sed substitution in Linux CLI - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the sed command do in Linux?

sed is a stream editor used to perform basic text transformations on an input stream (a file or input from a pipeline).

beginner

Explain the basic syntax of a sed substitution command.

The basic syntax is sed 's/pattern/replacement/' filename. It replaces the first occurrence of pattern with replacement on each line.

beginner

How do you replace all occurrences of a pattern in each line using sed?

Add the g flag at the end: sed 's/pattern/replacement/g' filename. This replaces every match on each line.

beginner

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

It replaces every occurrence of the word cat with dog in each line of file.txt, printing the result to the screen.

intermediate

How can you save the changes made by sed substitution back to the original file?

Use the -i option for in-place editing: sed -i 's/old/new/g' filename. This updates the file directly.

What does the g flag do in a sed substitution?

A Replaces only the first occurrence in the entire file

B Deletes the matching lines

C Replaces all occurrences of the pattern in each line

D Prints the matching lines only

The g flag means global replacement on each line, changing all matches, not just the first.

Which command replaces the first occurrence of 'apple' with 'orange' in each line of fruits.txt?

A sed 's/apple/orange/' fruits.txt

B sed 's/apple/orange/g' fruits.txt

C sed -i 's/apple/orange/g' fruits.txt

D sed 's/apple/orange/1' fruits.txt

Without the g flag, only the first occurrence per line is replaced.

How do you apply sed substitution and save changes directly to the file?

A sed -i 's/old/new/g' filename

B sed 's/old/new/g' filename > filename

C sed 's/old/new/g' filename

D sed -e 's/old/new/g' filename

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

What will this command do? sed 's/blue/red/2' colors.txt

A Replace the second line containing 'blue' with 'red'

B Replace all occurrences of 'blue' with 'red'

C Replace the first occurrence of 'blue' with 'red'

D Replace the second occurrence of 'blue' with 'red' on each line

The number after the substitution command specifies which occurrence to replace on each line.

Which delimiter can you use instead of / in sed substitution?

A *

B #

C &

D %

You can use other delimiters like # to avoid escaping slashes in patterns.

Describe how to use sed to replace all occurrences of a word in a file and save the changes.

Think about the command structure and flags needed.

Explain the difference between using sed 's/pattern/replacement/' and sed 's/pattern/replacement/g'.

Focus on how many matches get replaced per line.