Generate a random ASCII file (no spaces, punctuation, just A-Za-z0-9)

tr -dc A-Za-z0-9 </dev/urandom | head -c 1024

Generate a random sequence of numbers, tab separated, seven numbers per line

seq -w 0 999999 | shuf | paste - - - - - - - > tab_separated.txt

Replace tabs with commands

sed 's/\t/,/g' tab_separated.txt > comma_separated.txt
# Equivalent:
cat tab_separated.txt | tr '\t' ,