Use lines in a file as filenames for grep?

Question

I have a file which contains filenames (and the full path to them) and I want to search for a word within all of them. some pseudo-code to explain:

grep keyword <all files specified in files.txt>

or

cat files.txt > grep keyword
cat files txt | grep keyword

the problem is that I can only get grep to search the filenames, not the contents of the actual files.

Answer 1

cat files.txt | xargs grep keyword

or

grep keyword `cat files.txt`

or (equivalent to previous but harder to mis-read)

grep keyword $(cat files.txt)

should do the trick.

Pitfalls:

• If files.txt contains file names with spaces, either solution will malfunction, because "This is a filename.txt" will be interpreted as four files, "This", "is", "a", and "filename.txt". A good reason why you shouldn't have spaces in your filenames, ever.

  • There are ways around this, but none of them is trivial. (find ... -print0 / xargs -0 is one of them.)

• The second (cat) version can result in a very long command line (which might fail when exceeding the limits of your environment). The first (xargs) version handles long input automatically; xargs offers several options to control the details.

Answer 2

Both of the answers from DevSolar work (tested on Linux Ubuntu), but the xargs version is preferable if there may be many files, since it will avoid running into command line length limits.

so:

cat files.txt | xargs grep keyword

is the way to go