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: wherever it occurs in the line.
sed 's/DEBUG: /NOTICE: /' trace.txtIncorrect. It replaces the prefix instead of deleting it.
Incorrect. It replaces the prefix instead of deleting it.
sed 's/DEBUG: $//' trace.txtIncorrect. The pattern requires DEBUG: at the end of a line rather than at the start.
Incorrect. The pattern requires DEBUG: at the end of a line rather than at the start.
sed 's/^DEBUG: //' trace.txtCorrect. The empty replacement removes only a line-start DEBUG: prefix.
Correct. The empty replacement removes only a line-start DEBUG: 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
- Command output formatting can vary slightly by distribution or tool version.
- Run the example in a temporary directory or process when possible.
Foundation review
Read the scope first
Check whether the command acts on the current shell, a new process, an existing process, or a file.
Verify the observable result
Use the supplied command and compare the output with the expected result.