Replace only if string exists in current line

Question

I have a line such as:

sed -i \'s/mystring/newstring/\' $target

This command will change all mystring to newstring.

Answer 1

This is from the sed one-liners page:

OPTIMIZING FOR SPEED: If execution speed needs to be increased (due to large input files or slow processors or hard disks), substitution will be executed more quickly if the "find" expression is specified before giving the "s/.../.../" instruction. Thus:

sed 's/foo/bar/g' filename         # standard replace command
sed '/foo/ s/foo/bar/g' filename   # executes more quickly
sed '/foo/ s//bar/g' filename      # shorthand sed syntax

Speed is not the issue of the question at hand, but the syntax hints help formulating the solution:

sed -i '/searchstring/ s/mystring/1/; s/mystring/0/' $target