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):
Listing 1. Sending a selected number of lines of XF86Config to standard output
$ 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.
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!
- The introduction to text utilities in this series complements
information found in your system
manandinfopages. To learn more, open a new terminal window and typeman headorinfo tail-- or you can open a new browser window and view theheadandtailman 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.
Jacek Artymiak works as a freelance consultant, developer, and writer. Since 1991 he's been developing software for many commercial and free variants of UNIX and BSD operating systems (AIX, HP-UX, IRIX, Solaris, Linux, FreeBSD, NetBSD, OpenBSD, and others), as well as MS-DOS, Microsoft Windows, Mac OS, and Mac OS X. Jacek specializes in business and financial application development, Web design, network security, computer graphics, animation, and multimedia. He's a prolific writer on technology subjects and the coauthor of "Install, Configure, and Customize Slackware Linux" (Prima Tech, 2000) and "StarOffice for Linux Bible" (IDG Books, 2000). Many of Jacek's software projects can be found at SourceForge. You can learn more about him at his personal Web site and contact him at jacek@artymiak.com.
