Shell learns 33 days----about redirection

About redirects

Additional redirection operators

Use set-c collocation

Posix shell : execute set -c command to open shell The so-called forbidden override option simple > redirect encountered when the destination file already exists .>| operator can be another noclobber option failed

Provide in-line input << and <<-: use program<< Deli miter, input data can be provided within the Shell script body ; This data is called embedded file . in the default case , Shell variables can be made within the body of the embedded file . command and arithmetic substitution .

#!/bin/bash

Cd/home

Du-s * |

Sort-nr |

Sed 10q |

While read amount name

Do

Mail-s "Disk usage waring" $name <<eof

Greetings , you is one of thetop of ten consumers of disk ...

Unneeded files, as soon as possible

Thanks.

Eof

Done

Analysis : The content of the message is the input data .

If the delimiter is enclosed in any one form of quotation marks , the shell will not process the input Nevin , case :

[Email protected] tmp]# i=5

[email protected] tmp]# cat << ' E ' of

> This is the calue if I: $i

> here is a command sub: $ (echo Hello,world)

> EOF

This is the calue if I: $i

Here is a command sub: $ (echo Hello,world)

The delimiter is not separated by any quotation marks

Cat <<eof

> This is the calue if I: $i

> here is a command sub: $ (echo Hello,world)

> EOF

This is the Calue if I:5

Here is a command sub:hello,world

The second form of the embedded file redirector has a minus sign ending.In this case,all tabs at the beginning(Tab)before passing to the program as input,are removed from the embedded file and the end delimiter(Note:only the tabs that begin will be deleted,The opening space is not deleted).do this,LetShellscripts are easier to read.

Open a file with <> as input and output only

UseProgram<>file,available for read and write operations.The default is to open on standard inputfile.generally,<open a file in read-only mode,and>Open in read-only mode.<>operator opens the given file in both read and write modes.this is handed Programidentify and make full use of;actually,using this operator does not require much support..

File descriptor Processing

Linux uses file descriptors to mark each file object . The file descriptor is a non-negative integer , can uniquely indicate the file opened in the reply .

Bash retains 3 file descriptors

File descriptor	Abbreviation	Describe
0	Stdin	Standard input
1	STDOUT	Standard output
2	STDERR	Standard error

The STDIN file descriptor represents the standard input for the Shell , and for the terminal , the White bird quasi-input is the keyboard .

When using the input redirection symbol (<) , Linux replaces the standard input file descriptor with the specified file redirection .

the STDOUT file descriptor represents the standard shell output . On the terminal , The standard output is the display .

Using the output redirection symbol (>,>>), You can direct the content you want to output to the display from the specified file .

The STDERR file descriptor is used to handle error messages , which represent The standard error output of the shell .

By default STDOUT and STDERR point to the same place , by default , The error message is also output to the monitor output .

REDIRECT Error output

REDIRECT only error as follows : See the STDERR file descriptor set to 2 in the table above

[[email protected] tmp]# ls t 2>error

[[email protected] tmp]# Cat Error

LS: cannot access t: no file or directory

redirect Errors and data :

[[email protected] tmp]# mkdir task

[Email protected] tmp]# CD task/

[[email protected] task]# mkdir task

[[email protected] task]# ls task T 2>error 1>list

[email protected] task]# Cat list

Task

[[email protected] task]# Cat Error

LS: cannot access t: no file or directory

Analysis : If an error occurs , the error message will be placed in the wrong ; if the correct , The output information is placed in the List in .

You can also output STDOUT and STDERR to the same file :

[[email protected] tmp]# mkdir task

[[email protected] tmp]# ls task t&>out

[[email protected] tmp]# cat out

LS: cannot access t: no file or directory

Task

There are two ways to redirect output in a script :

1. Temporarily redirect each line of output

2. Permanently redirect all commands in the script

Look first ---- Temporary REDIRECT

To intentionally generate error messages in a script , you need to redirect the output of a separate line (Echo &"error msg>" &2) to STDERR.

Case :

#!/bin/bash

echo "Error msg" >&2

echo "Normal msg"

Execute script , output result :

Error MSG

Normal msg

Parsing : You must add a & symbols .

By default , Linux directs STDERR to STDOUT. but , If you redirect when you run the script STDERR, all the scripts are directed to STDERR the text will be redirected , Case :

[Email protected] tmp]#/test.sh 2>test

Normal msg

[email protected] tmp]# cat test

Error MSG

Parse : redirect the standard error output of the execution script to test, and in the previous step , "Error msg" is directed to the standard error output .

Second ---- Permanent redirection

If you have a large amount of data in your script that needs to be redirected , You can use the exec command to tell the shell Redirect to a specific file descriptor during script execution :

Bash code :

#!/bin/bash

EXEC 1>testout

echo "Error MSG2"

echo "Normal MSG2"

echo "Error Msg1"

echo "Normal MSG1"

Execution :

[Email protected] tmp]#./test1.sh

[email protected] tmp]# cat Testout

Error MSG2

Normal MSG2

Error MSG1

Normal MSG1

redirect input in the script :

Use the exec command to redirect STDIN to a file on a Linux system :
EXEC 0< testfile

This command tells the Shell to get input from the file testfile , not STDIN.

Extended exec command :

Syntax : exec [program [arguments ...]]

Use :

Replace the shell with a new program , or change the I/O settings of the Shell itself .

Main options :

No

Behavior :

The collocation parameter ---- is to use the specified program instead of the Shell to pass parameters to it . If you only use I/O redirect , It will change Shell the file descriptor itself .

For specific reference :

Http://www.cnblogs.com/peida/archive/2012/11/14/2769248.html

Copyright NOTICE: This article for Bo Master original article, without Bo Master permission not reproduced.

Shell learns 33 days----about redirection