A nightly job must run /usr/local/sbin/backup.sh and send mail to root only when the backup fails. Which line does that?
LPIC-1 Exam 102-500, objective 105. Shells and shell scripting medium
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The options
Correct /usr/local/sbin/backup.sh || echo "backup failed" | mail -s "backup" root
Correct. The right-hand side of || runs only when the left-hand command exits non-zero, so mail is sent on failure and nothing is sent on success.
Not correct /usr/local/sbin/backup.sh && echo "backup failed" | mail -s "backup" root
Wrong, and inverted. && runs the right-hand side only after a zero exit status, so root would be told the backup failed exactly when it succeeded.
Not correct /usr/local/sbin/backup.sh ; echo "backup failed" | mail -s "backup" root
Wrong. A semicolon only sequences the commands. The mail goes out every night whatever the backup did.
Not correct /usr/local/sbin/backup.sh | mail -s "backup" root
Wrong. A pipe connects the backup's standard output to mail's standard input, so mail is always sent and its body is whatever the script printed, regardless of the exit status.
Why
&& and || are short-circuit list operators that branch on the exit status of the command to their left: && continues on zero, || continues on non-zero. A pipe wires output to input and ignores exit status entirely, which is the distinction this item turns on. A pipeline binds tighter than the list operators, so the echo and the mail together form the right-hand operand of ||.
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 105 and space the ones you get wrong.
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