Home > Articles > Operating Systems, Server > Linux/UNIX/Open Source

Shell Programming FAQs

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

File and Directory Questions

This section looks at some questions about files and directories. These questions include issues with specific commands and examples that illustrate the use of commands to solve particular problems.

How do I determine the absolute pathname of a directory?

Shell scripts that work with directories often need to determine the absolute pathname of a directory to perform the correct operations on these directories.

You can determine the absolute pathname of a directory by using the cd and pwd commands as follows:

ABSPATH=´(cd dir 2> /dev/null && pwd ;)´

Here dir is the name of a directory. This command changes directories to the specified directory, dir, and then displays the full pathname of the directory using the pwd command. Then you assign the output of pwd, which is the full path to dir, to the variable ABSPATH. Because the cd command changes the working directory of the current shell, you execute it in a sub-shell. Thus the working directory of the shell script is unchanged.

The following function also provides this functionality:

abspath () { [ -n "$1" ] && ( cd "$1" 2> /dev/null && pwd ; ) }

Here, you determine whether the first argument is given and if it is, you cd to that directory and print its absolute path.

How do I determine the absolute pathname of a file?

Determining the absolute pathname of a file is slightly harder than determining the absolute pathname of a directory. You need to use the dirname and basename commands in conjunction with the cd and pwd commands to determine the absolute pathname of a file:

CURDIR=´pwd´
cd ´dirname file´
ABSPATH="´pwd´/´basename file´"
cd $CURDIR

Here file is the name of a file whose absolute pathname you want to determine. First you save the current path of the current directory in the variable CURDIR. Next you move to the directory containing the specified file, file.

Then you join the output of the pwd command and the name of the file determined using the basename command to get the absolute pathname. At this point the absolute pathname of the file is stored in the variable ABSPATH. Finally you change back to the original directory.

As an example, the following function implements this functionality:

absfpath () {
  if [ -z "$1" ] ; then 
   return 1
  fi
  CURDIR="´pwd´"
  cd "´dirname $1´"
  ABSPATH="´pwd´/´basename $1´"
  cd "$CURDIR"
}

How can I locate a particular file?

The structure of the UNIX directory tree sometimes makes locating files and commands difficult. To locate a file, often you need to search through a directory and all its subdirectories. The easiest way to do this is with the find command:

find dir -name file -print

Here dir is the name of a directory where find should start its search, and file is the name of the file it should look for.

The name option of the find command also works with the standard filename substitution operators covered in Chapter 9. For example, the command

find /home/ranga -name "*.txt" -print

displays a list of all the files in the directory /home/ranga and all its subdirectories that end with the string .txt.

How can I grep for a string in every file in a directory?

When you work on a large project involving many files, remembering the contents of the individual files becomes difficult. It is much easier to look through all the files for a particular piece of information.

You can use the find command in conjunction with the xargs command to look for a particular string in every file contained within a directory and all its subdirectories:

find dir -type f -print | xargs grep "string" 

Here dir is the name of a directory in which to start searching, and string is the string to look for. Here you specify the -type option to the find command so that only regular files are searched for the string. As an example, the following command searches all of the C language include files in /usr/include for the string pid_t:

$ find /usr/include -type f -print | xargs grep pid_t

How do I remove all the files in a directory matching a particular name?

Some editors and programs create large numbers of temporary files. Often you need to clean up after these programs, to prevent your hard drive from filling up. The simplest method to remove a set of files that matches a particular name is to use the find and xargs commands as follows:

find dir -type f -name "name" -print | xargs rm

Here dir is the pathname of a directory and name is the filename that you want to remove. For example, the following command removes all of the files that end with ~ from the directory /home/cvs:

find /home/cvs -type f -name "*~" -print | xargs rm

The only limitation in using find and xargs is that xargs cannot properly deal with pathnames that contain spaces. If you need to delete files whose pathnames contain spaces you will need to use the -exec option of find rather than xargs:

find dir -type f -name "name" -exec rm '{}' \; -print

What command can I use to rename all the *.aaa files to *.bbb files?

In DOS and Windows, you can rename all the *.aaa files in a directory to *.bbb by using the rename command as follows:

rename *.aaa *.bbb

In UNIX you can use the mv command to rename files, but you cannot use it to rename more than one file at the same time. To do this, you need to use a for loop:

OLDSUFFIX=aaa
NEWSUFFIX=bbb
for FILE in *."$OLDSUFFIX"
do
  NEWNAME=´echo "$FILE" | sed -e "s/${OLDSUFFIX}\$/$NEWSUFFIX/"´
  mv "$FILE" "$NEWNAME"
done

Here you generate a list of all the files in the current directory that end with the value of the variable OLDSUFFIX. Then you use sed to modify the name of each file by removing the value of OLDSUFFIX from the filename and replacing it with the value of NEWSUFFIX. You use the $ character in our sed expression to anchor the suffix in OLDSUFFIX to the end of the line; this ensures that the pattern is really a filename suffix. After you have the new name, you rename the file from its original name, stored in FILE, to the new name stored in NEWNAME.

To prevent a potential loss of data, you might consider modifying this loop to specify the -i option to the mv command. For example, if the files 1.aaa and 1.bbb exist prior to executing this loop, after the loops exits, the original version of 1.aaa will be overwritten when 1.bbb is renamed as 1.aaa. If mv -i is used, you will be prompted before 1.bbb is renamed:

mv: overwrite 1.aaa (yes/no)?

You can answer no to avoid losing the information in this file. The actual prompt produced by mv might be different on your system.

What command can I use to rename all the aaa* files to bbb* files?

The technique used in the last question can be used to solve this problem as well. In this case, you can use the variables OLDPREFIX to hold the prefix a file currently has and NEWPREFIX to hold the prefix you want the file to have. As an example, you can use the following for loop to rename all files that start with aaa to start with bbb instead:

OLDPREFIX=aaa
NEWPREFIX=bbb
for FILE in "$OLDPREFIX"*
do
  NEWNAME=´echo "$FILE" | sed -e "s/^${OLDPREFIX}/$NEWPREFIX/"´
  mv "$FILE" "$NEWNAME"
done

How can I set my filenames to lowercase?

When you transfer a file from a Windows or DOS system to a UNIX system, the filename can end up in all capital letters. You can rename these files to lowercase using the following command:

for FILE in * 
do
  mv -i "$FILE" ´echo "$FILE" | tr '[A-Z]' '[a-z]'´ 2> /dev/null
done

Here, you are using the mv -i command in order to avoid overwriting files. For example, if the files APPLE and apple both exist in a directory, you might not want to rename the file APPLE.

How do I eliminate carriage returns (^M) in my files?

If you transfer text files from a DOS machine to a UNIX machine, you might see a ^M (Ctrl-M) before the end of each line. This character corresponds to a carriage return. In DOS, a newline is represented by the character sequence \r\n, where \r is the carriage return and \n is newline. In UNIX a newline is represented by just \n. When text files created on a DOS system are viewed in UNIX, the \r is displayed as ^M. The ^M can be removed from a file by using the tr command as follows:

tr -d '\015' < file > newfile

Here file is the name of the file that contains the carriage returns, and newfile is the name you want to give the file after the carriage returns have been deleted. You are using the octal representation \015 for carriage return, because the escape sequence \r is not correctly interpreted by some versions of tr.

  • + Share This
  • 🔖 Save To Your Account

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT 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 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 information@informit.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 InformIT. 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.informit.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