Command Line – How Does ‘tail -f’ Parameter Work

command linemonitoringtail

$ 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)

Best Answer

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.

Related Solutions

How to tail/grep/awk the last N bytes of a file, rather than lines

Using the -c switch (2MB = 2 * 1024 * 1024 = 2097152 bytes):

tail -c 2097152 myapp.log

Thanks to Petr Uzel for the suggestion. Some tail implementations allow to add a unit to print the last kilobytes (k) or megabytes (m), like:

tail -c 2m myapp.log

However please note that it is not standard (in none of POSIX, UNIX (SUS) or Linux (LSB)) and not portable. Also note that since the terms "Mega"/"kilo"... and their abbreviations (M, k...) have ambiguous meanings (1000 vs 1024), there's not much guarantee of what this or that implementation of tail will mean by 2m (though the current versions of the current implementations that do support it seem to be going for the 1024 variant).

View only the new entries in a growing log file

You can try:

tail -n0 -f x.log

From man page:

-n, --lines=K
output the last K lines, instead of the last 10; or use -n +K to output lines starting with the Kth

Best Answer

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How to tail/grep/awk the last N bytes of a file, rather than lines

View only the new entries in a growing log file

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