Sed command in Linux

Substitution command stream editor for filtering and transforming text

sed 's/as/sd/' <file-name>

sd replace as in file-name output only in first occurrence of line

cat file-name | sed ‘s/as/sd/’

same as above use cat and pipe command in between

Here in this command, we used

s        --> used as substitute command
/        --> used as Delimiter
as        --> search pattern string
sd        --> replacement string

Sometime when we try sed on Unix path, we need to use backward slash(Esc character). Like

sed 's//etc/resolv.conf//etc//dns-settings/’ 

but this create confusion for some users. we can avoid this confusion and replace delimiter with our own choice

sed 's_/etc/resolv.conf_/etc/dns-settings_'

We can also use these settings with global occurrence instead of only first in one line

sed 's/as/sd/g' <file-name>

We can also use these settings for only second occurrence instead of only first in one line

sed 's/as/sd/2' <file-name>

sed 's/as/sd/2g' <file-name>

Second and after second occurrence

sed '/root/d' sudoers

Delete the lines that contain root

sed '3d' sudoers

Delete 3rd line

sed -n '/root/p' sudoers

Print only lines containing root

sed  ‘/^$/d’ suders

Delete blank lines from file