Connecting Filters with Redirection and Piping



Connecting Filters with Redirection and Piping

 

Piping

the standard output from the first command is

sent to the second command as standard input

before it goes to the output device

% who | more

% cat file1 | more == % more file1

% sort file1 | uniq | more

% who | grep jp107 | sort | uniq | more

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Redirection of Standard Input

allows you to send the contents of a text file

into a utility that expects standard input

% mail jplane < file.ltr

% mail -s "subject line" jplane < file.ltr

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Redirection of Standard Output

allows you to capture the standard output of a

command into a file for use later by creating a

new file or replacing the contents of a file

that already exists

% who > who.list

% man vi > man.file

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Redirection of Standard Output to Append

allows you to capture the standard output of a

command into a file that already exists allows

you to append the standard output of a command

to the end of a file that already exists or to

create the file if it does not

% man learn >> man.file

 

Example with cat

% cat > this

creates the file getting info from

standard input end with ^D

% cat this

displays the contents to standard output

% cat > that

creates the file getting info from

standard input end with ^D

% cat < that

displays the contents to standard output

(same as % cat that)

% cat this that > other

creates the file named other with the

contents from both this and that

% cat other

displays the contents to standard output

% cat this >> other

adds another copy of this to the contents

of the file named other

% cat other

displays the contents to standard output

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tee filter

redirection into a file, but a copy also

continues down the standard input stream

 

utility producing output------------,----------->

to standard output |

|

v

file

Examples

% who | tee who.list

% who | tee who.1 | more

% who | tee who.1 > who.2

 

Have the same results:

% cat f1 | tee f2

 

% cp f1 f2

% cat f1

 

%cat f1 > f2

%cat f1

 

bigger example:

 

% cat f1 | tee f2 | tee f3 | tee f4 > f5

here document

 

redirection within a script

cat Working dir $PWD

> EOF

Working dir /home/user

 

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noclobber shell variable

if it is turned on, it

will prevent overwriting a file when you use

the redirection of standard output

and it will prevent creating a file when you

use the appending redirection the ! addition

allows you to override the protection when

noclobber is turned on

 

commands:

% set noclobber

% unset noclobber

 

Examples:

% set noclobber % set noclobber

% cat this > that % cat this >> new.file

error error

% cat this >! that % cat this >>! new.file

% unset noclobber % unset noclobber

% cat this > that % cat this >> newer.file

Redirection of STDERR

“some command “ &2>1

“some command” &2>/dev/null

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