Clearing Process Logs

Haskell, But Quickly

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I’m writing a big hairy database import from a legacy system and not all of the records contain valid data. It’s important to me to see when any of the rows from the old datastore fail model validations so I can either fix my model logic, or insert corrected data later.

My workflow is basically to run the import, view logged exceptions in a tmux split, fix them, and repeat. I found it painful to have to clear the log/screen every time I wanted to rerun the import, so I automated it.

I’ve structured my import into a collection of Rake tasks so each table can be imported individually (after importing its respective dependencies). I have an additional task for running all of the import tasks, and this additional task depends on a task I use for truncating the log file.

task import: [:clear_import_log] + setup do
  Rake.application.tasks.each do |task|
    task.invoke if"import:")

task :clear_import_log do
  path = "#{File.dirname(__FILE__)}/../import.log"
  log =, "w+")
  log.puts "file truncated"

When I open the log file in the clear_import_log task, I use the w+ which truncates the existing file to zero length, or creates a new file if one doesn’t exist.

To actually clear the screen when I tail the log, I need to employ a touch more trickery. I wrote a bash script which clears the screen when it encounters the line “file truncated”, or prints the line to the screen in any other case.


set -e

while read LINE; do
  case "$LINE" in
    "file truncated")
      echo "$LINE"

In my tmux split, I tail the log file and pipe the output to my bash script, so now whenever I restart the database import, the screen clears and I see fresh information.

tail -F -n0 log/import/import.log | script/