GREP stands for Global Regular Expression Print. I think that every sysop loves grep, grepping and anything that has something in common with grep – this tool makes our lives really easier ;) If You’re not convinced than I think You’re in a good place – maybe the following text will convince You :)

1. Excluding irrelevant words: sometimes We have to grep for some word but We have to exclude some irrelevant string. E.g. let’s grep for ‘index.html’ but let’s also exclude ’404′ from this:

   grep 'index.html' access.log | grep -v 404

2. egrep (extended grep, same as grep -e or grep --regexp=) allows us to do more powerful search including regular expressions with metacharacters like +, ?, | and ()

   egrep "html|cgi" access.log

3. Counting results - If we just want to know the number of lines that matched our query - We would use:

   grep -c 'index.html' access.log

4. Case Insensitive search - by default grep is case sensitive, If we want to make case insensitive search than we use:

   grep -i 'Index' access.log

5. Matching eXact word only - by default grepping for Word will return lines containing SomeWord and . If We would like to find only those lines containing exact word Word We should use:

   grep -x '404' access.log

grep -w could be also useful here.

6. Matching left and right side of the word - to search for instances of string matching Word in the end or start We use \< or \>:

   Below would match any word starting with access, like access_entry:

   grep 'error\>'

   Below would match any word ending with error, like general_error:

   grep '\<access'

7. Showing context results - sometimes We would like to grep for some errors in logs, but we also would like to view the context of that log entry - e.g. grepping for 'Relay access denied' in Postfix logs to see If that error is occurring with some pattern:

   grep --context=3 'Relay access denied' maillog

8. zgrep - this one would grep in the compressed gzip file - just like gunzip -c flog.gz | grep Word:

   zgrep 'Relay access denied' maillog3.gz

9. Coloring matched words - We can highlight our matched words with some color (check man page to see how to set exact color):

   grep --color 'Relay access denied' maillog3.gz