I have a list of folders from which I have to delete .dec files. How to delete from them at one time. Any for loop or something has to be used?

Solutions:

for loop or while loop:

cat filename | while read line
do
cd $line
rm -f *.dec
done

Let says, you have folder:

/tmp

and subfolders /tmp/test1 and /tmp/test1

content in your filename looks like this:

/tmp/test1
/tmp/test2

while loop will read line, and cd $line which means cd /tmp/test1 and rm -f *.dec in /tmp/test1, next, read next line

Do it on test env first, or best advise, cp -rp /tmp/test1 /tmp/test1.bkup, just in case you screw up.
 

I need a script which basically deltes all files in folder a which are alreasy present in folder b

Say folder a has files 

Code:
abc.txt
pqr .txtand b has 

Code:
abc.txt
pqr.txt
rmr.txt

then file abc.txt and pqr.txt from a should be deleted

Solutions:

Check if this heps you get started: 

Code:
for file in /path/tp/b/*
do
    echo check if $file exists in directory a
done

or

Code:

for file in `ls  folderb`
do
find  foldera -type f -name "$file" -exec rm -f {} \;
done

or

You can also try diff command to get listing.

Code:
-bash-3.2$ diff fol1 fol2 | grep fol1
Only in fol1: b.txt
Only in fol1: c.txt
-bash-3.2$

or 

Code:
for i in $(ls -1 /home/pritish/a);do
        for j in $(ls -1 /home/pritish/b);do
                if [ $i ==  $j ]; then
                        $(rm -i /home/pritish/a/$i)
                fi
        done
done

or

Code:
for i in `ls dir1`;do
if [ -f dir2/$i ]; then
rm dir1/$i
fi
done