Remove reads from FASTQ file based on missing fixed base

Question

[c.heininger]

Hello everybody, I have a question regarding processing raw FASTQ files based on a specific UMI approach.

Basically, we employed a strategy to our paired-end sequencing experiment, where we use 6nt UMIs in our library. Following the UMI sequence is a fixed base, that is the same for every R1 and every R2 (different between R1 and R2 of course).
This results in most reads having the same base at position 7 (confirmed with FastQC).

An example of a read I want to keep is this:

@NB551018:503:HV5CTAFX2:1:11101:9572:1046 1:N:0:ACNTGA   
TTATTNACTAGCTGCGTTCTTCATCGACGCACGAGCCGAG   
+  
A/AA/#EEEEEEAEAEEEEEEEEEAEEEEEAEE//EEEEE  

Here the 7th base is an A, which marks this as a read that is to be kept.

An example of read to be discarded is this:

@NB551018:503:HV5CTAFX2:1:11101:14783:1047 1:N:0:ACNTGA  
GCGACNCTATCCACCCAAAGGATAAACATTTATCATACCA  
+  
AAA/A#EAEEEEEEEA<AEEEEEAEEAAEEEE/AEEEEAE  

The 7th base here is a C, meaning I want to discard this read.

My question now is: How do I remove all reads that do not meet the condition of this "fixed 7th base"? Is there a good way to do this in Linux (maybe with grep, awk...?) I am not yet very well versed in using Linux and it's built in tool for manipulating and processing files, hence the question.
Or ist there a specialized tool, that can do this, that I am not aware of?

I am very grateful for any help!
-Chris

edited: Added examples for what data looks like and what I want to keep/discard.

[cpad0112]

If you could post example data including UMI sequences and expected output, for few records, that would help in understanding the issue. You can use cutadapt to do the job.

[c.heininger]

I edited the post, thank you. Though I do not think cutadapt can do exactly this?

[cpad0112]

Try following code:

with seqkit:

$ seqkit grep -srip ^.{6}A file.fq

with cutadapt, try this:

$ cutadapt  -g ^"N{6}A" --discard-untrimmed --action=retain --quiet file.fq

Since data is PE, use appropriate command for PE. This is for SE.