A script must store the number of lines in /var/log/syslog in a variable, so the output has to be the bare number with no file name after it. Which command produces exactly that?
LPIC-1 Exam 101-500, objective 103. GNU and Unix commands 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 wc -l < /var/log/syslog
Correct. The shell opens the file and hands it to wc as standard input, so wc has no file name to report and prints only the count.
Not correct wc -l /var/log/syslog
Wrong. When wc is given a file name as an operand it prints the count followed by that name, so the variable would contain two fields.
Not correct wc --lines /var/log/syslog
Wrong. --lines is only the long spelling of -l; the operand is still a file name, so the name is printed alongside the count.
Not correct wc -L < /var/log/syslog
Wrong unit. -L prints the length of the longest line, not the number of lines, although the redirection does correctly suppress the file name.
Why
Many filters behave differently depending on whether they open a file themselves or read a stream: given operands they label the output with the file name, given standard input they have no name to print. Redirecting with < is therefore the standard trick when the count must be machine-readable, as in `lines=$(wc -l < file)`. The same distinction explains why `cat file | wc -l` also prints a bare number.
Where this comes from
- Cited
- manual page wc(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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