Advanced File
Processing
Objectives
• Use the pipe operator to redirect the output of one
command to another command
• Use the grep command to search for a specified
pattern in a file
• Use the uniq command to remove duplicate lines
from a file
• Use the comm and diff commands to compare two
files
• Use the wc command to count words, characters
and lines in a file
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Objectives (continued)
• Use manipulation and transformation commands,
which include sed, tr, and pr
• Design a new file-processing application by creating,
testing, and running shell scripts
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Advancing Your
File-Processing Skills
• Selection commands focus on extracting specific
information from files
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Advancing Your
File Processing Skills (continued)
• Manipulation and transformation commands alter
and transform extracted information into useful and
appealing formats
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Advancing Your
File Processing Skills (continued)
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Using the Selection Commands
• Using the Pipe Operator
– The pipe operator (|) redirects the output of one
command to the input of another
– An example would be to redirect the output of the ls
command to the more command
– The pipe operator can connect several commands on
the same command line
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Using the Pipe Operator
Using pipe
operators and
connecting
commands is
useful when
viewing directory
information
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Using the grep Command
• Used to search for a specific pattern in a file, such as a
word or phrase
• grep’s options and wildcard support allow for powerful
search operations
• You can increase grep’s usefulness by combining with
other commands, such as head or tail
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Using the uniq Command
• Removes duplicate lines from a file
• Compares only consecutive lines, therefore uniq requires
sorted input
• uniq has an option that allows you to generate output
that contains a copy of each line that has a duplicate
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Using the uniq Command (continued)
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Using the uniq Command (continued)
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Using the comm Command
• Used to identify duplicate lines in sorted files
• Unlike uniq, it does not remove duplicates, and it works
with two files rather than one
• It compares lines common to file1 and file2, and
produces three column output
– Column one contains lines found only in file1
– Column two contains lines found only in file2
– Column three contains lines found in both files
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Using the diff Command
• Attempts to determine the minimal changes needed to
convert file1 to file2
• The output displays the line(s) that differ
• Codes in the output indicate that in order for the files to
match, specific lines must be added or deleted
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Using the wc Command
• Used to count the number of lines, words, and bytes or
characters in text files
• You may specify all three options in one issuance of the
command
• If you don’t specify any options, you see counts of lines,
words, and characters (in that order)
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Using the wc Command (continued)
The options for the
wc command:
–l for lines
–w for words
–c for characters
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Using Manipulation and
Transformation Commands
• These commands are: sed, tr, pr
• Used to edit and transform the appearance of
data before it is displayed or printed
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Introducing the sed Command
• sed is a UNIX/Linux editor that allows you to make global
changes to large files
• Minimum requirements are an input file and a command
that lets sed know what actions to apply to the file
• sed commands have two general forms
– Specify an editing command on the command line
– Specify a script file containing sed commands
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Translating Characters
Using the tr Command
• tr copies data from the standard input to the standard
output, substituting or deleting characters specified by
options and patterns
• The patterns are strings and the strings are sets of
characters
• A popular use of tr is converting lowercase characters to
uppercase
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Using the pr Command to
Format Your Output
• pr prints specified files on the standard output in
paginated form
• By default, pr formats the specified files into singlecolumn pages of 66 lines
• Each page has a five-line header containing the file
name, its latest modification date, and current page, and
a five-line trailer consisting of blank lines
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Designing a New File-Processing
Application
• The most important phase in developing a new
application is the design
• The design defines the information an application
needs to produce
• The design also defines how to organize this
information into files, records, and fields, which are
called logical structures
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Designing Records
• The first task is to define the fields in the records and
produce a record layout
• A record layout identifies each field by name and data
type (numeric or nonnumeric)
• Design the file record to store only those fields relevant
to the record’s primary purpose
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Linking Files with Keys
• Multiple files are joined by a key: a common field that
each of the linked files share
• Another important task in the design phase is to plan a
way to join the files
• The flexibility to gather information from multiple files
comprised of simple, short records is the essence of a
relational database system
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Creating the Programmer
and Project Files
• With the basic design complete, you now implement your
application design
• UNIX/Linux file processing predominantly uses flat files
• Working with these files is easy, because you can create
and manipulate them with text editors like vi and Emacs
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Creating the Programmer
and Project Files (continued)
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Formatting Output
• The awk command is used to prepare formatted output
• For the purposes of developing a new file-processing
application, we will focus primarily on the printf action of
the awk command
Awk provides a
shortcut to other
UNIX/Linux
commands
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Using a Shell Script to
Implement the Application
• Shell scripts should contain:
– The commands to execute
– Comments to identify and explain the script so that
users or programmers other than the author can
understand how it works
• Use the pound (#) character to mark comments in a
script file
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Running a Shell Script
• You can run a shell script in virtually any shell that you
have on your system
• The Bash shell accepts more variations in command
structures that other shells
• Run the script by typing sh followed by the name of the
script, or make the script executable and type ./ prior to
the script name
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Putting it All Together to
Produce the Report
• An effective way to develop applications is to combine
many small scripts in a larger script file
• Have the last script added to the larger script print a
report indicating script functions and results
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Summary
• UNIX/Linux file-processing commands are (1)
selection and (2) manipulation and transformation
commands
• uniq removes duplicate lines from a sorted file
• comm compares lines common to file1 and file2
• diff tries to determine the minimal set of changes
needed to convert file1 into file2
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Summary (continued)
• tr copies data read from the standard input to the
standard output, substituting or deleting characters
specified
• sed is a file editor designed to make global
changes to large files
• pr prints the standard output in pages
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Summary (continued)
• The design of a file-processing application reflects
what the application needs to produce
• Use record layout to identify each field by name
and data type
• Shell scripts should contain commands to execute
programs and comments to identify and explain
the programs
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