Replace first occurrence in line
sed 's/old/new/' file.txt
Replace all occurrences (global)
sed 's/old/new/g' file.txt
Edit file in-place
sed -i 's/old/new/g' file.txt
macOS requires -i '':
sed -i '' 's/old/new/g' file.txt
Delete lines containing pattern
sed '/pattern/d' file.txt
Delete empty lines
sed '/^$/d' file.txt
Delete line number 5
sed '5d' file.txt
Delete lines 5-10
sed '5,10d' file.txt
Print only lines containing pattern
sed -n '/pattern/p' file.txt
Replace on specific line (line 3 only)
sed '3s/old/new/' file.txt
Add line after match
sed '/pattern/a New line here' file.txt
Multiple substitutions
sed -e 's/foo/bar/g' -e 's/hello/world/g' file.txt
Use different delimiter (useful with paths)
sed 's|/old/path|/new/path|g' file.txt
Capture groups
Replace "John Doe" with "Doe, John":
sed 's/\([A-Z][a-z]*\) \([A-Z][a-z]*\)/\2, \1/' file.txt