$ cat test.txt
123
456
789
abc
def
$ cat test.txt | sed -e "/789/d"
123
456
abc
def

$ cat test.txt
123
456
abc
def
$ cat test.txt | sed -e "/456/a 789"
123
456
789
abc
def

$ cat test.txt
123
456
abc
def
$ cat test.txt | sed -e "/456/i aaa"
123
aaa
456
abc
def

$  cat test.txt
Defaults          requiretty
$ cat test.txt |sed 's/Defaults.*requiretty/#Default\trequiretty/g'
#Default        requiretty

sed -i '/disable/ s/yes/no/' /etc/xinetd.d/tftp"

If it is enclosed by \(target character string \), it is partially reused such as \1, \2, \3, … of the replaced character string

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