xargs/find/grep a list of files from a list of directories

Question

I have a list of directories in a text file, wherein I want to find and change files that share a particular naming convention.

example file:

dir1
dir2
dir3

example folder structure with files

dir1/
   thing.txt
   thing.blah
dir2/
   rar.txt
   thing.blah
dir3/
   apple.txt
   another.text.file.txt
   thing.blah

First, I find the name of the .txt, but then I want to perform a change on it. For example, i want to perform a sed command on thing.txt, rar.txt and apple.txt, but not another.text.file.txt.

My question is, once I have all the file names, how do I perform a command on the files with those names? How can I then take those lines of text such as:

cat dirFile.txt | xargs ls | grep <expression>.txt 
    thing.txt
    rar.txt
    apple.txt
 !cat | some command

and run an action on the actual files under the directories?

What I'm getting is the above result,

But what I need is

dir1/thing.txt
dir2/rar.txt
dir3/apple.txt

Answer 1

Say you have a file named dirs which have all directories you need to search:

while IFS= read -r i; do
  find "$i" -name '<expression>' -print0
done < dirs | xargs -0 some_command

If you know the directories doesn't have spaces or another type of separators you can simplify a bit:

find $(<dirs) -name '<expression>' -print0 | xargs -0 some_command

Perhaps your some_command expect only one file at a time, in this case use -n1 :

... | xargs -0 -n1 some_command

or move some_command to find itself:

find $(<dirs) -name '<expression>' -exec some_command {} \;

• $(<dirs) is a comand substitution . It reads the content (like cat ) of dirs file and use it as the first arguments of find . Empty dirs is safe on GNU find (eg Linux) but you need at least one line - which is converted as one argument - on BSD (eg Mac OS X)
• -print0 separates files with null
• -0 expects these null chars.
• -n1 says xargs to send only one argument to some_command

Answer 2

I guess my comments were not adequately expressed. to get the output you desire;

cat dirfile.txt | xargs -I % find % -name <your file spec> or -regex <exp>