Substitution

Basic substitution — replace first occurrence per line

echo 'hello world hello' | sed 's/hello/goodbye/'

Global substitution — replace all occurrences per line

echo 'hello world hello' | sed 's/hello/goodbye/g'

Replace nth occurrence — third match only

echo 'a:b:c:d:e' | sed 's/:/-/3'

Alternate delimiter — useful when pattern contains slashes

echo '/usr/local/bin' | sed 's|/usr/local|/opt|'

Case-insensitive substitution (GNU sed)

echo 'Hello HELLO hello' | sed 's/hello/goodbye/Ig'

Backreference swap — reverse two captured groups

cat <<'EOF' > /tmp/names.txt
Modestus, Evan
Smith, John
Garcia, Maria
EOF
sed 's/\([^,]*\), \(.*\)/\2 \1/' /tmp/names.txt

Delete lines matching a pattern

cat <<'EOF' > /tmp/config.txt
# This is a comment
server=10.50.1.20
# Another comment
port=443
timeout=30
EOF
sed '/^#/d' /tmp/config.txt

Strip trailing whitespace from every line

printf 'hello   \nworld  \n' | sed 's/[[:space:]]*$//'

Replace only on lines matching a condition

cat <<'EOF' > /tmp/hosts.txt
10.50.1.20  ise-01   active
10.50.1.50  ad-dc    standby
10.50.1.1   pfsense  active
EOF
sed '/standby/s/ad-dc/ad-dc-backup/' /tmp/hosts.txt

Chain multiple substitutions with -e

echo 'foo bar baz' | sed -e 's/foo/FOO/' -e 's/baz/BAZ/'