Print file name in front of grep matched pattern output lines

Question

I want to print line content of a matched pattern from a input file to a output file. I am using:

    grep -hnr "pattern" ./input.txt > output.txt 

Output.txt file is like:

this line has the word pattern in it 

But I want to print name of the input file in the same line in output file. For example, I want the output file to be like:

input this line has the word pattern in it 

If the pattern matches multiple times in input file, I want to print filenames multiple times. For example:

input this line has the word pattern in it 
input this line also has same pattern

and finally, I want to do this in a directory with many input files (*.txt format) and generate one output.txt files with all results.

Answer 1

Simply use :

 grep -H "pattern" ./input.txt > output.txt 

From man grep :

-H, --with-filename
Print the file name for each match. This is the default when there is more than one file to search.

-h, --no-filename
Suppress the prefixing of file names on output. This is the default when there is only one file (or only standard input) to search.