Understand the standard equipment documents

Linux Treat all input and output devices as special files

Standard input file

When you need to execute the entered command ,Shell Translate commands and use the keyboard as the default input source . The keyboard is called a standard input file .
cat When the command is followed by a file name , All lines will be displayed , If there is no filename ,cat The command gets its input from the standard input file
cat Command waiting for keyboard input , Enter characters from the keyboard and press Eenter Key time , The characters will be displayed on the screen
Input ：
cat This is a test to depict
Output ：
This is a test to depict

Enter as many lines as you want , If you want to prompt for the end , Need to press Ctrl + d key

stay Linux in , Not all commands come from standard input files ,cd Commands do not use standard input files
stay Linux in , For all open files （ Including standard documents ） Assign a number called a file descriptor . File descriptor 0 Assigned to the standard input file

Standard output file

Shell Use the display as the default output device for any command it executes . The display is called a standard output file .
issue ls On command ,Shell Execute the command and output it （ List of directories ） Occurs to the standard output file
Not all commands generate output ,mkdir The command does not use the standard output file
File descriptor 1 Represents the standard output file

Standard error file

for example ：cat The command is followed by a nonexistent file name 、cp filel dirl, Yes file or dirl No authority
File descriptor 2 Indicates the standard error file

Redirect input or output

• Input redirection
• Output redirection
• Error redirection

Input redirection

cat < test1
< From a file test1 Redirect input .
cat Will file test1 Each line of is regarded as input and displayed on the display
The above command can be written through the file descriptor
cat 0< test1
0< Indicates input redirection

Output redirection

cat test1 > out_test
> Indicates redirecting output to a file out_test, Also is to cat The output of the command is written to the disk file out_test
If out_test file does not exist , First create an empty file , Write again
If out_test File exists , Then delete the previous content , Write again

cat test1 >> out_test Attach output to file out_test

Use file descriptors to write
cat test1 1> out_test
cat test1 1>> out_test

Error redirection

cat test_2 2> error-mesg
hypothesis test_2 Not in current directory , therefore , When the user tries to execute this command ,Linux An error message will be generated , Then write the error message to the file error-mesg

Use filters

A filter is a program , Get input from standard input file , Handle （ Or filtration ） It and sends the output to the standard output file

grep filter

grep The filter searches by regular expression

File name is optional , If there is no filename ,grep Wait for standard input , After entering a line ,grep Search for regular expressions in rows , If this particular expression is included, this line is displayed , On the contrary, it does not show .
Key Ctrl + d End of input

Specify regular expressions

character	usage	Example
[]	Match any one of a set of characters	grep “New[abc]”
[] Inside plus -	Matches any one of the characters in a range	grep “New[a-c]”
^	Subsequent styles must appear at the beginning of each line	grep “^New[abc]”
[] Inside plus ^	The style must not contain any characters in the specified set	grep “New[ ^ a-c]”
$	The previous style must appear at the end of each line	grep “New[abc]$”
.( spot )	Matches any character	grep “New.[abc]”
( The backslash )	Or the special meaning of the characters slightly followed	grep “New.[abc]”

grep Filter options
grep Filters also have some options , Can be used to change the output of a command

• -n： Print each line that matches the style and its line number . The line number is printed at the beginning of the line .
• -c： Print the number of lines that match the style
• -v： Print all lines that do not match the style specified by the regular expression

wc filter

wc Filters are used to calculate the number of lines in disk files or standard input files 、 Number of words and characters

Provide file name
wc test
Output ：1 5 22 test
Said file test There is 1 That's ok 、5 A word and 22 Characters

Do not provide file name
wc wc is a filter
Press Ctrl + d Output ：1 4 15

wc Filter options

Options	describe
-l	Display row number
-w	Show the number of words
-c	Show the number of characters

cut filter

When required from a specific command （ls or who） Extract a specific column from the output of , have access to cut filter

Options	describe
-f	Displays the specified column
-c	Display the specified characters
-d	Specify the column delimiter .Tab Is the default column delimiter

tr filter

1. tr Filters can be used to translate one set of characters into another .
2. It can also be used to delete duplicate characters .

Remove duplicate characters （ Compress ）
-s Used to compress repeated characters into one character

Convert case
tr "[a-z]" "[A-Z]"
Convert lowercase characters to uppercase characters

sort filter

sort The filter arranges each line in the standard input in ascending order .
Need to enter sort command , Then enter the data to sort , Then you need to press Ctrl + d Key sort data

sort Filter options

• -r Options ： In reverse order （ Descending ） Sort input
• -f Options ：sort The filter is in the form of characters ASCII The order of code values is numbers 、 Input sort consisting of letters and other characters
• -n Options ： In a ASCII Sequence numbers
• < filename >： Sort the contents of the specified file , Displays the rows of the file in sorted order
• +pos1 -pos2 Options ： Sort files in the order of specific columns
• -t Options ： Sort files based on specific column separators
• -o Options ： take sort The output of the filter is saved in a disk file , Not on the standard output

Use pipes

Linux With combined filters and other commands （ for example ls） The function of , In this way, the standard output of a filter or command can be sent as standard input to another filter or command
give an example
ls > tempfile
more tempfile
Besides , Store the list of directories in a file with the first command tempfile in , This file is then more Command display content
adopt Linux Pipeline function , These two steps can be combined and executed as one command , No need to create temporary files
ls | more
A vertical bar （ | ） It's a pipe character , instructions Shell take " | " The output of the previous command is sent as input to the subsequent command
Another advantage of pipelining is that you don't have to rewrite utilities to perform complex tasks

tee command

tee The command reads standard input and writes it to standard output and files .
If the file to write data does not exist , The file will be created .
If the file already exists , Just rewrite the contents of the file .
-a Option allows you to attach new content to an existing file