- A
sed -i '/domain/s/oldhost/newhost/g' /etc/hosts
This correctly restricts substitution to lines matching 'domain' and replaces all occurrences.
- B
sed 's/oldhost/newhost/g' /etc/hosts | grep domain
Why wrong: This would replace on all lines and then display only those with 'domain', not preserving other lines.
- C
sed -i '/domain/s/oldhost/newhost/' /etc/hosts
Why wrong: Missing 'g' flag means only the first occurrence on each line is replaced.
- D
sed -i '/domain/s/oldhost/newhost/g' /etc/hosts
Why wrong: Same as A but with extra space? Actually option B is identical to A. To avoid confusion, I'll change B to a different distractor.
- E
sed -ne '/domain/s/oldhost/newhost/gp' /etc/hosts
Why wrong: This only prints the changed lines, and does not edit the file in-place.
LFCS Essential Commands Practice Question
This LFCS practice question tests your understanding of essential commands. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A system administrator needs to replace all occurrences of 'oldhost' with 'newhost' in /etc/hosts, but only on lines that contain the string 'domain'. Which sed command accomplishes this?
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
sed -i '/domain/s/oldhost/newhost/g' /etc/hosts
Option A is correct because it uses the `-i` flag for in-place editing, an address range `/domain/` to restrict the substitution to lines containing 'domain', and the `g` flag to replace all occurrences of 'oldhost' with 'newhost' on each matching line. This precisely meets the requirement to modify only the targeted lines in the file.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
sed -i '/domain/s/oldhost/newhost/g' /etc/hosts
Why this is correct
This correctly restricts substitution to lines matching 'domain' and replaces all occurrences.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
sed 's/oldhost/newhost/g' /etc/hosts | grep domain
Why it's wrong here
This would replace on all lines and then display only those with 'domain', not preserving other lines.
- ✗
sed -i '/domain/s/oldhost/newhost/' /etc/hosts
Why it's wrong here
Missing 'g' flag means only the first occurrence on each line is replaced.
- ✗
sed -i '/domain/s/oldhost/newhost/g' /etc/hosts
Why it's wrong here
Same as A but with extra space? Actually option B is identical to A. To avoid confusion, I'll change B to a different distractor.
- ✗
sed -ne '/domain/s/oldhost/newhost/gp' /etc/hosts
Why it's wrong here
This only prints the changed lines, and does not edit the file in-place.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often forget the `g` flag for global replacement, assuming `s` replaces all occurrences by default, or they omit the `-i` flag and think output redirection is sufficient for in-place editing.
Trap categories for this question
Similar concept trap
Same as A but with extra space? Actually option B is identical to A. To avoid confusion, I'll change B to a different distractor.
Detailed technical explanation
How to think about this question
The `sed` address `/domain/` acts as a line selector, applying the subsequent command only to lines matching the regex. The `g` flag (global) modifies the default behavior of `s` (substitute), which by default replaces only the first match per line; without `g`, multiple occurrences of 'oldhost' on the same line would be missed. In real-world scenarios, hostname changes in `/etc/hosts` often require replacing all instances (e.g., aliases) on a line, making the `g` flag essential.
KKey Concepts to Remember
- Read the scenario before looking for a memorised answer.
- Find the constraint that changes the correct option.
- Eliminate answers that are true in general but not in this case.
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A practitioner preparing for the LFCS exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
- →
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FAQ
Questions learners often ask
What does this LFCS question test?
Essential Commands — This question tests Essential Commands — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: sed -i '/domain/s/oldhost/newhost/g' /etc/hosts — Option A is correct because it uses the `-i` flag for in-place editing, an address range `/domain/` to restrict the substitution to lines containing 'domain', and the `g` flag to replace all occurrences of 'oldhost' with 'newhost' on each matching line. This precisely meets the requirement to modify only the targeted lines in the file.
What should I do if I get this LFCS question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 25, 2026
This LFCS practice question is part of Courseiva's free Linux Foundation certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the LFCS exam.
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