hosts.txt mixes host names with raw IP addresses. Which command uses a sed address to print ONLY the lines that begin with a digit, each line exactly once?
LPIC-1 Exam 101-500, objective 103. GNU and Unix commands medium
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The options
Correct sed -n '/^[0-9]/p' hosts.txt
Correct. /^[0-9]/ is a regular expression address selecting lines whose first character is a digit, p prints them, and -n switches off the automatic printing so nothing else appears.
Not correct sed '/^[0-9]/p' hosts.txt
Wrong. Without -n sed still auto-prints every input line, so non-matching lines appear once and the matching lines appear twice.
Not correct sed -n '/^[0-9]/d' hosts.txt
Wrong twice over: d deletes the matching lines rather than printing them, and -n has already suppressed the automatic output, so this command prints nothing at all.
Not correct sed -n '/[0-9]$/p' hosts.txt
Wrong anchor. $ anchors the match to the END of the line, so this prints lines that finish with a digit instead of lines that start with one.
Why
A sed script is address{,address}command. An address can be a line number, $ for the last line, or a /regex/ that selects every matching line. ^ anchors a basic regular expression to the start of the line and $ to the end, and [0-9] is a bracket expression matching any one digit. The -n plus p idiom is how you make sed behave like grep: -n suppresses the automatic print of the pattern space and p prints it explicitly.
Where this comes from
- Cited
- LPI exam objective 103.7
- What it says
- Use regular expressions with sed to select lines in a text stream.
Practise this
Reading one question is not practice. The trainer will draw a set from objective 103 and space the ones you get wrong.