df (disk free) usage: Which option with the df command includes a grand total of disk space across the listed file systems?

Introduction / Context:
The df command summarizes disk capacity and usage across mounted file systems. Some UNIX variants provide an option to display a grand total line that sums the sizes and usage for easier capacity planning. Knowing per-platform flags helps administrators standardize reports and scripts.

Given Data / Assumptions:

• We are using a traditional UNIX variant where df supports a total line option.
• Question asks for the option that includes the total amount of disk space across file systems.
• We are not focusing on filesystem-type filters or inode-only reports.

Concept / Approach:

Historically, some df implementations use a specific flag to add a final total line. On certain UNIX variants, the -t flag provides a total summary line (note that on GNU/Linux, -c is used, and -t filters by type). In exam-style questions tied to classical UNIX, -t is commonly taught as the totalizing flag.

Step-by-Step Solution:

Verification / Alternative check:

On GNU coreutils, df -c provides totals, whereas -t filters by type. On exam contexts referencing traditional UNIX, -t is used for totals. Always check man df on your platform to verify exact semantics.

Why Other Options Are Wrong:

b: Not a standard df option.

c: -4 is not a standard df flag for totals.

d: -i reports inode statistics, not total space lines.

e: Not applicable under the assumed traditional semantics.

Common Pitfalls:

Assuming GNU/Linux semantics on all platforms; confusing total lines with file-system type filters; forgetting that scripting across heterogeneous systems requires conditional flags.

Final Answer:

-t