Chapter Four: UNIX/Linux File Processing Guide To UNIX Using Linux Third Edition

Chapter Four:UNIX/Linux File

Processing

Guide To UNIX Using Linux Third Edition

Guide to UNIX Using Linux, Third Edition 2

Objectives

• Explain UNIX and Linux file processing

• Use basic file manipulation commands to create, delete, copy, and move files and directories

• Employ commands to combine, cut, paste, rearrange, and sort information in files

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Objectives (continued)

• Create a script file

• Use the join command to link files using a common field

• Use the awk command to create a professional-looking report

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UNIX and Linux File Processing

• Based on the approach that files should be treated as nothing more than character sequences

• Because you can directly access each character, you can perform a range of editing tasks – this

offers flexibility in terms of file manipulation

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Reviewing UNIX/Linux File Types

• Regular files, also known as ordinary files– Create information that you maintain and manipulate,

and include ASCII and binary files• Directories

– System files for maintaining file system structure

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Reviewing UNIX/Linux File Types (continued)

• Special files– Character special files relate to serial I/O devices– Block special files relate to devices such as disks

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Understanding File Structures

• Files can be structured in many ways depending on the kind of data they store

• UNIX/Linux store data, such as letters and product records, as flat ASCII files

• Three kinds of regular files are– Unstructured ASCII character– Unstructured ASCII records– Unstructured ASCII trees

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Understanding File Structures (continued)

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Processing Files

• UNIX/Linux processes commands by receiving input from a standard input device (e.g. keyboard) and sending it to a standard output device (e.g. monitor)

• System administrators and programmers refer to standard input as stdin, standard output as stdout

• When UNIX/Linux detect errors, they send data to standard error (stderr, the monitor)

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Using Input and Error Redirection

• You can use redirection operators to retrieve input from something other than the standard input device and send output to something other than the standard output device

• Examples of redirection– Redirect the ls command output to a file, instead of to

the monitor (or screen)– Redirect a program that receives input from the

keyboard to receive input from a file instead– Redirect error messages to files, instead of to the

screen by default

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Manipulating Files

• When you manipulate files, you work with the files themselves, as well as their contents

• Create files using output redirection– cat command - concatenate text via output redirection– without a command - > filename– touch command - creates empty files

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Manipulating Files (continued)

• Delete files when no longer needed– rm command - permanently removes a file or an

empty directory– The -r option of the rm command will remove a

directory and everything it contains • Copy files as a means of back-up or as a means to

assist with new file creation– cp command - copies the file(s) specified by the

source path to the location specified by the destination path

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Manipulating Files (continued)

• Move files from directory to directory

– mv command - removes file from one directory and places it in another

• Finding a file helps you locate it in the directory structure

– find command - searches for the file that has the name you specify

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Manipulating Files (continued)

• Combining files using output redirection

– cat command - concatenate text of two different files via output redirection

– paste command - joins text of different files in side by side fashion

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Manipulating Files (continued)

The paste command joins text of different files in side by side fashion

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Manipulating Files (continued)

Extracting fields of a file using output redirection: the cut command removes specific columns or fields from a file

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Manipulating Files (continued)

• Sorting the contents of a file

– sort command - sorts a file’s contents alphabetically or numerically

– the sort command offers many options:

• You can sort the contents of a file and redirect the output to another file

• Utilizing a sort key provides the option of sorting on a field position within each line

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Manipulating Files (continued)

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Creating Script Files

• UNIX/Linux users create shell script files to contain commands that can be run sequentially as a set – this helps with the issues of command automation and re-use of command actions

• UNIX/Linux users use the vi editor to create script files, then make the script executable using the chmod command with the x argument

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Creating Script Files (continued)

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Using the join Command on Two Files

• Sometimes you want to link the information in two files

• The join command is often used in relational database processing

• The join command associates information in two different files on the basis of a common field or key in those files

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A Brief Introduction to theAwk Program

• Awk, a pattern-scanning and processing language helps to produce professional-looking reports

• Awk provides a powerful programming environment that can perform actions on files that are difficult to duplicate with a combination of other commands

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A Brief Introduction to theAwk Program (continued)

• Awk checks to see if the input records in specified files satisfy a pattern

• If so, awk executes a specified action

• If no pattern is provided, awk applies the action to every record

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Chapter Summary

• UNIX/Linux supports regular files, directories, and character and block special files

• File structures depend on data being stored

• UNIX/Linux receives input from the standard input device (keyboard, stdin) and sends output to the standard output device (monitor, stdout)

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Chapter Summary (continued)

• touch updates a file’s time and date stamps and creates empty files

• rmdir removes empty directories

• cut extracts specific columns or fields from a file

• paste combines two or more files

• sort sorts a file’s contents

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Chapter Summary (continued)

• To automate command processing, include commands in a script file

• join extracts data from two files sharing a common field and uses this field to join the two files

• Awk is a pattern-scanning and processing language useful for creating a formatted report with a professional look