Standard I/O and piping

Standard I/O and piping

Standard inputs and outputs

The program is made up of instruction + data

There are three types of data flow in the program:

Input data flow: <--standard input (stdin), usually the default is the keyboard

Output data stream:--> standard output (stdout), general default to terminal window

Error output stream:--> standard error (STDERR), general default to terminal window

Standard input: 0

Standard output: 1

Error Output: 2

I/O redirection : Changing the default location

Output redirection: REDIRECT standard output (stdout) and standard error (STDERR) to the specified location

Format: command action symbol file name

Supported Operation symbols:

>: Redirect the standard output (stdout) to a file, overwriting the original file contents

>>: Redirect standard output (stdout) to file, append content based on original content

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Description: Above is to say ll show the results output redirected to the file, when using >, file files are overwritten, when using >>, the original file and the redirected content are displayed, is the Append content

2>: Redirect the standard error (STDERR) to the file, overwriting the original content

2>>: Redirect standard error (stderr) to file, append content based on original content

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Description: The error results of the command execution are displayed in the file files, when using 2>, overwriting the original file output, when using 2>>, the original file content and standard errors are displayed, is the Append content

&>: Redirects all output to a file, regardless of the error output to the specified location, overwriting the output

&>>: Redirect all output to a file, regardless of the error output to the specified location, append output

2>&1

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Note: When using &> or &>>, when executing LL, is the standard output, LLS is the standard error, but no matter which output is redirected to the file,

2>&1 and &>,&>> the same, 2>&1 is used in the old version, to be used in the end, only effect, such as

Command >/path/toshmefile 2>&1

Command>>/path/to/somefile 2>&1

Understand: A and B, the output of A to B, if yes, the normal output to B, if the error, the content or output on B

SET-C: Prevents overwriting of existing files, but can append mandatory overrides: >|

Set +c: Allow overwrite

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Merge standard output from multiple programs

(command; command) >file

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Note: Redirect the standard output of ll and Echo Sha to the file files, so that this method can only be used for standard output,

Standard error cannot be used

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TR: converting and deleting characters

TR [option]: SET1 [set2]==>[] not required

Options:

-C: Take the complement of the character ==> replace all characters that are not part of the first character set

-D: Delete all characters belonging to the first character set

-S: Represents a character that repeats repeatedly in a single character

-T: Converts the first character set corresponding character to the character corresponding to the second character set

Some uses:

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As with the above effect, do not make special instructions

Pipeline:

Pipe Use symbols | To connect commands

Command1 |command2 |command3 ...

1. Send the standard output of command 1 to command 2, as standard input for command 2, and the standard output of command 2 to Command3 as standard input for command3 ....

2. Standard errors cannot be forwarded by pipeline by default, but can be implemented using 2>&1 or |&

3. The last command executes in the child shell process of the current shell process

4. Multiple tools can be combined

For example:

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Tee command:

Command1 |tee FileName | Command2

Save the standard output of the Command1 in a file, and then pipe the input to the Command2

Features: 1. Save different stages of output

2. Troubleshooting Complex Pipelines

3. Simultaneously view and record the output

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Practice:

1. Convert the contents of the/etc/issue file to uppercase and save to/tmp/issue.out

Instructions for use:

Cat/etc/issue |tr ' A-Z ' A-Z ' >/tmp/issue.outcat/etc/issuecat/tmp/issue.out

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2. Convert the current system login user's information to uppercase and save to/tmp/who.out

Instructions for use:

Who |tr ' A-Z ' A-Z ' >/tmp/who.out

W.H.O.

Cat/tmp/who.out

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3, a Linux user to the root email, request the message titled "Help", the message body is as follows:

Hello, I am Username, the system version is here,please help me to

Check it, thanks!

Operating system Version Information

Instructions:

Echo-e "Hello zhengwen \ n ' uname-r '" |mail-s "help" [email protected]

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4, the/root/file list, displayed as a line, and the file name separated by a space

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As you can see, in the file list, the file defaults to one row

So to display a line is to delete the swap line character "\ n"

Instructions:

LS |tr-d ' \ n '

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5. The contents of the File1 file are: "1 2 3 4 5 6 7 8 9 10"

Directive 1:

Echo 1 2 3 4 5 6 7 8 9 10|tr "' + ' |BC

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Directive 2:

Echo $[' echo 1 2 3 4 5 6 7 8 9 10|tr ' + '

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6. Remove the ' ^m ' character from the Windows text file

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File Test.txt is copied over the Windows, the Cat command using-a can see the text of the ' ^m ' character, to remove the ' ^m ' TR command is not recognized ' ^m ', ' ^m ' converted to octal to 015, the output is appended to the Hah file, the result is as follows

So the instructions are:

Cat test.txt|tr-d ' \015 ' >hahcat-a hah

7, processing string "Xt.,l 1 jr#! $mn 2 c*/fe 3 uz 4", keep only the numbers and spaces

Idea: To keep numbers and spaces, you want to erase all but the numbers and spaces

echo "Xt.,l 1 jr#! $mn 2 c*/fe 3 uz 4" |tr-c-d ' 0-9 \ Echo ' xt.,l 1 jr#! $mn 2 c*/fe 3 uz 4 ' |tr-c-d ' 0-9 \ n '

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8. Display the path variable in a separate row for each directory

The directory in the path variable is separated by default with ":", to display a separate line, you need to replace ":" With a newline character "\ n"

echo $PATH |tr ': ' \ n ' independent line echo $PATH |tr ': ' ' space separated

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9. Delete blank lines of the specified file

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10. Display each word (letter) in the file in a separate line with no blank lines

Cat/etc/init.d/functions |tr-cs ' [: Alpha:] ' \ n '

Replaces all characters except the letter with a newline character, the-s option represents the repeated characters in a single

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Standard I/O and piping