sed (stream editor) basics in Linux CLI - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is sed used for in Linux?

sed is a tool to find and change text in files or input streams automatically. It works like a robot that reads text line by line and edits it based on rules you give.

beginner

How do you replace the first occurrence of 'apple' with 'orange' in each line using sed?

Use: sed 's/apple/orange/' filename. This changes only the first 'apple' in each line to 'orange'.

intermediate

What does the -n option do in sed?

The -n option tells sed to not print anything by default. You then use the p command to print only the lines you want.

beginner

Explain the command: sed '2d' filename

This command deletes the second line from the file output. It reads the file and skips printing line 2.

intermediate

How can you save changes made by sed directly to the file?

Use the -i option: sed -i 's/old/new/g' filename. This edits the file in place without showing output on screen.

What does the command sed 's/cat/dog/' file.txt do?

A Replaces the first 'cat' with 'dog' in each line

B Deletes all lines containing 'cat'

C Prints lines containing 'dog'

D Replaces all 'cat' with 'dog' in the file

The s/cat/dog/ command replaces only the first 'cat' found in each line with 'dog'.

Which option prevents sed from printing all lines by default?

A -n

B -e

C -i

D -p

The -n option tells sed not to print lines unless explicitly told to.

How do you delete the 3rd line of a file using sed?

A sed 'delete 3' filename

B sed 'd3' filename

C sed -n '3d' filename

D sed '3d' filename

The command sed '3d' filename deletes the third line from the output.

What does the g flag do in sed 's/old/new/g'?

A Deletes the line if match found

B Changes only the first match per line

C Changes all matches in each line

D Prints the line if match found

The g flag means 'global', so it replaces all matches in each line, not just the first.

How do you edit a file directly with sed?

A sed -n 's/a/b/' file

B sed -i 's/a/b/' file

C sed 's/a/b/' file > newfile

D sed 's/a/b/' file -o

The -i option edits the file in place, saving changes directly.

Describe how to use sed to replace text in a file and save the changes directly.

Think about the substitution syntax and how to apply changes to the file itself.

Explain how sed processes input and how you can control what lines get printed.

Consider how <code>sed</code> normally prints all lines and how to change that.