How does the “tail” command’s “-f” parameter work?

$ tail -f testfile

the command is supposed to show the latest entries in the specified file, in real-time right? But that’s not happening. Please correct me, if what I intend it to do is wrong…

I created a new file “aaa” and added a line of text and closed it. then issued this command (first line):

$ tail -f aaa
xxx
xxa
axx

the last three lines are the contents of the file aaa. Now that the command is still running (since I used -f), I opened the file aaa via the GUI and started adding a few more lines manually. But the terminal doesn’t show the new lines added in the file.

What’s wrong here? The tail -f command only shows new entries if they are written by system only? (like log files etc)

Answers:

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Method 1

From the tail(1) man page:

   With  --follow  (-f),  tail  defaults to following the file descriptor,
   which means that even if a tail’ed file is renamed, tail will  continue
   to  track  its  end.   This  default behavior is not desirable when you
   really want to track the actual name of the file, not the file descrip-
   tor (e.g., log rotation).  Use --follow=name in that case.  That causes
   tail to track the named file  in  a  way  that  accommodates  renaming,
   removal and creation.

Your text editor is renaming or deleting the original file and saving the new file under the same filename. Use -F instead.

Method 2

Your editor has its own buffer for the file. When you modify the text in the editor, nothing is written to the file itself.

When you save your changes, chances are that the editor simply deletes the old file and create a new one. tail -f will still be connected to the deleted file, so it won’t show anything new.

Method 3

tail “refresh” each 1 second by default, not realtime.

Try with this (you need bash4):

• Open 2 terminals.
• In the first terminal execute touch ~/output.txt and tail -f ~/output.txt.
• In the second terminal execute for i in {0..100}; do sleep 2; echo $i >> ~/output.txt ; done
• Look at the output of tail in the first terminal.

All methods was sourced from stackoverflow.com or stackexchange.com, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0