Question

I would like to have a shell script that searches two files and returns a list of strings:

File A contains just a list of unique alphanumeric strings, one per line, like this:

accc_34343
GH_HF_223232
cwww_34343
jej_222

File B contains a list of SOME of those strings (some times more than once), and a second column of infomation, like this:

accc_34343 dog
accc_34343 cat
jej_222 cat
jej_222 horse

I would like to create a third file that contains a list of the strings from File A that are NOT in File B.

I’ve tried using some loops with grep -v, but that doesn’t work. So, in the above example, the new file would have this as it’s contents:

GH_HF_223232
cwww_34343

Any help is greatly appreciated!

Answer 1

Here’s what you can do:

grep -v -f <(awk '{print $1}' file_b) file_a > file_c

Explanation:

• grep -v : Use -v option to grep to invert the matching
• -f : Use -f option to grep to specify that the patterns are from file
• <(awk '{print $1}' file_b): The <(awk '{print $1}' file_b) is to simply extract the first column values from file_b without using a temp file; the <( ... ) syntax is process substitution.
• file_a : Tell grep that the file to be searched is file_a
• > file_c : Output to be written to file_c