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In the documentation for logstash, they mention the command tail -0a

I have never heard of the -a flag and I can't find any information on google, tail --help or the man pages.

What is the difference between tail -0a foo.bar and tail foo.bar


After testing this further, I wonder if it is a typo. Neither command works on my mac.

[root@interactive adaptive]# tail -0a /var/log/messages
tail: option used in invalid context -- 0

[root@interactive adaptive]# tail -a0 /var/log/messages
tail: invalid option -- 'a'
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Came here because this site mentioned "tail -0a": logstash.net/docs/1.4.2/tutorials/getting-started-with-logstash –  antak Sep 12 '14 at 1:48

2 Answers 2

up vote 3 down vote accepted

tail -0 is a historical, deprecated way of writing tail -n 0, i.e. print the last 0 lines of the file. (So tail -0 doesn't produce any output.)

I don't know of any tail implementation that has a -a option. From context, it looks like -f was meant. tail -f makes tail keep the file open when it reaches the end, and keep watching forever in case some other program appends more data at the end of the file, and prints this appended data as it comes. So tail -0f starts at the current end of the file, and prints any data that is subsequently appended to the file.

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The -0 means 0 lines, in other words, don't print any existing stuff.

None of the tail man pages mention a -a option. Perhaps they meant -f or -F, which would print any new contents written after tail started.


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