What can you learn from production.log?
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Many developers fail to realize that using the basic Unix tools, you can find on any server, you can find and collect valuable data from your logs. Often the data you need can be found without using any external services, including but not limited to service measurements.
This is a start of a series of posts that will try to show some of the nice things you can achieve with the Rails log from the command line.
Let’s star with the simplest one — following the incoming logs:
tail -f log/production.log
We could of course show the complete log with
cat, but often it is far longer
than we would expect and following the logs tends to be much nicer. On the other
cat can be handy when combined with
The first thing we want to learn is to filter the log from the information we don’t need for the current measurement. Filtering is all about recognizing the patterns in the text and cutting out everything that doesn’t match it. A common thing is to display only the CRUD request logs, or to filter everything out except database actions.
A nice scheme for creating complex filters (and the one we will be using in the examples) is the following:
cat log/production.log | <filters> | tail
In the above command the
cat command pushes the logs into the
filters, and the
tail command shows a portion of the result. An
alternative for the
tail - that shows the end of the results - can
head command that shows the start. By default, both commands
display only 10 results, but can be tweaked to show more with the
option. For example to list the last 20 lines we can use the following
cat log/production.log | <filters> | tail -n 20
But enough with the boring details, let’s define a filter. The following
example shows only incoming
cat log/production.log | grep 'method=POST' | tail -n 20
In the above example we used the
grep command that returns only the lines
that have the provided string or regular expression in them. We can also
grep commands, where the second
grep will only work
on the lines that the first
For example, the following command shows only the
POST requests that had
a HTTP status
200 as their answer:
cat log/production.log | grep 'method=POST' | grep 'status=200' | tail -n 20
Similarly to the previous ones, we can easily show only the logs that were created yesterday:
cat log/production.log | grep 'method=POST' | grep 'status=200' | grep 'time=2015-02-10' | tail -n 20
As the final example I will demonstrate something more complicated. We will list only the POST requests that had a duration longer than 10 milliseconds. Unfortunately, we can’t use the previous technique anymore because the string that we are matching are different on each line (somewhere the duration is 13 milliseconds, somewhere it is 114 milliseconds…).
Fortunately, we can use regular expressions with the
What we need is actually more than one digit in the duration that will
give us a number larger or equal to 10. To match a digit we will use the
[0-9] notation, and to match 0 or more of them we will write
cat log/production.log | grep 'method=POST' | grep 'duration=[0-9]*[0-9][0-9]' | tail -n 20
In the above example we used
[0-9][0-9] to match two digits and
to match the remaining.
I hope you enjoyed this tutorial, I will give my best to finish the next one soon, where I will be talking about counting.