sed (stream editor) basics in Linux CLI - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

What is the output of this sed command?

Given a file data.txt with content:

apple
banana
cherry
banana
apple

What is the output of:

sed 's/banana/orange/' data.txt

Linux CLI

sed 's/banana/orange/' data.txt

A

apple
orange
cherry
orange
apple

B

apple
orange
cherry
banana
apple

C

apple
banana
cherry
banana
apple

D

apple
orange
cherry
cherry
apple

💡 Hint

Remember, sed replaces only the first match per line by default.

The command replaces the first occurrence of 'banana' with 'orange' on each line. Only the second line has 'banana' as the first occurrence, so only that line changes.

💻 Command Output

intermediate

What does this sed command output?

Given file.txt content:

cat
bat
rat
mat

What is the output of:

sed -n '/at/p' file.txt

Linux CLI

sed -n '/at/p' file.txt

A

tam
tar
tab
tac

B

cat
bat
rat

C

at
bat
rat
mat

D

cat
bat
rat
mat

💡 Hint

The -n option suppresses automatic printing; /pattern/p prints matching lines.

The command prints only lines matching 'at'. All lines contain 'at', so all lines print exactly as in the file.

📝 Syntax

advanced

Which sed command correctly deletes lines 2 to 4?

You want to delete lines 2, 3, and 4 from a file using sed. Which command is correct?

A sed '2,4d' filename

B sed '2-4d' filename

C sed '2;4d' filename

D sed '2d4d' filename

💡 Hint

Ranges in sed use a comma, not a dash or semicolon.

The correct syntax for a range is 'start,end' followed by the command. '2,4d' deletes lines 2 through 4.

🔧 Debug

advanced

Why does this sed command fail?

You run:

sed 's/\(foo\)\(bar\)/\2\1/' file.txt

but it outputs the original text unchanged. Why?

Linux CLI

sed 's/\(foo\)\(bar\)/\2\1/' file.txt

A The command is correct; the file has no 'foobar' to match.

B The backreferences \1 and \2 are not recognized; sed needs single backslashes.

C The pattern does not match because the parentheses are escaped incorrectly.

D The replacement syntax is wrong; it should use $1 and $2 instead of \1 and \2.

💡 Hint

Check if the input file contains the pattern 'foobar'.

The command syntax is correct. If the file does not contain 'foobar', no substitution occurs, so output is unchanged.

🚀 Application

expert

How to swap two words on each line using sed?

You want to swap the first two words on each line of a file using sed. Which command achieves this?

A sed 's/\b(\w+) (\w+)\b/\2 \1/' filename

B sed 's/(\S+) (\S+)/\2 \1/' filename

C sed 's/\([^ ]*\) \([^ ]*\)/\2 \1/' filename

D sed 's/\(\w+\) \(\w+\)/\2 \1/' filename

💡 Hint

sed uses basic regular expressions; \w and \b are not supported by default.

Option C uses basic regex supported by sed: \([^ ]*\) matches first word, \([^ ]*\) second word, then swaps them.