started and it will automatically detect process them accordingly.

A use case for this tool is to create a docker image with this tool and to put a docker logging driver plugin on it. By running tail_folders as PID 1 in this container, its stdout/stderr will be sent to the desired log system. This way you can easily aggregate log files generated from other containers running on the same host by just creating volumes for folders where the logs are written and to share these volumes with the tail_folders container.

tail_folders also can start a process and it can tail a log file (or some) generated by the process. Note that tail_folders should be configured with the right parameters in order to scan the target folder where the process will create their log file/s. When tail_folders is launched in this way, it will end as soon as the process ends and the exit code of tail_folders will be the one from the process.

A use case for this could be to scope a program to be used in OpenFaas. There are times when you have a process that write garbage along with the actual output to stdout. Some log warnings, some legacy code that is difficult to change are examples of it. tail_folders can scope the program and will output from its stdout the content of a log file where the process will only write the actual information that needs to be sent.

Available parameters:

  -content_filter string
        Filter expression to apply on tailed lines
  -content_filter_by string
        Content filter type: Either 'include', 'exclude', 'regex' or 'no-filter' (default "no-filter")
  -discard-files-older-than int
        Discard tailing files not recently modified (seconds) (default -1)
  -filter string
        Filter expression to apply on filenames (default "*.log")
  -filter_by string
        Expression type: Either 'glob' or 'regex' (default "glob")
  -folders string
        Paths of the folders to watch for log files, separated by comma (,). IT SHOULD NOT BE NESTED (default ".")
  -output string
        Output type: Either 'raw' or 'json' (default "json")
  -recursive
        Whether or not recursive folders should be watched (default true)
  -tag string
        Optional tag to use for each line
  -timeout int
        Time to wait till stop tailing when no activity is detected in a folder (seconds) (default -1)
  -version
        Print the version

tail_folders generates a log file that is found in working_dir/.logdir/taillog.log. Note: if tail_folders starts a new process, the stdout/stderr of that process will be written to tail_folders's log.

JSON output follows this data structure:

// Entry models a line read from a source file
type Entry struct {
      // Tag is user-provided setting for different tail_folders processes running
      // in a single host
      Tag string `json:"tag,omitempty"`
      // Hostname is the hostname where tail_folders is running
      Hostname string `json:"host,omitempty"`
      // Folders is a list of folder names where the source file is
      Folders []string `json:"dirs,omitempty"`
      // Filename is the base filename of the source file
      Filename string `json:"file,omitempty"`
      // File is the whole filepath. This field is for internal use only
      File string `json:"-"`
      // Message is the actual payload read from the source file
      Message string `json:"msg,omitempty"`
      // Timestamp is the time where the log is read
      Timestamp time.Time `json:"time,omitempty"`
}

So an example of a output line can be:

{"host":"MacBook-Pro.local","dirs":["tmp"],"file":"hola.log","msg":"aaaa","time":"2019-05-05T20:26:59.596488+02:00"}

tail_folders offers settings to control how to deal with old log files that are not expected to receive more log data:

• timeout

This setting establishes an activity monitor on each subfolder that is aware of files being written. When there is no write in any file during the timeout amount of time, then active tail processes are killed. This is interesting when you do not expect that log files to be written any more

• discard-files-older-than

During initial scan for files that matches the provided filter, this settings allows to not track files which modification time is older than the discard-files-older-than amount

./tail_folders -tag=node1 -folders="./aa,./bb"

You can now create folder ./aa and ./bb. Then you can touch ./aa/app_1.log and touch ./bb/app_2.log.

Then open two new terminal sessions and execute while true; do echo "aaaa" >> ./aa/app_1.log; sleep 1; done and while true; do echo "bbbbbbbb" >> ./bb/app_2.log; sleep 1; done in each.

And the stdout of tail_folders should be similar to:

...
[node1] [bb/app_2.log] bbbbbbbb
[node1] [aa/app_1.log] aaaa
[node1] [aa/app_1.log] aaaa
[node1] [bb/app_2.log] bbbbbbbb
[node1] [aa/app_1.log] aaaa
...

Do not forget to CTRL-C both while true processes and remove the files and folders.

This sample assumes the process will write its log into current folder and the log file will have the .log extension. If it is not the case, please use the parameters to tweak the configuration.

./tail_folders -folders /tmp -recursive=false -output raw -- ./app.sh
[/tmp/test.log] aaaa
[/tmp/test.log] aaaa
[/tmp/test.log] aaaa