Save space for in-place sed with zstd

zstd-header-change.sh

# compress $FILE with zstdmt
# remove $FILE
# print the header, decompress $FILE.zst afterwards and write them into $FILE
# delete $FILE.zst
FILE=myfile; zstdmt $FILE -f && rm $FILE && echo NEW_HEADER | cat - <(unzstd -c $FILE.zst | tail -n +2) > $FILE && rm $FILE.zst

zstd-sed.sh

# compress $FILE with zstdmt
# remove $FILE
# decompress $FILE.zst and perform the sed operation (substituting "FROM" to "TO")
# delete $FILE.zst
FILE=myfile; zstdmt $FILE -f && rm $FILE && unzstd -c $FILE.zst | sed 's/FROM/TO/' > $FILE && rm $FILE.zst