Grep is one of the most powerful utility we have with UNIX Shell. This is used for various purposes like finding a text in a file or finding files containing certain text in a path. This comes very handy when scanning through large log files for specific information.
Overview
The description for grep says, it prints lines that matches certain pattern.
The normal grep usage looks like this.
1grep 'something' /path/to/logfile
Finding lines before a keyword
This will find all lines containing word something in the file /path/to/logfile. However, when scanning for errors in log files, it’s usually important to be able to see what happened before the error or even in certain other conditions it may be important to see what were the lines before this line. In these situations, we can pass -B or --before-context flag to see given number of lines before certain keyword in a file.
Let’s create sample server log file using numbers, so that we can see how these options work.
1for i in {1..100}
2do
3 echo "This is line $i"
4done > server.log
Verify the number of lines in the file.
cat server.log | wc -l # 100
If we check some content of the file,
head -n 5 server.log
This is line 1
This is line 2
This is line 3
This is line 4
This is line 5
Now, let’s suppose, I am looking for word 50 and I want to see 4 lines before this as well. I can run below.
cat server.log | grep -B 4 50
The same thing can be achieved like this.
grep -B 4 50 server.log
Output:
This is line 46
This is line 47
This is line 48
This is line 49
This is line 50
Finding lines after a keyword
Similarly, to find lines after a certain keyword, we can use -A or --after-context flag with grep command.
Let’s see 3 lines after line containing word 60.
grep -A 3 60 server.log
Output:
This is line 60
This is line 61
This is line 62
This is line 63
Conclusion
I have used these commands quite often when digging through logs, especially those with distributed systems containing like millions of lines of code to find the root cause of an error. I hope it will be helpful to you in your search through logs.
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