Find & Replace Multiple Sequence Headers in Multiple FASTA Files - bash

Question

Here's my problem (using a Mac OS X):
I have about 35 FASTA files with 30 sequences in each one. Each FASTA file represents a gene, and they all contain the same individuals with the same sequence headers in each file. The headers are formatted as "####_G_species," with the numbers being non-sequential. I need to go through every file and change 4 specific headers, while also keeping the output as 35 discrete files with the same names as their corresponding input files, preferably depositing the outputs into a separate subdirectory.
For example: Every file contains a "6934_Sergia_sp," and I need to change
every instance of that name in all of the 35 files to "6934_R_robusta." I need to do the same with "8324_Sergestes_sp," changing every instance in every file to "8324_P_vigilax." Rinse and repeat 2 more times with different headers. After changing the headers, I need to have 35 discrete output files with the same names as their corresponding input files.
What I've found so far that seems to show the most promise is from the following link:
https://askubuntu.com/questions/84007/find-and-replace-text-within-multiple-files
using the following script:
find /home/user/directory -name \*.c -exec sed -i "s/cybernetnews/cybernet/g" {} \;
Changing the information to fit my needs, I get a script like this:
find Path/to/my/directory -name \*.fas -exec sed -i 's/6934_Sergia_sp/6934_R_robusta/g' {} \;
Running the script like that, I get and "undefined label" error. After researching,
https://www.mkyong.com/mac/sed-command-hits-undefined-label-error-on-mac-os-x/
I found that I should add '.fas' after -i giving:
find Path/to/my/directory -name \*.fas -exec sed -i '.fas' 's/6934_Sergia_sp/6934_R_robusta/g' {} \;
because on Macs you need to specify an extension for the output files. Running the script like that, I get very nearly what I'm looking for with each input file being duplicated, the correct header in each being correctly substituted for the new name, and the outputs being placed in the same directory. However, this only substitutes one header at a time, and the output files have a .fas.fas extension.
Moving forward, I would have to rename the output files to remove the second " .fas " in the extension, and rewrite and rerun the script 3 more times to get everything changed how I want it, which wouldn't be the end of the world, but definitely wouldn't be ideal.
Is it possible to set up a script so that I can run all 4 substitutions at the same time, while also exporting the outputs to a new subdirectory?

Answer 1

Your approach is good, but I would prefer a more verbose approach where I don't have to fight so much with the quotes. Something like:
for fasta in $(find Path/to/my/directory -name "*.fas")
do
new_fasta=$(basename $fasta .fas).new.fas
sed 's/6934_Sergia_sp/6934_R_robusta/g; s/Another_substitution/Another_result/' $fasta > $new_fasta
done
Here, you fed the list of FastA file to loop over, you compute a new fasta name (and location, if needed), and finally run sed over the input and leave the output in a new file. Observe that you can give more than one substitution in sed, separated by semicolons.
BTW, as #Ed Morton said, for the next question please, include a concise description of the problem and sample input and expected output.