Shell learns 33 days----about redirection

Tag: Terminal message exists time archive consumer Ror mit Mail

About redirects

Additional redirection operators

Use set-c collocation

POSIX Shellprovides options to prevent accidental file truncation:Runset-ccommand to openShellthe so-called forbidden override option,When it is open again,simple.>REDIRECT encountered when destination file already exists,it will fail .. >|the operator can alsoNoclobberInvalid option.

Provide in-line input << and <<-: use program<< Deli miter, ability to provide input data within the shell script body ; This data is called embedded file . in the default case , Shell ability to make variables within the body of an 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 message content is the input data .

Assuming that the delimiter is enclosed in any one form of an argument , the shell does not process the input Nevin , the 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 of the arguments

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

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

Opening a file with <> as input and output is only used

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 Descriptive descriptor processing

Linux uses file description descriptors to mark each file object . the descriptive descriptor of a file is a non-negative integer , ability to uniquely identify the file opened in the reply .

Bash retains 3 descriptive descriptors of the file

Descriptive descriptors for documents	Abbreviation	Descriptive narrative
0	Stdin	Standard input
1	STDOUT	Standard output
2	STDERR	Standard error

STDIN File Description descriptor represents The standard input of the Shell, for the terminal , the White bird quasi-input is the keyboard .

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

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

Use the output redirection symbol (>,>>) to direct the content that will be output to the display from the specified file .

The STDERR file Description 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

Only redirect Errors , such as the following : See the STDERR File Description descriptor in the table above is set to 2

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

[[email protected] tmp]# Cat Error

LS: unable to access T: no file or folder

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: unable to access T: no file or folder

Analysis : Assuming an error , the error message is put into error ; assuming correct , The output information is placed in the List in .

It is also possible to 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: unable to access T: no file or folder

Task

There are two ways to redirect output in a script :

1. Temporarily redirect each line of output

2. All commands in the permanent redirect script

Look first ---- temporarily redirect

For the benefit of generating error messages in the script , a separate line (Echo &error msg> &2) output will need to be redirected to STDERR.

Case :

#!/bin/bash

echo "Error msg" >&2

echo "Normal msg"

Run the script and output The result :

Error MSG

Normal msg

Parsing : You have to add a between the file description descriptor number and the output redirect symbol. Span style= "Font-family:times New Roman" >& symbols .

By default , Linux directs STDERR to STDOUT. but , Assuming that the script was executed with the redirect STDERR, all 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

Analysis : redirect the standard error output from the running script to test, and in the previous step , "Error msg" is directed to the standard error output .

Another ---- Permanent redirection

Assuming that there is a lot of data in the script that needs to be redirected, you can use the exec command to tell the shell Redirect to a specific file descriptive descriptor during script run :

Bash code :

#!/bin/bash

EXEC 1>testout

echo "Error MSG2"

echo "Normal MSG2"

echo "Error Msg1"

echo "Normal MSG1"

Run :

[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 pairing ---- is to use the specified program instead of the shell to pass the parameter to it . Suppose it's just a I/O redirect , It will change Shell The file description descriptor itself .

Detailed references :

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

Shell learns 33 days----about redirection