Parsing df Output Reliably: Avoid Human-Readable Pitfalls

Answer analysis

Option-by-option breakdown

For each option: why learners choose it and why it is or isn't the right answer here.

• ✗

  Use 'df -h / | tail -1 | sed 's/.* //' | tr -d '%'

  Why it's wrong here

  Option A is fragile because it parses the human-readable output from 'df -h' with sed. The formatting of 'df -h' can vary across systems; for example, the percentage field may not be the last field, or there may be multiple spaces, making 's/.* //' unreliable. Additionally, using 'tail -1' assumes the output has only two lines, which may not hold if there are multiple mount points in /? This approach is not portable.

• ✗

  Use 'df -h / | tail -1 | cut -d' ' -f5'

  Why it's wrong here

  This option also uses `-h` and attempts to use `cut` with space delimiter. However, `df -h` output may have multiple spaces or different column ordering, so `cut -d' ' -f5` may not always extract the correct field. This is unreliable for portability.

• ✓

  Use 'df / | awk 'NR==2 {print $5}' | tr -d '%'

  Why this is correct

  This option uses `df /` without `-h`, which produces a stable, machine-parseable output. The `awk` command extracts the fifth field (the percentage used) and `tr` removes the percent sign. This is portable across different Unix/Linux systems.

  Related concept

  Read the scenario before looking for a memorised answer.

• ✗

  Use 'df -h / | grep -oP '\d+%'

  Why it's wrong here

  This option uses `df -h` and `grep` with a Perl regex to extract digits followed by '%'. While it may work on some systems, the human-readable output can vary, and the regex might not match all cases (e.g., if the percentage is like '100%' or has a leading space). It is less reliable than the approach in C.

Detailed technical explanation

How to think about this question

The `df` command without `-h` outputs fixed-width columns with the percentage in the 5th field, making it safe for `awk` parsing. The `-h` flag triggers human-readable formatting that can vary between systems (e.g., different units, alignment), breaking scripts that rely on column positions. In real-world automation, using `df --output=pcent /` (on GNU coreutils) is even more portable, but `df / | awk 'NR==2 {print $5}'` is a classic POSIX-compliant pattern.

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