Especially when accessing logfiles or large configfiles you often look for a particular pattern in it.

Everybody working with Linux shell usually appreciates grep.
I will just give you a short overview before getting around with the really worth knowing things:

All lines that match contain “EE” in general.log or mylog.log

grep -i 'd' general.log mylog.log

To lists the names of all files in the current directory whose contents mention “EE”.

grep -l 'EE' *.log


grep -lv lists the names of all files containing one or more lines that do not match. To list the names of all files that contain no matching lines, use the -L or --files-without-match option.

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You also can use regular expressions if needed.

For exclusively displaying lines starting with the string “root” just type:

grep ^root /etc/passwd

A cool addon to grep is egrep which can be used like sed (which shouldn’t be an issue here) to find & manipulate at a single blow.

This should delete all comments in the apache config.

egrep '^[^#]' /etc/apache2/apache2.conf

At least this is hardly correct, to match the comments it needs a bit more because they are most times followed by some whitespaces.
Effectively remove those lines:

egrep -v '^ *(#|$)' /etc/apache2/apache2.conf

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If you’re a fan of ruby you might like this.
In ruby you need only request that your string be tested against the regular expression:

ruby -pe 'next unless $_ =~ /regex/' < test.txt

For instance to get every line of test.txt that contains a time in the given format (HH:MM:SS):

ruby -pe 'next unless $_ =~ /(^|\s)[0-9]{2}:[0-9]{2}(:[0-9]{2})(\s)/' < test.txt

To print only lines of 30 characters or greater:

ruby -pe 'next unless $_.chomp.length >= 30' < test.txt

It’s just the beginning of easy matching with ruby, as you might expect,there are many more undreamed-of possibilities.