Archiving Directories and Files Linux Tips by Burleson Consulting

There are several reasons you may want to create an archive of a file or directory.  Here are some of the most common ones:

* Archive and compress unused directories to conserve disk space

* Create an archive of a directory and files before an upgrade allowing you to restore the original contents if there is a problem

* Archive a directory before making any major changes

* Create an archive to move several files and folders to another system as one

* Create an archive as a means of software distribution

One of the most useful utilities for archiving a set of files and directories is tar.  The name tar is short for Tape ARchiver because tar was originally written to write data to a tape backup device.

The following is a basic example of archiving a directory into a tar archive file and also extracting the archive into its original structure.

$ tar -cvf examples.tar examples
examples/
examples/declaration.txt
examples/gettysburg.txt
examples/preamble.txt
$ rm ?r examples
$ ls
examples.tar
$ tar ?xvf examples.tar
examples/
examples/declaration.txt
examples/gettysburg.txt
examples/preamble.txt
$ ls
examples  examples.tar

In this example we have demonstrated the two most common uses of tar.  The first tar command combines the ?c (create) option to create a new archive, the ?v (verbose) option to list the files and directories it's archiving and the ?f option to write to a file rather than to tape.  Remember that tar was originally written for use with tape drives and it still defaults to tape if you do not use the ?f option.

The two arguments for this tar command are the destination file (example.tar in our example here) and the files which should be added to that file.  This can be confusing since most other Linux commands take a source argument before the destination.  tar takes them in the order of destination then source so you can list multiple source files to be archived into a single file.  Also not that we have to specify the file extension (.tar) if we want our new archive to have an extension.  You can name a tar archive (typically called a tarfile or tarball) anything you want, but the .tar extension is a widely accepted convention.

In the second tar command the ?v and ?f options have the same result and the ?x (extract) option tells tar that we want to extract the contents of a tar file rather than create one.  We then give tar the name of the archive to be extracted and it goes to work restoring our files.

A Warning about Relative and Absolute Paths in tar

As with other commands tar can be used with either relative or absolute paths.  When specifying the tarfile to be created or extracted there is little difference between these two methods; however, if you use an absolute path when listing files to be archived you might get a surprise when you extract them!

If an absolute path is used to tell tar which files to archive, like with the command below the archive will record the absolute path and restore those files to that path, no matter where tar is run from or where the tarfile is.

 $ tar -cf examples.tar /home/tclark/examples

If an absolute path is not specified on archiving the files will be extracted into the working directory or the appropriate subfolder of the working directory.

Combining Files

On occasion, the need to combine or concatenate two or more files may arise.  The cat (short for concatenate) command is a nice simple way to get this done.  The cat command can take several file names as arguments and will output them all as one file in the order they are listed.

By default cat will output the concatenation of the files to the screen but it is easy to redirect the output to a file with the > redirection operator.

In this example we use cat to concatenate file1.dat, file2.dat and file3.dat into the file combined.dat.

$ cat file1.dat file2.dat file3.dat > combined.dat

In the next section we'll see how the cat command can be useful for more than just putting files together.

Displaying file contents

Oddly enough, the cat command is also used to display the contents of a file or to redirect the contents of a file to another command for additional processing, which is presented in further detail in the chapter on shell scripting.

The following is a demonstration of how we can use the cat command to display the contents of preamble.txt.

$ cat preamble.txt

U.S. Constitution: Preamble

We the People of the United States, in Order to form a more perfect Union,
establish Justice, insure domestic Tranquility, provide for the common defense,
promote the general Welfare, and secure the Blessings of Liberty to ourselves
and our Posterity, do ordain and establish this Constitution for the United
States of America.

The more command or the less command can be used to display larger files which will not all fit on the screen at once.  The more command is antiquated and has been largely abandoned in favor of the less command.  The less command actually has more options than more.  In other words, less honors all of the more options plus additional options only available with less, such as the ability to navigate to a particular line of a file or to a particular percentage of a file.  To exit the less display, the q subcommand can be used.

So since less is better than more we'll use that.  The following is an example of using less to display a file called preamble.txt.  The output has been truncated to save trees and the ellipsis (...) is not part of the less output.

$ less declaration.txt

The Declaration of Independence of the Thirteen Colonies
In CONGRESS, July 4, 1776

The unanimous Declaration of the thirteen united States of America,
When in the Course of human events, it becomes necessary for one people to dissolve the political bands which have connected them with another, and to assume among the powers of the earth, the separate and equal station to which the Laws of Nature and of Nature's God entitle them, a decent respect to the opinions of mankind requires that they should declare the causes which impel them to the separation.

We hold these truths to be self-evident, that all men are created equal, that they are endowed by ...

Great Britain [George III] is a history of repeated injuries and usurpations, all having in direct object the establishment of an absolute Tyranny over these States. To prove this, let Facts be submitted to a candid world.

declaration.txt (END)

In less you can typically use the arrow keys to move up and down one line at a time or use the b and space bar to move up and down (respectively) one page at a time.  Remember, when you're done viewing a file with less all you need to do is type q to quit.

Displaying Beginning Lines of a File

Sometimes a user might have a large file for which they only need to display the first few lines.  For instance, perhaps the user would like to see the error code on a dump file and the code and error messages appear within the first fifteen lines of the dump file.  The following example demonstrates how to display the first fifteen lines of a file using the head command.  The head command takes a number as an option and uses it as the number of lines to be displayed.  The default is 10.

$ head -15 declaration.txt

The Declaration of Independence of the Thirteen Colonies
In CONGRESS, July 4, 1776

The unanimous Declaration of the thirteen united States of America,

When in the Course of human events, it becomes necessary for one people to dissolve the political bands which have connected them with another, and to assume among the powers of the earth, the separate and equal station to which the Laws of Nature and of Nature's God entitle them, a decent respect to the opinions of mankind requires that they should declare the causes which impel them to the separation.

We hold these truths to be self-evident, that all men are created equal, that they are endowed by their Creator with certain unalienable Rights, that among these are Life, Liberty and the pursuit of Happiness. --That to secure these rights, Governments are instituted among Men, deriving their just powers from the consent of the governed, --That whenever any Form of Government becomes

In this example and often in use it may seem like head is displaying mroe lines than you asked for.  That typically is because the lines are too long for the display so a single line may be continued on the next line.

Displaying Ending Lines of a File

The need might arise to see only the last lines of a file.  A good example of this might be an error log file where the user would like to see the last few messages written to the log.  The tail command can be used to display the last lines of a file, while passing the number of lines to be displayed.  The following example requests the last eight lines in the file called declaration.txt. 

$ tail -8  declaration.txt

they are accustomed. But when a long train of abuses and usurpations, pursuing invariably the same Object evinces a design to reduce them under absolute Despotism, it is their right, it is their duty, to throw off such Government, and to provide new Guards for their future security.

?Such has been the patient sufferance of these Colonies; and such is now the necessity which constrains them to alter their former Systems of Government. The history of the present King of Great Britain [George III] is a history of repeated injuries and usurpations, all having in direct object the establishment of an absolute Tyranny over these States. To prove this, let Facts be submitted to a candid world.

Again it appears we are getting more than eight lines, but this is just the result of long lines wrapping onto two lines.

Display Active Writes to a File

Sometimes you need to go one step further and watch as lines are being written to a file.  Perhaps, for example, an application is compressing and copying files to an alternate location, writing messages to a log file called message.log as it processes each file.  A curious user might want to observe the progress of the application.  In this case, the tail command with the ?f (follow) option can be used to read the messages as they are written to a file.  The following example assumes that the current working directory is the same directory where the log file resides.

$ tail -f  message.log

* A clever Linux user can also use the less command to display the beginning lines of a file, the ending lines of a file, or to follow active writes to a file like tail ?f does.  See the man entry for the less command to see how this is done.

These commands are useful for viewing a file in a human readable format.  Occasionally you may need to view a file in hex format as shown in the next section.

This is an excerpt from "Easy Linux Commands" by Linux guru Jon Emmons.  You can purchase it for only $19.95 (30%-off) at this link.