Publishers of technology books, eBooks, and videos for creative people

Home > Articles > Apple > Operating Systems

  • Print
  • + Share This
This chapter is from the book

Project 27 Compress Files

“How do I compress and uncompress files?”

This project shows you how to compress (or zip) files. It covers the commands gzip, gunzip, bzip2, bunzip2, zip, and unzip.

Compress and Uncompress

Zipping is a cross-platform way to compress files. The compression is lossless, meaning that the original file can be reconstructed verbatim from the compressed file. Specialized compression techniques, such as the JPEG image compression format, are lossy, meaning that some information from the original image is lost in the compressed image.

Two compression formats are in widespread use:

  • The original Lempel-Ziv coding (LZ77), implemented by the commands zip and unzip, and the GNU equivalents gzip and gunzip. We’ll concentrate on the GNU equivalents in this project.
  • The newer Burrows-Wheeler algorithm, implemented by the commands bzip2 and bunzip2. Compression generally is considerably better than that achieved by LZ77.

Many files can be compressed into a single archive file with command zip or the Unix “tape archiver” tar, which is covered in Project 28.

Let’s simply compress and uncompress a file to demonstrate gzip and gunzip.

$ ls -lh
   -rw-r--r-- 1 saruman saruman 1M ... list-all.txt
   $ gzip list-all.txt
   $ ls -lh
   -rw-r--r-- 1 saruman saruman 282K ... list-all.txt.gz

You’ll notice three things: The compressed file is considerably smaller than the original, it has replaced the original, and it sports the extension .gz.

Now let’s uncompress the file (extension .gz is assumed if not given).

$ gunzip list-all.txt
   $ ls -lh
   -rw-r--r-- 1 saruman saruman  1M ... list-all.txt

You may want to keep the original file when, for example, you compress a file to email it. Use option -c, which sends the compressed file to standard out, and redirect standard out to an appropriately named file.

$ gzip -c list-all.txt > list-all.txt.gz
   $ ls -lh
   -rw-r--r-- 1 saruman saruman   1M ... list-all.txt
   -rw-r--r-- 1 saruman saruman 282K ... list-all.txt.gz

For the reverse case, in which you want to expand the compressed file and keep the original compressed copy, use gunzip with option -c. You must include the .gz extension in the filename when an uncompressed file with the same filename also exists.

$ gunzip -c list-all.txt > copy-of-list-all.txt
   
   gunzip: list-all.txt: not in gzip format
   $ gunzip -c list-all.txt.gz > copy-of-list-all.txt
   $ ls -lh
   -rw-r--r-- 1 saruman saruman   1M ... copy-of-list-all.txt
   -rw-r--r-- 1 saruman saruman   1M ... list-all.txt
   -rw-r--r-- 1 saruman saruman 282K ... list-all.txt.gz

Options -1 through -9 are used to set compression levels in gzip. Higher settings yield smaller compressed files but also increase compression times. The default setting is -6, so specify an option in the range -7 to -9 for better but slower compression, or use a setting from -5 to -1 for faster compression but larger compressed files.

$ gzip -9 -c list-all.txt >best.gz
   $ gzip -1 -c list-all.txt >worst.gz
   $ ls -lh
   -rw-r--r-- 1 saruman saruman 271K ... best.gz
   -rw-r--r-- 1 saruman saruman   1M ... list-all.txt
   -rw-r--r-- 1 saruman saruman 345K ... worst.gz

Option --best is equivalent to -9, and --fast is equivalent to -1.

Create Compressed Archives

Many files can be compressed into a single file with a command like

$ gzip -c *.txt > all.gz

Be warned, however, that when all.gz is uncompressed, it will not be split back into its constituent files.

If you want to archive many files into a single compressed file and be able to recover them as individual files, either use zip and unzip, or archive them first by using the tar command (see Project 28).

Here’s an example that uses zip to compress all the files in a directory called week1 into a single file. Command zip takes the name of the archive as its first argument, followed by a list of files to be deflated into the archive file. The wildcard pathname week1/* denotes every file in directory week1.

$ zip week1.zip week1/*
   adding: week1/friday.ws (deflated 48%)
   adding: week1/monday.ws (deflated 47%)
   adding: week1/thursday.ws (deflated 48%)
   adding: week1/tuesday.ws (deflated 46%)
   adding: week1/wednesday.ws (deflated 46%)

We can examine the contents of a zip file by giving option -l to unzip.

$ unzip -l week1.zip
   Archive: week1.zip
   Length    Date    Time   Name
   ------    ----    ----   ----
   1712  05-03-104  17:22  week1/friday.ws
   1593  05-03-104  17:22  week1/monday.ws
   1546  05-03-104  17:22  week1/thursday.ws
   1598  05-03-104  17:22  week1/tuesday.ws
   1545  05-03-104  17:22  week1/wednesday.ws
   ------                   -------
   7994                   5 files

(These files were apparently created in the year 104!)

To unzip the archive, use

$ unzip week1.zip
   Archive: week1.zip
   inflating: week1/friday.ws
   inflating: week1/monday.ws
   inflating: week1/thursday.ws
   inflating: week1/tuesday.ws
   inflating: week1/wednesday.ws

To find out more about zip and unzip, run them without any arguments. Versions of Mac OS X older than 10.4 do not have man pages for either of them.

Use bzip2

The bzip2 and bunzip2 commands are very similar to gzip and gunzip but use the newer Burrows-Wheeler algorithm to provide better compression. They use the extension .bz2 or sometimes just .bz.

Here’s a quick demonstration of gzip versus bzip2.

$ gzip -9 -c list-all.txt > list-all.txt.gz
   $ bzip2 -9 -c list-all.txt > list-all.txt.bz2
   $ ls -lh
   -rw-r--r-- 1 saruman saruman 1M ... list-all.txt
   -rw-r--r-- 1 saruman saruman 222K ... list-all.txt.bz2
   -rw-r--r-- 1 saruman saruman 271K ... list-all.txt.gz

If you attempt to uncompress a damaged bzip2 file, bunzip2 will warn you of data corruption. There’s a chance that you can recover the compressed file by using the bzip2recover command.

  • + Share This
  • 🔖 Save To Your Account

Peachpit Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from Peachpit and its family of brands. I can unsubscribe at any time.

Overview


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about Peachpit products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information


To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.

Surveys

Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites; develop new products and services; conduct educational research; and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.

Newsletters

If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email ask@peachpit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information


Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.

Security


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.

Children


This site is not directed to children under the age of 13.

Marketing


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information


If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.

Choice/Opt-out


Users can always make an informed choice as to whether they should proceed with certain services offered by Adobe Press. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.peachpit.com/u.aspx.

Sale of Personal Information


Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents


California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure


Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.

Links


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact


Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice


We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020