What are the differences among grep, awk & sed?

> Possible Duplicate:
> What are the differences between Perl, Python, AWK and sed?
> What is the difference between sed and awk?

Maybe not a very specific question, but I am confused about the differences between grep, awk and sed in terms of their role in Unix/Linux system administration and text processing.

Short definition:

grep: search for specific terms in a file

#usage
$ grep This file.txt
Every line containing "This"
Every line containing "This"
Every line containing "This"
Every line containing "This"

$ cat file.txt
Every line containing "This"
Every line containing "This"
Every line containing "That"
Every line containing "This"
Every line containing "This"

Now awk and sed are completly different than grep. awk and sed are text processors. Not only do they have the ability to find what you are looking for in text, they have the ability to remove, add and modify the text as well (and much more).

awk is mostly used for data extraction and reporting. sed is a stream editor
Each one of them has its own functionality and specialties.

Example
Sed

$ sed -i 's/cat/dog/' file.txt
# this will replace any occurrence of the characters 'cat' by 'dog'

Awk

$ awk '{print $2}' file.txt
# this will print the second column of file.txt

Basic awk usage:
Compute sum/average/max/min/etc. what ever you may need.

$ cat file.txt
A 10
B 20
C 60
$ awk 'BEGIN {sum=0; count=0; OFS="\t"} {sum+=$2; count++} END {print "Average:", sum/count}' file.txt
Average:    30

I recommend that you read this book: Sed & Awk: 2nd Ed.

It will help you become a proficient sed/awk user on any unix-like environment.

Grep is useful if you want to quickly search for lines that match in a file. It can also return some other simple information like matching line numbers, match count, and file name lists.

Awk is an entire programming language built around reading CSV-style files, processing the records, and optionally printing out a result data set. It can do many things but it is not the easiest tool to use for simple tasks.

Sed is useful when you want to make changes to a file based on regular expressions. It allows you to easily match parts of lines, make modifications, and print out results. It's less expressive than awk but that lends it to somewhat easier use for simple tasks. It has many more complicated operators you can use (I think it's even turing complete), but in general you won't use those features.

I just want to mention a thing, there are many tools can do text processing, e.g. sort, cut, split, join, paste, comm, uniq, column, rev, tac, tr, nl, pr, head, tail.....

they are very handy but you have to learn their options etc.

A lazy way (not the best way) to learn text processing might be: only learn grep , sed and awk. with this three tools, you can solve almost 99% of text processing problems and don't need to memorize above different cmds and options. :)

AND, if you 've learned and used the three, you knew the difference. Actually, the difference here means which tool is good at solving what kind of problem.

a more lazy way might be learning a script language (python, perl or ruby) and do every text processing with it.