2.18 Archiving and Compressing Files
Now that you’ve learned about files, permissions, and possible errors, you need to master gzip and tar, two common utilities for compressing and bundling files and directories.
2.18.1 gzip
The program gzip (GNU Zip) is one of the current standard Unix compression programs. A file that ends with .gz is a GNU Zip archive. Use gunzip file.gz to uncompress <file>.gz and remove the suffix; to compress the file again, use gzip file.
2.18.2 tar
Unlike the ZIP programs for other operating systems, gzip does not create archives of files; that is, it doesn’t pack multiple files and directories into a single file. To create an archive, use tar instead:
$ tar cvf archive.tar file1 file2 ...
Archives created by tar usually have a .tar suffix (this is by convention; it isn’t required). For example, in the previous command, file1, file2, and so on are the names of the files and directories that you wish to archive in <archive>.tar. The c flag activates create mode. The v and f flags have more specific roles.
The v flag activates verbose diagnostic output, causing tar to print the names of the files and directories in the archive when it encounters them. Adding another v causes tar to print details such as file size and permissions. If you don’t want tar to tell you what it’s doing, omit the v flag.
The f flag denotes the file option. The next argument on the command line after the f flag must be the archive file for tar to create (in the preceding example, it is <archive>.tar). You must use this option followed by a filename at all times, except with tape drives. To use standard input or output, set the filename to a dash (-).
Unpacking .tar Files
To unpack a .tar file with tar use the x flag:
$ tar xvf archive.tar
In this command, the x flag puts tar into extract (unpack) mode. You can extract individual parts of the archive by entering the names of the parts at the end of the command line, but you must know their exact names. (To find out for sure, see the table-of-contents mode described next.)
When using extract mode, remember that tar does not remove the archived .tar file after extracting its contents.
Using Table-of-Contents Mode
Before unpacking, it’s usually a good idea to check the contents of a .tar file with the table-of-contents mode by using the t flag instead of the x flag. This mode verifies the archive’s basic integrity and prints the names of all files inside. If you don’t test an archive before unpacking it, you can end up dumping a huge mess of files into the current directory, which can be really difficult to clean up.
When you check an archive with the t mode, verify that everything is in a rational directory structure; that is, all file pathnames in the archive should start with the same directory. If you’re unsure, create a temporary directory, change to it, and then extract. (You can always use mv * .. if the archive didn’t create a mess.)
When unpacking, consider using the p option to preserve permissions. Use this in extract mode to override your umask and get the exact permissions specified in the archive. The p option is the default when you’re working as the superuser. If you’re having trouble with permissions and ownership when unpacking an archive as the superuser, make sure that you’re waiting until the command terminates and you get the shell prompt back. Although you may only want to extract a small part of an archive, tar must run through the whole thing, and you must not interrupt the process because it sets the permissions only after checking the entire archive.
Commit all of the tar options and modes in this section to memory. If you’re having trouble, make some flash cards. This may sound like grade school, but it’s very important to avoid careless mistakes with this command.
2.18.3 Compressed Archives (.tar.gz)
Many beginners find it confusing that archives are normally found compressed, with filenames ending in .tar.gz. To unpack a compressed archive, work from the right side to the left; get rid of the .gz first and then worry about the .tar. For example, these two commands decompress and unpack <file>.tar.gz:
$ gunzip file.tar.gz
$ tar xvf file.tar
When starting out, it’s fine to do this one step at a time, first running gunzip to decompress and then tar to verify and unpack. To create a compressed archive, do the reverse: run tar first and gzip second. Do this frequently enough, and you’ll soon memorize how the archiving and compression process works. But even if you don’t do it all that often, you can see how tiresome all of the typing can become and you’ll start looking for shortcuts. Let’s take a look at those now.
2.18.4 zcat
The method just shown isn’t the fastest or most efficient way to invoke tar on a compressed archive, and it wastes disk space and kernel I/O time. A better way is to combine archival and compression functions with a pipeline. For example, this command pipeline unpacks <file>.tar.gz:
$ zcat file.tar.gz | tar xvf -
The zcat command is the same as gunzip -dc. The -d option decompresses and the -c option sends the result to standard output (in this case, to the tar command).
Because it’s so common to use zcat, the version of tar that comes with Linux has a shortcut. You can use z as an option to automatically invoke gzip on the archive; this works both for extracting an archive (with the x or t modes in tar) and creating one (with c). For example, use the following to verify a compressed archive:
$ tar ztvf file.tar.gz
However, try to remember that you’re actually performing two steps when taking the shortcut.
A .tgz file is the same as a .tar.gz file. The suffix is meant to fit into FAT (MS-DOS-based) filesystems.
2.18.5 Other Compression Utilities
Two more compression programs are xz and bzip2, whose compressed files end with .xz and .bz2, respectively. While marginally slower than gzip, these often compact text files a little more. The decompressing programs to use are unxz and bunzip2, and the options of both are close enough to their gzip counterparts that you don’t need to learn anything new.
Most Linux distributions come with zip and unzip programs that are compatible with the ZIP archives on Windows systems. They work on the usual .zip files as well as self-extracting archives ending in .exe. But if you encounter a file that ends in .Z, you have found a relic created by the compress program, which was once the Unix standard. The gunzip program can unpack these files, but gzip won’t create them.