How do you use sed for text processing?

sed (stream editor) processes text line by line. Most common use: substitution: sed 's/old/new/' file replaces the first occurrence per line; sed 's/old/new/g' replaces all. sed -i 's/foo/bar/g' file edits the file in-place (add a backup suffix: sed -i.bak). Delete lines: sed '/pattern/d' file. Print specific lines: sed -n '5,10p' file. Insert before a line: sed '/pattern/i\new line'. Append after: sed '/pattern/a\new line'. Multiple expressions: sed -e 's/a/b/' -e 's/c/d/'. Use sed -n to suppress default output and only print explicitly with p.