04 April 2021
Return all occurrences of a string (I will be using 'appsignal' as an example) from the log.
grep appsignal log/production.log
Very similar to the first command but pass the -m (max count) option to limit the number of results returned.
grep -m 5 appsignal log/production.log
Returning a single line from a log without any of the logs that occured before or after it isn't particularly useful. Thankfully you can pass the -A or -B options with a number as an argument to also print the lines above or below the matching string. Passing -A 10 will print the matching line as well as the 10 lines preceeding it.
grep -A 10 appsignal log/production.log
While passing -B 10 will print the matching line as well as the 10 lines after it.
grep -B 10 appsignal log/production.log
A shortcut to returning the lines both above and below is to use the -C (context) option which will return both the preceeding and subsequent lines following a match.
grep -C 10 appsignal log/production.log
You may have noticed that the man command defaults to searching from the top of the file, this is not ideal if you want to print out the occurrences of a match that were logged most recently to your file. A quick and easy way to find the most recent matches is essentially to reverse the file and then search through the result. The cat command will dump the contents of your file to STDOUT and the tac command will do exactly the same but from the bottom! We can then pipe this output to grep and search it as we would in the previous examples.
To find the most recent log of your match (appsignal) and display the 5 lines above and below it you can use the following command.
tac log/production.log | grep -m 1 -C 5 appsignal
There are most certainly other ways to do this but I have found this solution to be simple and reliable. I hope it can be useful to you too. Please do drop me a message on twitter if you have any different or better mechanisms for searching logs!