This EX200 practice question tests your understanding of essential tools. 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 'enabled' with 'disabled' in /etc/ssh/sshd_config, but only on lines that do not start with '#'. Which sed command accomplishes this?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
sed '/^#/!s/enabled/disabled/g' /etc/ssh/sshd_config
Option A is correct because it uses an address range `/^#/!` to negate lines starting with `#` (comments), then applies the substitution `s/enabled/disabled/g` only to non-comment lines. The `!` operator inverts the match, so the command acts on lines that do NOT match the pattern, which is exactly what the requirement specifies.
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 '/^#/!s/enabled/disabled/g' /etc/ssh/sshd_config
Why this is correct
Correctly skips comment lines and replaces all occurrences on non-comment lines.
Related concept
Read the scenario before looking for a memorised answer.
✗
sed 's/enabled/disabled/g' /etc/ssh/sshd_config
Why it's wrong here
This replaces on all lines, including comments.
✗
sed -n '/^#/!s/enabled/disabled/gp' /etc/ssh/sshd_config
Why it's wrong here
The -n suppresses auto-print, and 'p' prints only changed lines, but it does not modify the file; it prints to stdout.
✗
sed '/^#/s/enabled/disabled/g' /etc/ssh/sshd_config
Why it's wrong here
This applies the substitution only on lines starting with '#', which is the opposite of what is needed.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates may confuse the `!` negation operator with the `-n` suppress-print option, or mistakenly apply the substitution to commented lines instead of non-commented lines, leading to incorrect configuration changes.
Detailed technical explanation
How to think about this question
The `sed` address `/^#/!` uses the `!` operator to invert the match, meaning the subsequent command is executed only on lines that do NOT match the pattern. The `g` flag ensures all occurrences on a line are replaced, not just the first. In real-world use, editing `/etc/ssh/sshd_config` often requires preserving comments for documentation, so this pattern is essential for safe automated configuration changes.
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 small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
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.
Essential Tools — This question tests Essential Tools — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: sed '/^#/!s/enabled/disabled/g' /etc/ssh/sshd_config — Option A is correct because it uses an address range `/^#/!` to negate lines starting with `#` (comments), then applies the substitution `s/enabled/disabled/g` only to non-comment lines. The `!` operator inverts the match, so the command acts on lines that do NOT match the pattern, which is exactly what the requirement specifies.
What should I do if I get this EX200 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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Question Discussion
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