You run `sed 's/http:/https:/' urls.txt` and find that on lines holding two URLs only the first was rewritten. Which command fixes it?
LPIC-1 Exam 101-500, objective 103. GNU and Unix commands easy
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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 's/http:/https:/g' urls.txt
Correct. The g flag after the closing delimiter makes the substitution apply to every match on the line rather than only the first.
Not correct sed -g 's/http:/https:/' urls.txt
Wrong. The repetition flag belongs to the s command, not to sed itself; sed has no -g option.
Not correct sed 's/http:/https:/2' urls.txt
Wrong. A numeric flag replaces only the nth match, so this rewrites the second occurrence and leaves the first alone.
Not correct sed -E 's/http:/https:/' urls.txt
Wrong. -E only switches the pattern to extended regular expression syntax; it has no effect on how many matches are replaced.
Why
The s command is written s/regex/replacement/flags, and the flags after the final delimiter control its behaviour: g for every match on the line, a number n for the nth match, gn for the nth onwards, i for case-insensitive matching and p to print the result. Without any flag sed replaces the first match per line only. The delimiter need not be a slash, which is why paths are often rewritten with s|old|new|.
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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