abc,abc,keepline,target,abc,abc
abc,abc,skipline,target,abc,abc

abc,abc,keepline,replace,abc,abc
abc,abc,skipline,target,abc,abc

sed 's/target/replace' file.csv > newfile.csv

% sed '/skipline/!s/target/replace/g' input
abc,abc,keepline,replace,abc,abc
abc,abc,skipline,target,abc,abc

$ cat ip.txt
abc,abc,keepline,target,abc,abc
abc,abc,skipline,target,abc,abc

$ sed '/skipline/! s/target/replace/' ip.txt
abc,abc,keepline,replace,abc,abc
abc,abc,skipline,target,abc,abc

awk '!/skipline/ {gsub(/target/,"replace")}1' input
    abc,abc,keepline,replace,abc,abc
    abc,abc,skipline,target,abc,abc

awk '!/skipline/ {gsub(/target/,"replace")}1' input >input.tmp && mv input.tmp input

i=abc,abc,keepline,target,abc,abc
if [[ ! $i =~ "skipline" ]] ; then
  i=${i//target/replace}
fi
echo $i
abc,abc,keepline,replace,abc,abc

i=abc,abc,skipline,target,abc,abc
if [[ ! $i =~ "skipline" ]] ; then
  i=${i//target/replace}
fi
echo $i
abc,abc,skipline,target,abc,abc