A nightly backup script should record its progress messages in /var/log/backup.out and its error messages in a separate file, /var/log/backup.err. Which command line does that?
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 backup.sh > /var/log/backup.out 2> /var/log/backup.err
Correct. > (equivalently 1>) redirects descriptor 1, standard output, and 2> redirects descriptor 2, standard error, each to its own file.
Not correct backup.sh 1> /var/log/backup.err 2> /var/log/backup.out
Wrong. The syntax is valid but the descriptors are swapped, so the progress messages land in backup.err and the errors in backup.out.
Not correct backup.sh > /var/log/backup.out 2>&1 > /var/log/backup.err
Wrong. 2>&1 makes standard error follow standard output to backup.out, and the later > only moves standard output on to backup.err. Nothing is left writing to backup.out except the error stream.
Not correct backup.sh &> /var/log/backup.out > /var/log/backup.err
Wrong. &> sends both streams to backup.out, then the trailing > moves standard output to backup.err, leaving errors in backup.out and truncating both files for no useful result.
Why
Every process starts with descriptor 0 for standard input, 1 for standard output and 2 for standard error. Because the two output streams are separate descriptors, they can be sent to different destinations in the same command line, which is the usual way to keep a clean result file and a readable error log. A number in front of > selects the descriptor; > alone means 1>.
Where this comes from
- Cited
- manual page bash(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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