For each trace.txt line, remove DEBUG: only when it appears at the beginning.
Do not change DEBUG: appearing inside the message body.
Which command is appropriate?
sed substitution accepts regular-expression anchors. Putting ^ before DEBUG: protects occurrences elsewhere in the line.
Detailed explanation
sed 's/DEBUG: //' trace.txtIncorrect. It removes the first DEBUG: wherever it occurs in the line.
Incorrect. It removes the first DEBUG text wherever it appears, including inside a message.
sed 's/DEBUG: /NOTICE: /' trace.txtIncorrect. It replaces the prefix instead of deleting it.
Incorrect. It changes the text to NOTICE instead of deleting the prefix.
sed 's/DEBUG: $//' trace.txtIncorrect. The pattern requires DEBUG: at the end of a line rather than at the start.
Incorrect. It matches a DEBUG prefix only when it reaches the end of the line.
sed 's/^DEBUG: //' trace.txtCorrect. The empty replacement removes only a line-start DEBUG: prefix.
Correct. The start anchor protects the body and the empty replacement removes the prefix.
Try it yourself
An example you can run in a temporary verification environment.
printf 'DEBUG: start\nmessage DEBUG: keep\nINFO: ok\n' | sed 's/^DEBUG: //'Expected result
start
message DEBUG: keep
INFO: okKey points
- s performs substitution
- ^ restricts the match to line start
- Nonmatching lines pass unchanged
Notes
- Environment: GNU sed 4.x
- Use temporary files and harmless commands for pattern experiments.
- Regular-expression details should be checked against the target GNU implementation.
Foundation review
Place the condition
Anchoring the substitution protects occurrences in the message body while replacing the optional prefix.
Check the result
Include one line with a DEBUG prefix and another with DEBUG in its message body.