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.

Practice

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

easy

A. Deletes all lines containing 'cat' in file.txt

B. Replaces the first occurrence of 'cat' with 'dog' on each line of file.txt

C. Prints only lines containing 'dog' from file.txt

D. Appends 'dog' at the end of each line containing 'cat' in file.txt

Solution

1. Step 1: Understand the substitution command

   The s/pattern/replacement/ syntax in sed replaces the first match of 'pattern' with 'replacement' on each line.

2. Step 2: Apply to the given command

   The command replaces the first occurrence of 'cat' with 'dog' on every line of file.txt.

3. Final Answer:

   Replaces the first occurrence of 'cat' with 'dog' on each line of file.txt -> Option B

4. Quick Check:

   sed substitution = replace first match [OK]

Hint: Remember: s/pattern/replacement/ changes first match per line [OK]

Common Mistakes:

• Thinking it deletes lines instead of substituting
• Assuming it replaces all occurrences without 'g' flag
• Confusing printing lines with substitution

2. Which of the following is the correct syntax to delete all lines containing the word 'error' using sed?

easy

A. sed '/error/d' filename

B. sed 'd/error/' filename

C. sed 's/error/d/' filename

D. sed 'delete /error/' filename

Solution

1. Step 1: Identify the delete command syntax in sed

   To delete lines matching a pattern, the syntax is sed '/pattern/d'.

2. Step 2: Match the correct option

   sed '/error/d' filename uses sed '/error/d', which deletes lines containing 'error'. Other options have incorrect syntax.

3. Final Answer:

   sed '/error/d' filename -> Option A

4. Quick Check:

   Delete lines with pattern = /pattern/d [OK]

Hint: Use '/pattern/d' to delete matching lines in sed [OK]

Common Mistakes:

• Placing 'd' before the pattern
• Using substitution syntax instead of delete
• Writing 'delete' instead of 'd'

3. What is the output of the command echo 'apple banana apple' | sed 's/apple/orange/g'?

medium

A. orange banana orange

B. orange banana apple

C. apple banana apple

D. apple banana orange

Solution

1. Step 1: Understand the 'g' flag in substitution

   The 'g' flag means replace all occurrences of the pattern on each line, not just the first.

2. Step 2: Apply substitution to the input string

   All 'apple' words are replaced by 'orange', so 'apple banana apple' becomes 'orange banana orange'.

3. Final Answer:

   orange banana orange -> Option A

4. Quick Check:

   Substitute all matches with 'g' flag = replaced all [OK]

Hint: Add 'g' to replace all matches, not just first [OK]

Common Mistakes:

• Forgetting the 'g' flag replaces only first occurrence
• Confusing output with original input
• Misreading the pipe input

4. Identify the error in this sed command: sed 's/hello/goodbye' file.txt

medium

A. No error; command is correct

B. Incorrect command; 's' cannot be used with text files

C. The pattern 'hello' is invalid

D. Missing delimiter after replacement text

Solution

1. Step 1: Check the substitution syntax

   The substitution command requires delimiters: s/pattern/replacement/. The final '/' is missing here.

2. Step 2: Identify the missing delimiter

   The command ends with 's/hello/goodbye' missing the closing '/' delimiter after 'goodbye'.

3. Final Answer:

   Missing delimiter after replacement text -> Option D

4. Quick Check:

   Substitution needs closing '/' delimiter [OK]

Hint: Check that substitution ends with '/' delimiter [OK]

Common Mistakes:

• Omitting the last '/' in substitution
• Assuming 's' can't be used on files
• Thinking pattern must be a regex error

5. You want to replace the word 'cat' with 'dog' only on lines 2 to 4 in a file named pets.txt. Which sed command achieves this?

hard

A. sed 's/cat/dog/ 2,4' pets.txt

B. sed '/2,4 s/cat/dog/' pets.txt

C. sed '2,4s/cat/dog/' pets.txt

D. sed 's/cat/dog/ 2-4' pets.txt

Solution

1. Step 1: Understand address range in sed

   To apply a command only on specific lines, use start,endcommand syntax, e.g., 2,4s/pattern/replacement/.

2. Step 2: Match the correct syntax

   sed '2,4s/cat/dog/' pets.txt correctly uses 2,4s/cat/dog/ to replace 'cat' with 'dog' on lines 2 to 4.

3. Final Answer:

   sed '2,4s/cat/dog/' pets.txt -> Option C

4. Quick Check:

   Use line range before command to limit sed action [OK]

Hint: Use 'start,end' before 's' to limit substitution lines [OK]

Common Mistakes:

• Placing line numbers after substitution command
• Using incorrect range syntax like '2-4'
• Adding slashes around line numbers