Linux Basic Concepts-----Linux I/O redirection, piping

Standard input: Keyboard

Standard output: Display

Error Output: Display

FD: File Descriptor: Allows the program to interact with the file, and facilitates the kernel to identify the file, each open file has a descriptor

The program interacts with the file interactively, through the file descriptor, rather than the file name, and the file name is user-friendly separately.

Linux all files, so standard input, standard output has its own file descriptor

Standard input Descriptor: 0

Standard Output Descriptor: 1

Standard error Output Descriptor: 2

To change its default data stream to another device: IO redirection

Output redirection

> Overwrite redirects

>> Chasing heavier orientation

/dev/null Black Hole

For the current shell only

sed-c filename Turn off output redirection: If the output file exists, overwrite is not allowed

Forced Overwrite: >|

sed +c filename Start output redirection function

Also define standard output and standard error redirection to different files

COMMAND >/tmp/aa.txt 2>/tmp/bb.txt

REDIRECT standard input and standard error output to the same file

COMMAND & >/dev/null

COMMAND & >>/dev/null

Input redirect

<

COMMAND </filename

<< Create file here

Cat > FileName << EOF is typically used for production files in scripts

Adasd

Asdasd

EOF End Character

Cat << EOF is typically used to output a large segment of information to the user

Sdfsf

Sdfsdf

EOF End Character

Pipeline | : The output of one command is redirected to another command as a parameter using the

COMMAND | COMMAND |

The last command of the pipeline, executed in the child shell of the current shell

Tee

The multi-channel output is output on the screen, and the output is saved in only one file

cat/etc/passwd | Tee filename

#######################################################

Cat-n filename Display file line number

Head-n fetching n rows of file contents: From the first line to the nth row

Tail-n fetching n rows of file contents: from the last line to the nth row

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