A colleague needs to search a file for the literal string `1.2.3.4` and is worried the dots will match any character. Which command treats the pattern as a fixed string with no regular expression meaning at all?
LPIC-1 Exam 101-500, objective 103. GNU and Unix commands medium
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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
Not correct grep -x '1.2.3.4' file
Wrong. -x requires the pattern to match the entire line. Again the dots keep their regular expression meaning.
Not correct grep -E '1.2.3.4' file
Wrong. -E selects extended regular expressions, in which an unescaped dot still matches any single character. It makes the problem no better.
Not correct grep -w '1.2.3.4' file
Wrong. -w requires the match to fall on whole-word boundaries. The pattern is still interpreted as a regular expression, so the dots remain wildcards.
Correct grep -F '1.2.3.4' file
Correct. -F (the same as the historical fgrep) treats the pattern as fixed text, so every character including the dots is literal.
Why
grep -F (fgrep) disables regular expressions entirely and is also the fastest mode. grep -E (egrep) selects extended regular expressions. Plain grep uses basic regular expressions. The separate fgrep and egrep binaries still exist on most systems but are deprecated in favour of the -F and -E options.
Where this comes from
- Cited
- LPI exam objective 103.7
- What it says
- Use grep, egrep and fgrep appropriately.
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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