replace names in fasta - bash

Question

I want to change the sequence names in a fasta file according a text file containing new names. I found several approaches but seqkit made a good impression, anyway I can´t get it running. Replace key with value by key-value file
The fasta file seq.fa looks like
>BC1
ATGCATGCATGCATGCATGCATGCATGCATGCATGCATGC
ATGCATGCATGCATGCATGCATGCATGCATGCATGCATGC
>BC2
TGCATGCATGCATGCATGCATGCATGCATGCATGCATGCG
GCATGCATGCATGCATGCATGCATGCATGCATGCG
>BC3
GCATGCATGCATGCATGCATGCATGCATGCATGCCCCCCC
TGCATGCATGCATG
and the ref.txt tab delimited text file like
BC1 1234
BC2 1235
BC3 1236
using siqkit in Git Bash runs trough the file but doesn´t change the names.
seqkit replace -p' (.+)$' -r' {kv}' -k ref.txt seq.fa --keep-key
I´m used to r and new to bash and can´t find the bug but guess I need to adjust for tab and _ ?
As in the example https://bioinf.shenwei.me/seqkit/usage/#replace part 7. Replace key with value by key-value file the sequence name is tab delimited and only the second part is replaced.
Advise how to adjust the code?
Desired outcome should look like: Replacing BC1 by the number in the text file 1234
>1234
ATGCATGCATGCATGCATGCATGCATGCATGCATGCATGC
ATGCATGCATGCATGCATGCATGCATGCATGCATGCATGC
>1235
TGCATGCATGCATGCATGCATGCATGCATGCATGCATGCG
GCATGCATGCATGCATGCATGCATGCATGCATGCG
>1236
GCATGCATGCATGCATGCATGCATGCATGCATGCCCCCCC
TGCATGCATGCATG

Answer 1

could you please try following.
awk '
FNR==NR{
a[$1]=$2
next
}
($2 in a) && /^>/{
print ">"a[$2]
next
}
1
' ref.txt FS="[> ]" seq.fa
Explanation: Adding detailed explanation for above code.
awk ' ##Starting awk program here.
FNR==NR{ ##FNR==NR is condition which will be TRUE when 1st Input_file named ref.txt will be read.
a[$1]=$2 ##Creating an array named a whose index is $1 and value is $2 of current line.
next ##next will skip all further statements from here.
} ##Closing BLOCK for FNR==NR condition here.
($2 in a) && /^>/{ ##Checking condition if $2 of current line is present in array a and starts with > then do following.
print ">"a[$2] ##Printing > and value of array a whose index is $2.
next ##next will skip all further statements from here.
}
1 ##Mentioning 1 will print the lines(those which are NOT starting with > in Input_file seq.fa)
' ref.txt FS="[> ]" seq.fa ##Mentioning Input_file names here and setting FS= either space or > for Input_file seq.fa here.
EDIT: As per OP's comment need to add >1234_1 occurrence number too in output so adding following code now.
awk '
FNR==NR{
a[$1]=$2
b[$1]=++c[$2]
next
}
($2 in a) && /^>/{
print ">"a[$2]"_"b[$2]
next
}
1
' ref.txt FS="[> ]" seq.fa

Answer 2

awk solution that doesn't require GNU awk:
awk 'NR==FNR{a[$1]=$2;next}
NF==2{$2=a[$2]; print ">" $2;next}
1' FS='\t' ref.txt FS='>' seq.fa
The first statement is filling the array a with the content of the tab delimited file ref.txt.
The second statement prints all lines of the second files seq.fa with 2 fields given the > as field delimiter.
The last statement prints all lines of that same file.