Hi sidpatil,
I believe gacanepa already explained how the paste command portion of the AND list:

Code:

cp file1 tmp.$$ && paste -d " " tmp.$$ /dev/null file2 > file1 && rm tmp.$$

works.

If I had known the name of the script that Bhrigu was creating (assume scriptname for demonstration purposes), I would have chosen the name scriptname.$$ instead of tmp.$$ to make it more obvious what script created the temp file. Note that using scriptname.$$ rather than something like scriptname.tmp (or myAux) allows multiple copies of this script to be run concurrently without interfering with each other (assuming that the files being processed were parameterized rather than hard-coded into the script).

The AND list:

Code:

command1 && command2 && command3

executes command1. If and only if it completes successfully (terminates with exit code 0), then command2 will be run. And, if and only if, command2 also completes successfully, then command3 will be run. In this case, if the cp command completes successfully, we have a backup copy of file1. If the paste completes successfully, file1 has been replaced with the contents of file2 appended to the ends of the lines from the original file1. And if both of them succeed, we remove the backup copy of the initial contents of file1. So, if the cp fails, file1 has not been changed. If cp succeeds and paste fails, file1 can be returned to its original contents by copying the backup file back into file1.

This could also be done by:

Code:

paste -d " " file1 /dev/null file2 > tmp.$$ && mv tmp.$$ file1

but the permissions and ownership of file1 might change, and if there were any hard links to file1 when this script started, they would no longer be linked to file1 if this script completes successfully.

To parameterize the script and make sure that it was invoked with two pathname operands, I would tend to write a slightly longer version of this script:

Code:

#!/bin/ksh
IAm=${0##*/}     # basename of the path used to invoke this script
if [ $# -ne 2 ]  # verify arg count
then    printf "Invalid number of operands.\nUsage: %s path1 path2\n" "$IAm" >&2
fi
tmpf="$IAm.$$"   # create temp file name

# Do the work...
cp "$1" "$tmpf" && paste -d " " "$tmpf" /dev/null "$2" > "$1" && rm "$tmpf"

I usually use the Korn shell, but any shell that accepts basic POSIX conforming shell syntax (including, but not limited to, bash and ksh) will be just fine for this simple script.