Normally we just using GREP in following way:

# grep 'ERROR' /var/log/project.log
# cat project.log | grep 'ERROR'

That’s ok, but usually we do need more in real work.

Let’s say,

1. We want to count ERROR in log file:

# grep "ERROR" /var/log/project.log | wc -l
# grep -c "ERROR" /var/log/project.log 

We are also able to search word start with ERROR or end with ERROR by:

# grep "\<ERROR" /var/log/project.log
# grep "ERROR\>" /var/log/project.log

We were trying to search word ERROR, but infact previous command can retrieve lines contains ERROR, that means if there are words ERROR1, ERROR2, ERROR3 …

They also counted, that’s not we want.

2. So we need search by word:

We just want to see ERROR, not ERROR1, ERROR2 ….

# grep -w 'ERROR' /var/log/project.log

Locate the line where contains ERROR, that’s great!! but I’m not able to see the error details, so I need see the lines AHEAD and BELOW, to get more informaion. Let’s say,

3. Check 10 lines around matched search criteral

# grep -C 10 'ERROR' /var/log/project.log

I can just view the lines Ahead or Below for sure:

# grep -A 10 'ERROR' /var/log/project.log  # ahead
# grep -B 10 'ERROR' /var/log/project.log  # below

And Don’t forget

4. we can use RegExpression too.

If I want to check the Thread-1 to Thread-8, I can do :

# grep 'Thread-[1-8]' /var/log/project.log

we can also use extended regexp

like locate IP address:

# grep -E '\b[0-9]{1,3}(\.[0-9]{1,3}){3}\b' /var/log/project.log

Another senario is when you working with folder, there may a lot of different file types: .java, .xml, .properties, .conf …

we may need do

5. Regression Grep

#  grep -rl 'SOMETHING' .

But my maven project, I just need find things in pom.xml. The quickest way is to

6. Integrate with FIND will be much more effecient

#  find ~/project-dir -name pom.xml -exec grep 'SOMETHING' {} +

7. Invert Match

Invert Match is also quite helpful, for example, we don’t want to see comment in Java file

#  with lowcase -v
#  grep -v "#" ~/some.project/src/main/java/Test.java

8. Multiple Patterns

We may want to search multiple patterns at one time, -E will work, and aslo we can use egrep :

# grep -E "foo|bar" *.txt
# egrep "foo|bar" *.txt

9. Search in Compressed Files

Usually our log files are backup to compressed gzip files, zgrep will help us:

# zgrep -i error /var/log/syslog.2.gz