Beginner Linux Standard I/O and pipelines

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

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

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

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 files, 2>&1 and &> &>> the same, 2>&1 is used in the old version, to be used at the end, only effect.

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: Prohibits overwriting existing files, but can append mandatory overrides:
Set +c: Allow overwrite

Merge standard output from multiple programs
(command; command) >file

Description : Redirects the standard outputs of ll and echo shells to the file files, which can only be used for standard output,
Standard error cannot be used

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 character of the first character set to the corresponding word in the second character set

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

Tee command:

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

Characteristics:

1. Save different stages of output
2. Troubleshooting Complex Pipelines
3. Simultaneously view and record the output

Practice

1./root/the list of files, displaying them as a line, separated by a space between the filenamesls/root/|tr ' \ n '2. Calculate 1+2+3+. The sum of +99+100echo {1..100}|tr ' + ' |BC3. Display the path variable in a separate row for each directoryecho $PATH |tr ":" \ n "4. Replace 0-9 of the specified files into A-jcat/etc/issue | tr ' 0-9 ' a-j '5. Display each word in the file/etc/centos-release (composed of letters) on a separate line with no blank linescat/etc/centos-release | tr "" \ n "

Beginner Linux Standard I/O and pipelines