Suppose you want to process only a part of some file, say a few lines from the start or end of it. What can you do? Use either head (which sends the first 10 lines to the standard output) or tail (which sends the last 10 lines to the standard output).

You can change the number of lines that these commands send to their standard output with the -n option (of course, your results will vary depending on the contents of your XF86Config file):

		
		$ head -n 4 /etc/X11/XF86Config

# File generated by anaconda.

# **********************************************************************

# Refer to the XF86Config(4/5) man page for details about the format of

# this file.


$ tail -n 4 /etc/X11/XF86Config

Modes       "1600x1200"

ViewPort    0 0

EndSubsection

EndSection

What if you'd rather tell head or tail to use bytes instead of lines? You can use the -c option instead of -n. So, to display the first 200 characters, use head -c 200 file , or use tail -c 200 file to display the last 200 characters. If you follow that number with b (for blocks), it will be multiplied by 512. Similarly, k (for kilobytes) multiplies the given number by 1024, and m (for megabytes) multiplies the given number by 1048576 bytes.

Remember that there is an important difference between head file1 file2 file3 and cat file1 file2 file3 | head. The former will print the specified number of lines from each file separating them with the header beginning with ==> followed by the name of the file. The latter will print the specified number of lines from the input stream made up of the files listed after the cat command, but treated as one single file. It is possible to switch the file name headers off with the -q (for quiet) option. The opposite of -q is -v (for verbose).

If the files you process keep on growing during processing (for example, when you are telling head or tail to read data from a file that is still being written to by another command), using the -f option tells tail to keep on reading data from the specified file and feeding it to its own standard output. It is ignored if data is sent through a pipe. Therefore, cat file | tail -f will not work as expected, but tail -f file will.

(If more than one file is being read by tail, lines will be separated with the standard header, beginning with ==>, to indicate where they are coming from.)

This option is perfect for monitoring system logs, for example, tail -f /var/log/access.log executed in a separate terminal window (or on a separate console) will keep on printing new Apache access log entries as they are being added after every hit until you stop it with Ctrl-C.

By combining head and tail, you can read a chunk of data of the given length from the middle of the file! Here is how it is done: suppose that you want to read a chunk of 789 bytes starting from byte 1000 counted from the beginning of that file. You can solve that problem with cat file | head -c 1788 | tail -c 789.

Reversing files with tac

What if you wanted to reverse the order of lines in a file? That is the job of the tac command. (Note that tac is cat spelled backwards.) It reverses the order of the lines or fields in a list of files.

It does not reverse the order of files -- this you must do yourself by listing them in reverse order after the tac command. For an example of how tac works, compare results of ls -l | tail and ls -l | tail | tac on some files in your home directory.

Questions or comments? I'd love to hear from you -- send mail to jacek@artymiak.com.

Next time, we'll take a look at the sort and tsort commands. See you then!

Resources

• The introduction to text utilities in this series complements information found in your system man and info pages. To learn more, open a new terminal window and type man head or info tail -- or you can open a new browser window and view the head and tail man pages at gnu.org.
  
  
  
  
• Learn more than you ever wanted to know about tail recursion from the Jargon file.
  
  
  
  
• Find even more info on these tools we know and love as textutils in the GNU text utilities manual (an expanded view of the same TOC lives at MIT, where you can also find a great list of even more useful GNU tools).
  
  
  
  
• Windows users can find these tools in the Cygwin package.
  
  
  
  
• Mac OS X users may want to try Fink, which installs a rich UNIX environment under the Mac OS X.
  
  
  
  
• Something just not working for you? Try checking the Frequently asked questions for GNU textutils.
  
  
  
  
• Need more introductory info before delving in to the tools we've covered here? Try starting with UNIXhelp for users.
  
  
  
  
• Or try the introductory article in this series "Tip: Get to know your textutils" (developerWorks, October 2002).
  
  
  
  
• Last time in this space, we met cat -- in "Tip: Concatenating files with cat" (developerWorks, October 2002).
  
  
  
  
• The classic work in this field is UNIX Power Tools, by Jerry Peek, Tim O'Reilly, and Mike Loukides (O'Reilly and Associates, 1997); ISBN 1-56592-260-3.
  
  
  
  
• Find the Linux resource you're looking for in the developerWorks Linux zone.
  
  

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• Reversing files with tac
• Resources
• About the author
• Comments