CompTIA Linux+ XK0-006 (V8) Practice Question

The here-string operator <<< is a shell redirection that expands its word and delivers the resulting single string (with a terminating newline) to the standard input of the preceding command. Therefore tr '[:lower:]' '[:upper:]' <<< "$DATA" meets every requirement: only tr is executed, no temporary files are created, and the variable's contents are provided via a here string.

<< "$DATA" is the opening token for a here-document; Bash would interpret the variable's value as the end-delimiter and wait for additional input, so it does not pass the variable to tr.

< "$DATA" redirects the contents of a file named by the expansion of $DATA; unless the variable contains a valid filename this fails and still violates the "no temporary file" rule.

echo "$DATA" | tr … works functionally but invokes the extra command echo, which the constraints forbid, and it uses a pipeline rather than the operator designed for feeding a short string to standard input.

Hence only the command that employs the <<< here-string is correct.