10. Standard i/o redirection and pipeline

Standard input and output

linux There are three kinds of I/O equipment

The standard input stdin -0 Accept input from the terminal by default

standard output stdout -1 Default output to terminal window

The standard error stderr -2 Default output to terminal window

I/O Redirect redirect

I/O Redirect ： The default input , Output or error corresponding to device change , Point to a new goal

Standard output and error redirection

stdout and stderr Can be redirected to a specified file , Instead of the default current terminal ; Format ： command     The operator file name

1>    or    > hold stdout Redirect to file

2>            hold stderr Redirect to file

&>            Redirect all output to a file         

If the above file already exists , The contents of the file will be overwritten

set -c It is forbidden to overlay existing files with content , But the best , utilize >| You can still force override

set +c Allow to override , default setting

Use   >> You can add content to the original content （>>            2>>     &>>）

Standard output and error output to different locations

Combine standard output and error input to redirect to the same data stream （ have access to &>>    '1>file   2>&1'）

Merge multiple program outputs   （cmd1;cmd2） perhaps { cmd1;cmd2;...;} Merging multiple programs stdout {} A semicolon and... Are required at the end of the command cmd1 You need a space in front

Clear large ​ File command （ The previous article explained ）

cat /dev/null >/data/file.log

Redirect separately

Discard the output file （\dev\null    Empty device file ）

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Standard input redirection

Import from a file stdin, Replace the input device of the current terminal , Use < To redirect standard input , Some commands can accept... Imported from a file stdin

tr command （ Convert characters and delete characters ）

tr [option] .. set1  [set2]

Options

-d  --delete： Delete all characters that belong to the first character set

-s  --squeeze-repeats: The continuous repeated characters are represented by a single character , That is to say, to remove the weight （ notes : It needs to be continuous , Discontinuities do not ）

-t --truncate-setl: Convert the character corresponding to the first character set to the character corresponding to the second character set

-c -C --complent: Take the complement of the character set

Standard input redirection

You can use < Redirect

Single line redirection   ctrl+d The input can be terminated

Multiline redirection

Use "<< Stop words ” The command directs multiple lines from the keyboard to stdin, All text before the position of the terminator is known is sent to stdin; The terminator may be one or more occurrences such as ！,@,$,EOF  Wait for EOF More commonly used

The Conduit

The Conduit （ Use | Express ,） Can be used to connect multiple commands

Format   command 1 | command 2 | command 3

Functional specifications ：

Will command 1 Of stdout Send to command 2 Of stdin, command 2 Of stdout Send to command 3 Of stdin

All commands will be in the current shell The child of the process shell In process execution

Combine the functions of multiple tools

Be careful ;stderr By default, it cannot be forwarded through the pipeline , You can use 2&>1 or | & Realization , The format is as follows ： command 1 2>&1 | command 2    command 1 |& command 2

In the pipe - Symbol

tar -cvf - /home | tar -xvf -

take home The files inside are packed , But the packaged data is not recorded to a file , It's transmitted to stdout, After passing through the pipe , take tar -cvf - /home To the back tar -xvf -, This one in the back - Is to go to the previous command stdout, So there is no need for temporary file

tee command

You can use tee Command redirects to multiple targets

command 1 | tee [-a] file name | command 2

-a Additional function ： Save the output of different stages   Troubleshooting of complex pipelines   View and record the output at the same time