Every run of digits in report.txt is to be wrapped in square brackets, so that `code 42 rev 7` becomes `code [42] rev [7]`. Which command does that?
LPIC-1 Exam 101-500, objective 103. GNU and Unix commands hard
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How these questions are written — where each question comes from, what the verification ledger records, and what happens when one is found wrong.
The options
Correct sed -E 's/[0-9]+/[&]/g' report.txt
Correct. In the replacement, an unescaped & stands for the entire text that the pattern matched, and g applies the rule to every run on the line.
Not correct sed -E 's/[0-9]+/[\1]/g' report.txt
Wrong. \1 refers to the first capture group, and the pattern defines none, so sed rejects the script as an invalid reference.
Not correct sed -E 's/[0-9]+/[$0]/g' report.txt
Wrong. $0 has no special meaning to sed; the output would contain the literal characters [$0] in place of every number.
Not correct sed -E 's/[0-9]+/[&]/' report.txt
Wrong only in reach. Without the g flag the first run of digits on each line is bracketed and any later ones are left alone.
Why
Three things are special on the right-hand side of an s command: & for the whole match, \1 to \9 for the text captured by the corresponding group, and \n or \ escapes. A literal ampersand must therefore be written \&. Capture groups are written ( ) under -E and \( \) without it, so the same substitution in basic syntax would be s/[0-9][0-9]*/[&]/g.
Where this comes from
- Cited
- manual page sed(1)
Practise this
Reading one question is not practice. The trainer will draw a short set from objective 103 and space the ones you get wrong.
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