2

I'm not entirely sure how to categorize this question. It feels like I discovered a bug in tail and/or bash, but obviously it's much more likely this is a bug in my understanding!

I ran the following command:

tail -f -n`wc -l scrape.log` scrape.log

in order to cat the entire file and any new material that is added. However, this tailed the file twice. Running this command:

tail -f -n`wc -l scrape.log`

gave me the desired outcome, tailing the file once. I thought backticks are a Bash escape tool, to "preprocess" a command before running the next one. What is going on here?

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3 Answers 3

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9

When you use backticks you're essentially passing the output of one command to another. Now let's see what wc -l does:

[cnicutar@fresh ~]$ wc -l /etc/passwd
11 /etc/passwd

It outputs the number of lines and the filename. So your command becomes:

tail -f -n 11 scrape.log scrape.log
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1 Comment

D'oh! This should have been obvious. Thank you for correcting me.
4

If your tail supports it, you can use

tail -f -n +1 scrape.log

to begin the tail with line 1 of the file.

(I am purposefully not answering your question, which cnicutar has already done. I'm just presenting a possible alternative.)

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Thank you for the tip. I missed this part of the man page, and it is a more straight-forward way of achieving my desired result.
1

Alternative solution

tail -f -n $(wc -l < scrape.log) scrape.log

Input redirection prevents the printing of the file name.

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