XK0-006 System Management Practice Question
An administrator wants to replace all occurrences of 'oldstring' with 'newstring' in a configuration file named config.cfg, and save the changes. Which sed command should be used?
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
sed -i 's/oldstring/newstring/g' config.cfg
It uses -i for in-place editing and g to replace all occurrences on each line. Option A has -i but lacks g, so it replaces only the first occurrence per line. Option C adds an unnecessary i flag for case-insensitive matching, which deviates from the requirement to replace 'oldstring' exactly as given. Option D lacks -i, so changes are only printed to stdout and not saved.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
sed -i 's/oldstring/newstring/' config.cfg
Why it's wrong here
Missing g flag, only replaces first occurrence per line.
- ✓
sed -i 's/oldstring/newstring/g' config.cfg
Why this is correct
The -i flag edits config.cfg in place, satisfying the requirement to save changes, and the g flag replaces every occurrence of oldstring on each line with newstring. Omitting g would substitute only the first match per line, leaving later instances unchanged.
- ✗
sed -i 's/oldstring/newstring/gi' config.cfg
Why it's wrong here
The i flag makes matching case-insensitive, so 'OldString' would also be replaced, altering content beyond the stated requirement. Case-insensitivity is tempting for normalising text, but here only exact lowercase 'oldstring' occurrences should change; the g flag alone handles all occurrences.
- ✗
sed 's/oldstring/newstring/' config.cfg
Why it's wrong here
Omitting -i sends the substituted text to standard output, leaving config.cfg untouched on disk. It is tempting because the substitution itself is correct and useful for previewing changes, but saving the file requires the in-place flag.
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