Tar on

The tar (tape archive) command is a frequently used command on linux that allows you to store files into an archive.

The commonly seen file extensions are .tar.gz and .tar.bz2 which is a tar archive further compressed using gzip or bzip algorithms respectively.

In this tutorial we shall take a look at simple examples of using the tar command to do daily jobs of creating and extracting archives on linux desktops or servers.

Using the tar command

The tar command is available by default on most linux systems and you do not need to it separately.

With tar there are 2 compression formats, gzip and bzip. The "z" option specifies gzip and "j" option specifies bzip. It is also possible to create uncompressed archives.

1. Extract a tar.gz archive

Well, the more common use is to extract tar archives. The following command shall extract the files out a tar.gz archive

$ tar -xvzf tarfile.tar.gz

Here is a quick explanation of the parameters used –

x - Extract files v - verbose, print the file names as they are extracted one by one z - The file is a "gzipped" file f - Use the following tar archive for the operation

Those are some of the important options to memorise

Extract tar.bz2/bzip archives

Files with extension bz2 are compressed with the bzip algorithm and tar command can deal with them as well. Use the j option instead of the z option.

$ tar -xvjf archivefile.tar.bz2

2. Extract files to a specific directory or path

To extract out the files to a specific directory, specify the path using the “-C” option. Note that its a capital C.

$ tar -xvzf abc.tar.gz -C /opt/folder/

However first make sure that the destination directory exists, since tar is not going to create the directory for you and will fail if it does not exist.

3. Extract a single file

To extract a single file out of an archive just add the file name after the command like this

$ tar -xz -f abc.tar.gz "./new/abc.txt"

More than once file can be specified in the above command like this

$ tar -xv -f abc.tar.gz "./new/cde.txt" "./new/abc.txt"

4. Extract multiple files using wildcards

Wildcards can be used to extract out a bunch of files matching the given wildcards. For example all files with “.txt” extension.

$ tar -xv -f abc.tar.gz --wildcards "*.txt"

5. List and search contents of the tar archive

If you want to just list out the contents of the tar archive and not extract them, use the “-t” option. The following command prints the contents of a gzipped tar archive,

$ tar -tz -f abc.tar.gz

Pipe the output to grep to search a file or less command to browse the list. Using the “v” verbose option shall print additional details about each file.

For tar.bz2/bzip files use the “j” option

Use the above command in combination with the grep command to search the archive. Simple!

$ tar -tvz -f abc.tar.gz | grep abc.txt
-rw-rw-r-- enlightened/enlightened 0 2015-01-13 11:40 ./new/abc.txt

6. Create a tar/tar.gz archive

Now that we have learnt how to extract existing tar archives, its time to start creating new ones. The tar command can be told to put selected files in an archive or an entire directory. Here are some examples.

The following command creates a tar archive using a directory, adding all files in it and sub directories as well.

$ tar -cvf abc.tar ./new/

The above example does not create a compressed archive. Just a plain archive, that puts multiple files together without any real compression.

In order to compress, use the “z” or “j” option for gzip or bzip respectively.

$ tar -cvzf abc.tar.gz ./new/
The extension of the file name does not really matter. "tar.gz" and tgz are common extensions for files compressed with gzip. ".tar.bz2" and ".tbz" are commonly used extensions for bzip compressed files.

7. Ask confirmation before adding files

A useful option is “w” which makes tar ask for confirmation for every file before adding it to the archive. This can be sometimes useful.

Only those files would be added which are given a yes answer. If you do not enter anything, the default answer would be a “No”.

# Add specific files

$ tar -czw -f abc.tar.gz ./new/*
add ‘./new/abc.txt’?y
add ‘./new/cde.txt’?y
add ‘./new/newfile.txt’?n
add ‘./new/subdir’?y
add ‘./new/subdir/in.txt’?n

# Now list the files added
$ tar -t -f abc.tar.gz 

8. Add files to existing archives

The r option can be used to add files to existing archives, without having to create new ones. Here is a quick example

$ tar -rv -f abc.tar abc.txt
Files cannot be added to compressed archives (gz or bzip). Files can only be added to plain tar archives.

9. Add files to compressed archives (tar.gz/tar.bz2)

Its already mentioned that its not possible to add files to compressed archives. However it can still be done with a simple trick. Use the gunzip command to uncompress the archive, add file to archive and compress it again.

$ gunzip archive.tar.gz
$ tar -rf archive.tar ./path/to/file
$ gzip archive.tar

For bzip files use the bzip2 and bunzip2 commands respectively.

10. Backup with tar

A real scenario is to backup directories at regular intervals. The tar command can be scheduled to take such backups via cron. Here is an example –

$ tar -cvz -f archive-$(date +%Y%m%d).tar.gz ./new/

Run the above command via cron and it would keep creating backup files with names like –

Ofcourse make sure that the space is not overflown with larger and larger archives.

11. Verify archive files while creation

The “W” option can be used to verify the files after creating archives. Here is a quick example.

$ tar -cvW -f abc.tar ./new/
Verify ./new/
Verify ./new/cde.txt
Verify ./new/subdir/
Verify ./new/subdir/in.txt
Verify ./new/newfile.txt                                                                                                                              
Verify ./new/abc.txt

Note that the verification cannot be done on compressed archives. It works only with uncompressed tar archives.

Thats all for now. For more check out the man page for tar command, with “man tar”.

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