How to used the tail command in unix?

I want to use tail -f with sleep and number of lines, to display at a time.

I am using:
tail -f -s 20 -n 5 filename.
This is displaying correct result for 1st time, but as tail -f is appending, I am not able to view 5 lines.

Answer:

You need to put it in an infinite loop and not use the -f option. Try this, using the syslog file as an example:

while true
do
tail -n 5 /usr/adm/syslog
sleep 20
done 
 

About tail

Delivers the last part of the file.

Syntax

tail [+ number] [-l] [-b] [-c] [-r] [-f] [-c number | -n number] [file] 

+number
-number 
This option is only recognized if it is specified first. COUNT is a decimal number optionally followed by a size letter (`b', `k', `m') as in `-c', or `l' to mean count by lines, or other option letters (`cfqv'). 

-l 
Units of lines. 

-b 
Units of blocks. 

-c 
Units of bytes. 

-r Reverse. 
Copies lines from the specified starting point in the file in reverse order. The default for r is to print the entire file in reverse order. 

-f Follow. 
If the input-file is not a pipe, the program will not terminate after the line of the input-file has been copied, but will enter an endless loop, wherein it sleeps for a second and then attempts to read and copy further records from the input-file. Thus it may be used to monitor the growth of a file that is being written by some other process. 

-c number 
The number option-argument must be a decimal integer whose sign affects the location in the file, measured in bytes, to begin the copying:  
+ Copying starts relative to the beginning of the file. 
- Copying starts relative to the end of  the file. 
none Copying starts relative to the end of the file. 
  
The origin for counting is 1; that is, -c+1 represents the first byte of the file, -c-1 the last.
 
-n number 
Equivalent to -c number, except the starting location in the file is measured in lines instead of bytes. The origin for counting is 1; that is, -n+1 represents the first line of the file, -n-1 the last. 

file 
Name of the file you wish to display 

Examples

tail myfile.txt

The above example would list the last 10 (default) lines of the file myfile.txt.

tail myfile.txt -n 100

The above example would list the last 100 lines in the file myfile.txt.

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