A configuration file is to be displayed with every empty line and every line that begins with # in the first column removed. Select the TWO commands that do this.
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
Choose 2.
Correct grep -Ev '^(#|$)' /etc/ssh/sshd_config
Correct. -E enables alternation without backslashes, ^(#|$) matches a line starting with # or a line with nothing in it, and -v inverts the selection.
Correct sed -E '/^(#|$)/d' /etc/ssh/sshd_config
Correct. The same pattern used as a sed address, with the d command deleting the matching lines from the output stream.
Not correct grep -E '^(#|$)' /etc/ssh/sshd_config
Wrong. Without -v this prints exactly the comments and blank lines that were to be discarded.
Not correct sed '/^#/d' /etc/ssh/sshd_config
Wrong, though half right. Comments go, but empty lines survive because nothing in the address matches them.
Not correct grep -v '^#|^$' /etc/ssh/sshd_config
Wrong. In a basic regular expression | is an ordinary character, so the pattern only matches a line consisting of the three characters #|^, and virtually everything is printed.
Why
Alternation is the clearest illustration of the basic and extended split: in an extended regular expression | separates alternatives and ( ) group them, while in a basic one those characters are literal unless escaped as \| and \( \). The other half of the item is that grep selects lines while sed transforms a stream, so an inverted match in grep and a d command in sed reach the same result. Note that both patterns anchor # to column one; leading whitespace before a comment would need ^[[:space:]]*#.
Where this comes from
- Cited
- manual page grep(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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