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8/9/2019 Filter PDF (en)

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Redirection, pipeline and filter

Standard input, output and error

Redirection

Pipeline

Filter utilities

Version 1.0 linuxslides.blogspot.com


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Standard input, output, error

processstandard input

(stdin)standard output

(stdout)

process is a runninglinux command linestdin is an input fora process which istyped from keyboard

stdout is an outputthat shows up onthe screen

standard error

(stderr)

stderr is an errormessage that showsup on the screen



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Standar output (stdout)

Let's say we executed a command:

$ ls

Maildir public_html data.txt

$ whoami

joni

$ cat /etc/passwdroot:x:0:0:root:/root:/bin/bashdaemon:x:1:1:daemon:/usr/sbin:/bin/shbin:x:2:2:bin:/bin:/bin/shsys:x:3:3:sys:/dev:/bin/sh

...

process

stdout

proses

stdout

process

stdout



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Standar error (stderr)

Let's say we executed another command:

$ hello

bash: hello: command not found

$ pure-ftpd

The program 'pure-ftpd' is currently notinstalled. You can install it by typing:

sudo apt-get install pure-ftpdbash: pure-ftpd: command not found

process

stderr

process

stderr



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Standar input (stdin)

Let's say we executed another command:

$ cat

Hallo world...

process

stdin (input from keyboard)

Hallo world... stdout (output shows up on the screen)

Ctrl+D EOF (end of file)



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Redirection

normal redirection symbol

from keyboard from file or 1>

to screen to file 2>

stdin

stdout

stderr



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Redirection stdin

Without redirection:$ cat

Hallo world...

process

stdin (input from keyboard)

Hallo world... stdout (output shows up on the screen)

With redirection stdin (input from file, not from keyboard):

$ cat < /etc/passwd

process

root:x:0:0:root:/root:/bin/bashdaemon:x:1:1:daemon:/usr/sbin:/bin/shbin:x:2:2:bin:/bin:/bin/shsys:x:3:3:sys:/dev:/bin/sh...

stdout

redirection stdin

The command above is the same with: $ cat /etc/passwdVersion 1.0 linuxslides.blogspot.com


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Redirection stdout

$ cat < /etc/passwd

process

root:x:0:0:root:/root:/bin/bashdaemon:x:1:1:daemon:/usr/sbin:/bin/sh

bin:x:2:2:bin:/bin:/bin/sh stdout

redirection stdin

Redirection stdin (input from file, not from keyboard):

$ cat > ~/hallo.txt

process

Hallo world...How're you? Bye.. :)

stdin

redirection stdout

Redirection stdout (output to file, not to screen):

Ctrl+D EOF (end of file)

To display the content of file, type: $ cat halo.txt

QUIZ

Try following command:$ cat >> ~/hallo.txt(type some lines and press Ctrl+D)Display the content. What happen?



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Redirection stdout usage

Redirection stdout generally used for capturing anddocumenting command's results:

$ ls -l > result-ls.txt

$ cat result-ls.txt

$ ifconfig > ip-address.txt

$ cat ip-address.txt

QUIZIs the command below valid? What it uses for?$ cat < hallo.txt > hallo2.txt



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Homework

Using redirection technique, assemble commands that willcreate a file which is recursively increasing its own sizeinfinitely. It won't stop until terminated by user, or the hard disk

is full.



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Redirection stderr

Without redirection stderr:

$ hallo

bash: hallo: command not found

process

stderr

$ hallo 2> error.txt

With redirection stderr:

redirection stderr

$ cat error.txt

bash: hallo: command not found

Notice that the error message didn't show up on the screen,but redirected and saved in file named error.txt. To see:



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Redirection stdout and stderr

$ find / -name xyz > result.txt 2>&1

The meaning of command above is:

Search results will be redirected in file named result.txt If any error messages show up, it will also redirected to the same

file result.txt



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Pipeline

Pipeline is a technique for flowing a command's output asan input of another command.

process1 output1 = input2 process2 output2

In command line:

$ process1 | process2 | process3 | ...

Pipeline example:

$ cat /var/log/messages | more

$ ls /etc | grep passwd

$ dpkg -l | grep apache

...

Command that able to accept an input andproduce an output at the same time called filter



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Filter utilities

more

less

grep

head

tail

cut

paste

split

sort

tr

wc

pr



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more

more is a filter for paging through text one screenful at a time.For navigations use enter to scroll per line, or space to scrollper screen.

$ cat /var/log/messages$ more /var/log/messages

$ cat /var/log/messages | more



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less

less is like more, but besides standard enter and spacenavigations, less is also have additional navigations like uparrow, down arrow, Page Up, and Page Down.

$ cat /var/log/messages$ less /var/log/messages

$ cat /var/log/messages | less



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grep

grep searches lines containing a match to the given pattern. Forexample if we want to search all lines containing root:

$ grep root /etc/passwd

$ cat /etc/passwd | grep root

To search all lines, NOT containing root:

$ grep -v root /etc/passwd

To count how many lines matched:

$ grep -c root /etc/passwd

Search with case sensitive:

$ grep -i ROOT /etc/passwd



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head and tail

head: Print the first 10 lines of each FILE to standard output:$ head -5 /etc/passwd

$ cat /etc/passwd | head -2

$ cat -n /etc/passwd | head -3

tail: Print the last 10 lines of each FILE to standard output:

$ tail -5 /etc/passwd

$ cat /etc/passwd | head -2

$ cat -n /etc/passwd | head -3

QUIZUsing head, tail and pipeline,

assemble commands forprinting only the third row.



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cut

cut is an utility to print selected column with a specific delimiter.For example /etc/passwd filled with columns separated by : .How many columns are in /etc/passwd?

$ cat /etc/passwd

To print the first column:$ cut -d: -f1 /etc/passwd

To print the first and the fifth column:

$ cut -d: -f1,5 /etc/passwd

To print the first to the fifth column:

$ cut -d: -f1-5 /etc/passwd



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paste

paste is an utility for merging columns and print it to standardoutput.

Take the first column and save it:

$ cut -d: -f1 /etc/passwd > col1.txt

Take the second column and save it:

$ cut -d: -f2 /etc/passwd > col2.txt

Merge the two files above with paste:$ paste col1.txt col2.txt

Merge with delimiter :

$ paste -d: col1.txt col2.txt Version 1.0 linuxslides.blogspot.com


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splitsplit is an utility for splitting a file into pieces with certain size.

Split /etc/passwd into files each contain 10 rows:

$ cat -n /etc/passwd | split -10

$ split -10 /etc/passwd

The results are files with prefix xa: xaa, xab, xac:

$ cat xaa

If we want different prefix like xxx:

$ split -10 /etc/passwd xxx

To merge again back to original:

$ cat xxx* > passwd.txt



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trtr (translate) is an utility for translate or delete characters.

Translate character a into o:

$ tr a o

Hallo

HolloCtrl+D

Translate file's contents into capital:

$ cat /etc/passwd | tr a-z A-Z



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wcwc (word count) is an utility for printing newline, word, and byte

counts for each file.

To count lines from /etc/passwd:

$ cat /etc/passwd | wc -l

To count words from /etc/passwd:

$ cat /etc/passwd | wc -w

To count characters from /etc/passwd:

$ cat /etc/passwd | wc -c

To count files in your home directory:

$ ls ~ | wc -l



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prpr is a utility for converting text files for printing.

To format /etc/passwd:

$ cat /etc/passwd | pr

To format /etc/passwd with header:$ cat /etc/passwd | pr -h File /etc/passwd

QUIZ: Try to format /var/log/messagesfor printing and

redirect it to~/messages.txt


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