GREP FUNCTION: EASY SEARCH COMMAND

 Friends,

     Now we are trying to explain a Linux command which helps you a lot in your researches. Its nothing but a simple command called as GREP. It is a powerful command which helps you to search a particular string or words from a file or from a folder and also in some cases it helps during trace file analysis.

SYNTAX

Here the syntax is;

grep <option> <string> <filename>

Here, this command search string within the file named as filename. If it is not given, then the command waits for the input from the keyboard and search for string.  For more about this Click here 

There are two important notes for <string>

• If it contains a white space,  embrace it with quotation marks ("")
• If it contains a special character such as “ or &, you can suppress it using backslashes (\” or \&)  to indication that this is a character

For example, If u need to search a word "apple" in a file named as word.txt. Then;

grep apple word.txt

In case of searching words in sub directories, Simply write as; 

                                                                      grep -r apple *

If you want to look a function is available with ns2 folder or not, simply you can search by using this command. For example if you want to find function  className::rt_resolve(ReturnType variableName)

You can find this function using grep:  grep -r ::rt_resolve( *

In case of searching a string with special character "&hdr_aodv", we can search it as;

 grep -r \&hdr_aodv *

if you’re interested only in the ns-2.35/aodv directory. Now ypu can search by using this:

grep rt_resolve aodv/*.*

Now you can filter the search by providing  the files extension along with the search.For example;

grep rt_resolve *.h  searches rt_resolve in .h files only.

While in the case of trace file analysis we can use it as follows;

cat out.tr | grep " 2 3 cbr " | grep ^r | column 1 10 | awk '{dif = $2 - old2; if(dif==0) dif = 1; if(dif > 0) {printf("%d\t%f\n", $2, ($1 - old1) / dif); old1 = $1; old2 = $2}}' > jitter.txt

This means that the "CBR packet receive" event at n3, selects time (column 1) and sequence number (column 10), and calculates the difference from last packet receive time divided by difference in sequence number (for loss packets) for each sequence number. i .e  for selecting particular value we can use this.