Using Shell Variables in awk and sed

For sed - use double quotes around the variable.

sed '/'"$pattern"'/d' filename

Another option is to enclose entire sed command in double quotes.

sed "/$pattern/d" filename

For awk - use combination of double quotes single quotes double quotes

awk '{ if ($(1) == "'"$pattern"'" ) {print $2}}'

Optionally, you could create an awk variable from shell variable.

awk '{ if ($(1) == ipattern ) {print $2}}' ipattern=$pattern filename

Router Transformation Groups

Router Transformation's Groups are not the same as CASE statement conditions or what you may also call waterfall logic.

Router transformation routes data to all the groups that evaluate to TRUE unlike the CASE statement that executes the code only for the first true condition.

Awk one liner IF

Here is a quick one liner AWK for field comparison.

awk -F, '{if ($(1)=="0") {print $1}}' myfilename

What does it do?
From the comma separated file every record with first field as zero is printed.

It's important to keep parentheses as indicated. Although "print $1" is good; "if $1" poops.

Python - All Set for Comparison

I spent entire evening looking for a way to sort the characters in a string and figured it out.

1. Turn the String into List. 
2. Sort it. 
3. Join it back as a String.

And then, I stumbled upon a better way to do string comparison. Convert them to sets!!!

set(stringA).difference(stringB)

Note that it may not be same as
set(stringB).difference(stringA)

Details on set theory here:

http://docs.python.org/library/sets.html#set-objects

Discussion here:

http://stackoverflow.com/questions/1388818/how-can-i-compare-two-lists-in-python-and-return-matches

BASH shell only - brace expansion

You might have used brace expansion if you have tried this very popular way of backing up files - 

cp filename{,.bk}

The command above will copy filename to filename.bk. It saves you from writing the filename again. 

This neat stuff is not compatible with korn shell.

Some other examples to reinforce how this works.

echo John{Doe,Smith}
  JohnDoe JohnSmith
echo {John,Jane}{Doe,Smith}
  JohnDoe JohnSmith JaneDoe JaneSmith
  

Note the lack of space after comma.

vi or sed - Delete till end of line from pattern

Here's the regex that will match everything from the pattern until a newline is found.

Say you want to remove everything after "here" from this sample file.

code here    where?
code here   right here!
code here         look every where.

Used regex

s/here.*//

Explanation:

.*  

dot matches any character including spaces, numbers, alphabets but not new line.

asterisk matches any number of occurrences of a character

s/here.*/meow/ will result in

code meow
code meow
code meow




Shell script for multiple SED edits

Following shell script

1.    Reads the names of the files to be edited from the file "filelist".

2.    Executes two sed statements on each of those files and

3.    Overwrites the original file. 

#!/usr/bin
while read line
do
sed 's/0512/0505/g' $line > temp
sed 's/05\/13/05\/06/g' temp > $line
done < filelist