sed is a linux command that you can use to modify or filter text file. using sed command we can delete specific lines from the file also we can replace the string or text from the file. in the below examples i will show you how i used different types of sed commands with decodingdevops.txt file to replace the strings.
$ cat decodingdevops.txt

1 docker python docker
2 ant shell bash
3 gradle ant shell
4 linux gitlab github
5 maven github linux
6 git shell docker
7 jenkins devops jfrog
8 aws mavne ant
9 nagios splunk datadog
10 devops ant linux
11 docker python git

Delete the lines with SED command

Example 1:

if you want to print the file by deleting first line from the file you can use below sed command. it will not delete the line from the file. just it will print the file by removing that particular line.

$ sed '1d' decodingdevops.txt

2 ant shell bash
3 gradle ant shell
4 linux gitlab github
5 maven github linux
6 git shell docker
7 jenkins devops jfrog
8 aws mavne ant
9 nagios splunk datadog
10 devops ant linux
11 docker python git

Example 2:

to remove particulars lines from the file we use below sed command

$ sed '1,5d' decodingdevops.txt

6 git shell docker
7 jenkins devops jfrog
8 aws mavne ant
9 nagios splunk datadog
10 devops ant  linux
11 docker python git

in above example it will print the file by removing the lines from 1 to 5. i,e it will print remaining lines in the file, here those are 6 to 11

Print the lines with SED command

Example 3:

by using below sed command we can print only particular lines from the file.

$ sed -n '1,5p' decodingdevops.txt

1 docker python docker
2 ant  shell bash
3 gradle ant shell
4 linux gitlab github
5 maven github linux

in above example it printed the lines from only 1 to 5

Remove the line with particular string using SED  Command

in the blow example it will print the file by removing the line which is having the docker string

so we have docker string in 1,6,11 lines it will remove those lines and print the remaining file.

$ sed '/docker/d' decodingdevops.txt

2 ant  shell bash
3 gradle ant shell
4 linux gitlab github
5 maven github linux
7 jenkins devops jfrog
8 aws mavne ant
9 nagios splunk datadog
10 devops ant  linux

Print The line with Particular string using SED  Command

it will print the lines with which is having docker strings

$ sed -n -r '/docker/p' decodingdevops.txt

1 docker python docker
6 git shell docker
11 docker python git

this command works exacly like linux grep command.

$  grep docker decodingdevops.txt

1 docker python docker
6 git shell docker
11 docker python git

Linux Sed Command Substitution

Replace a string in a file In linux with SED Command

$ sed 's/docker/dfd/' decodingdevops.txt

1 dfd python docker
2 ant  shell bash
3 gradle ant shell
4 linux gitlab github
5 maven github linux
6 git shell dfd
7 jenkins devops jfrog
8 aws mavne ant
9 nagios splunk datadog
10 devops ant  linux
11 dfd python git

This only replace first occurrence of the string in each line. if the same docker strings are there more than 1 time in any line it will replace only first occurrence of docker with 'dfsd' . and this is case sensitive, if you mention DOCKER it will not replace.

Replace all occurrences of strings

by adding some extra flags we can replace all occurrences of strings

$ sed 's/docker/dfd/g' decodingdevops.txt

1 dfd python dfd
2 ant  shell bash
3 gradle ant shell
4 linux gitlab github
5 maven github linux
6 git shell dfd
7 jenkins devops jfrog
8 aws mavne ant
9 nagios splunk datadog
10 devops ant  linux
11 dfd python git

here you can see i added g in the command which represents global. so by using above command we can replace all occurrences of stings

for case insensitive you mention i flag in the command

$ sed 's/DOCKER/dfd/gi' decodingdevops.txt

1 dfd python dfd
2 ant  shell bash
3 gradle ant shell
4 linux gitlab github
5 maven github linux
6 git shell dfd
7 jenkins devops jfrog
8 aws mavne ant
9 nagios splunk datadog
10 devops ant  linux
11 dfd python git

Remove a String from the file using SED Command

To remove a particular string from the entire file we can use below sed command

$ sed 's/DOCKER//gi' decodingdevops.txt

1  python
2 ant  shell bash
3 gradle ant shell
4 linux gitlab github
5 maven github linux
6 git shell
7 jenkins devops jfrog
8 aws mavne ant
9 nagios splunk datadog
10 devops ant  linux
11  python git

just dont mention anything in the second string place.

Replace A string in file using sed command in specific lines

$ sed -r '1,+5 s/docker/dfgfdf/gi' decodingdevops.txt

1 dfgfdf python dfgfdf
2 ant  shell bash
3 gradle ant shell
4 linux gitlab github
5 maven github linux
6 git shell dfgfdf
7 jenkins devops jfrog
8 aws mavne ant
9 nagios splunk datadog
10 devops ant  linux
11 docker python git

Here it will replace the 'docker' string in lines from the 1st to 6(1+5=6) with 'dfgfdf'