Module Linux essentials - Module 8: Pipes, redirection and REGEX

Module Linux essentials - Module 8 introduce pipes, redirection and REGEX. After studying this chapter you should be able to understand pipes, redirection and partial POSIX; understand how to search and extrac data from files. Inviting you to refer.

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This slide deck is for LPI Academy instructors to use for lectures for LPI Academy courses

Module 8 Pipes, Redirection and

REGEX

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©Copyright Network Development Group 2013

Exam Objective 3.2 Searching and Extracting

Data from Files

Objective Summary

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Command Line and

Redirection

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Command Line Pipes

• The pipe character ( | ) can be used

between two commands to send the

output of the first as input to the second:

• The output of ls /etc is sent to head

as input.

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Command Line Pipelines

pipelines The order in which commands are

added to the pipeline can affect the output:

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– Standard Error (STERR) is the is the output produced

by the command when an error has occurred

STDOUT normally appears in the same window as

where command executed.

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I/O Redirection Symbols

shell:

– < /path/to/file (Redirect STDIN from file)

– > /path/to/file (Redirect STDOUT overwriting file)

– >> /path/to/file (Redirect STDOUT appending file)

– 2> /path/to/file (Redirect STDERR overwriting file)

– 2>> /path/to/file (Redirect STDERR appending file)

– &> /path/to/file (Redirect STDERR and STDOUT

overwriting file)

– &>> /path/to/file (Redirect STDERR and STDOUT

appending file)

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The null device

/dev/null file (Otherwise known as the “Bit Bucket”)

• This file is very useful in redirection of input and output

– any output redirected to /dev/null is discarded.

– /dev/null can be used for input to provide a stream

of null values.

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STDIN, STDOUT, and

STDERR

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STDIN or 0

the keyboard but can be redirected with the < symbol

them to process

providing data by the keyboard via STDIN, type CTRL-D

translates from one set of characters to another

• If you were the user typing the data to be

translated by the tr command, you would type CTRL-D when finished

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STDIN from keyboard

translates from lowercase to uppercase after the

user typed the command and pressed Enter

• Then, "alpha" was typed and Enter pressed

Finally, the user typed CTRL-D

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Redirecting STDIN from file

uppercase with STDIN being redirected from the /etc/hosts file:

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STDOUT or 1

command when operating correctly

the command is executed

to STDOUT

be redirected, as shown on the following slide

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Redirecting STDOUT

command is executed and the output appears

on STDOUT

redirects the output to the file a.txt

contents to STDOUT, so the output is shown

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Appending STDOUT redirection

will clobber, or overwrite, the file specified

• Using the double arrow >> for STDOUT

redirection will either create a new file or append

an existing one:

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STDERR or 2

command after an error has occurred

• It is normally sent to the console/terminal where the command is executed

to be output to STDERR because the /fake file does not exist

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Redirecting STDERR

that would cause an error to be sent to STDERR which is then redirected to the /tmp/err.msg file

contents of the file to STDOUT to display the file:

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Disposing of STDERR

would cause STDERR to be redirected to the /dev/null file, in effect disposing of the error message

output

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Working with STDERR and

STDOUT

filesystem

locates a file that matches your criteria

access a directory

redirecting both STDOUT and STDERR on the following slides

later in this chapter

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STDERR and STDOUT

Example

command searching recursively the /etc/pki directory for any files matching "*.crt"

messages appear:

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Isolating STDERR

redirected to the /dev/null file, so the

STDERR output alone is sent to the terminal window:

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Isolating STDOUT

redirected to /dev/null file, so the STDOUT output alone is sent to the terminal window:

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Redirecting Multiple Streams

Separately

to the crt.err file and the STDOUT output is sent

to the crt.txt file:

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Redirecting Multiple Streams

Combined

redirected into the same file, crt.all:

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find Command

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Searching with find command

thousands of files making finding files

challenging

to search for files in different ways including:

– ownership

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Syntax of find command

• If the starting directory (start_dir) is not

specified, then the current directory is assumed

search will be done For example, use the

-name option to search by name

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Syntax of find command (cont'd)

• The search criteria (criteria) is the data to be

used with the search option So, if the search

option was -name, then the search criteria would

be the name of the file to find

which will output the names of the files that are found Other result options can perform actions

on the files that are found

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Searching by file name

find /etc/pki -name "*.crt"

/etc/pki directory

• Output any file names that match "*.crt" (anything that ends in ".crt“)

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Displaying file detail

• The option -ls will create output similar to the

ls -l command (show both)

permissions, link count, user owner, group

owner, size, date/time, and file name

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Searching by file size

by its size

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Useful options for find command

directory and its immediate subdirectories

the payroll group

by filename

the last ten minutes or less

the user bob

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less Command

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Viewing files with less command

designed to display only one page of data at a time

that has less features than the less command

and forth with movement commands to view one page at a time

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The help screen in less

• Once in the less program, pressing the "h" key will display the help screen:

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less movement commands

has many movement commands The most

common commands are:

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less searching commands

• Type / to search from cursor to end of file.

• Type ? to search from cursor to beginning of file.

next match or N to go to previous match.

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head or tail

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Filtering with head

a file by default

be displayed to be specified

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head with negative lines

number of lines specified from the top of the file

many lines from the bottom to not show

/etc/passwd except the last thirty-two

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Filtering with tail

a file by default

be displayed to be specified:

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tail with positive lines

• If the -n option specifies the number of lines to

be displayed with a plus ( + ) prefix, then the tail command interprets that to mean to

display from that line number to the end of the file:

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Following with tail

a file and print them out as they occur by using -f option

• System administrators frequently follow log files

in order to troubleshoot system problems

when following with the –f option by using

CTRL-C

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sort Command

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Sorting files or input

lines according to one or more fields you specify for sorting

with the –t option, you can specify the delimiter

can use the -r option to reverse the sorting of a field

• The default sort is a dictionary sort, but you can use the -n option to make it a numeric sort

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Example of sort

is sorted using a : character as a delimiter, by the fourth field numerically and then the third

field numerically in reverse:

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File Statistics

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File statistics with wc command

for each file it is given as an argument

bytes contained in each file

• If provided more than one file, then it also

calculates the totals of all files

• To view individual statistics, specify -l for lines, -w for words or -c for bytes

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Example of wc command

in the /etc/passwd and /etc/passwd- files, the following wc command could be executed:

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Using wc with pipes

that the output of a command can be analyzed

will count how many lines of output was

produced

directories are in the /etc directory, you could execute: ls /etc | wc -l

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cut Command

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Filtering with cut command

• If you want to extract columns of text, then the cut command provides two simple techniques:

– By delimiter, where whitespace is the default The -d option can let you specify other delimiters and -f is used to indicate which fields to extract.

– By character position, using the -c option with the

range of the column to extract.

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Example of cut command

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grep Command

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Filtering with grep command

standard input or the contents of a file for lines matching a specified pattern

word, appears within a file, then the grep

command is useful for that purpose

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Common grep options

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Basic Regular Expressions

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Basic Regular Expressions

used with the grep command without requiring

an option to use them (unlike Extended Regular Expression show later)

alphabetic or numeric characters that match

themselves

meaning of regular expression meta-characters, including the backslash itself

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