A script must continue only when /etc/app.conf exists and is a regular file, and must stop if that name is a directory or absent. Which test does exactly that?
LPIC-1 Exam 102-500, objective 105. Shells and shell scripting medium
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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 [ -f /etc/app.conf ]
Correct. -f is true only when the name exists and, after following any symbolic links, refers to a regular file. A directory makes it false.
Not correct [ -e /etc/app.conf ]
Wrong. -e is true for anything that exists, including a directory, a device node or a socket, so it does not exclude the directory case.
Not correct [ -d /etc/app.conf ]
Wrong, and inverted. -d is true precisely when the name is a directory, which is the case the script must reject.
Not correct [ -s /etc/app.conf ]
Wrong. -s tests only that the file exists and has a size greater than zero; it says nothing about the file type, and it rejects a legitimate empty configuration file.
Why
The file-type tests are distinct predicates: -e for existence of any type, -f for a regular file, -d for a directory, -L for a symbolic link, -s for non-zero size, and -r, -w, -x for the caller's access rights. -f already implies existence, so [ -e f ] && [ -f f ] is redundant. Quote the operand when it comes from a variable, because an unquoted empty value collapses the test into a one-argument form that changes its meaning.
Where this comes from
- Cited
- manual page test(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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