1. The standard input 、 standard output 、 The standard error

Text stream exists in Linux In every process . When Linux When starting a process , Will automatically open three ports ： mark Quasi input （Standard Input）、 standard output （Standard Output） And standard mistakes （Standard Error）. Processes usually input and output through these three ports .

With bash Process, for example , A running bash It's a process . By default ,bash The standard input is connected to the keyboard , Standard output and standard error are connected to the screen . For a program , Although it always opens these three streams , But he will use it as needed , It doesn't have to be used .

Such as ：

$ ls           #  After getting the name of the folder under the current path , Combine these documents into a piece of text , Print on the terminal with standard output 
$ rm none-exits-file    #  Delete a nonexistent file , The error message will be output through the standard error port ：rm: cannot remove 'none-exits-file': No such file or directory

2. Reorientation

When bash When running a command , This bash A child process will be created to run the command . By default , Because the standard output of the subprocess is the same as bash identical , So the output appears in bash window . If you want text to flow to a file instead of being displayed on the screen , Then we can use Redirect （redirect） The mechanism of .

• standard output

$ ls > output.log          #  Use  >  take ls Import the output text stream into a file 
$ ls >> output.log 		   #  Use  >>  take ls Import the output text stream into a file 

Be careful ： > and >> stay output.log When there is no , The two are the same , They are all new output.log And import the text stream into . If output.log When there is no , Use >> Text will be added at the end . Use > A new file will be created , Overlay existing files .

• The standard error

Single > and >> By default, symbols will only redirect standard output . Use 1 For standard input ,2 Represents standard error . Or use &> Redirect both standard input and standard output .

$ rm none-exit-file 2> error.log     #  Output the error to error.log In file 
$ ls 1> output.log 2> error.log		#  Import the standard output stream into a file output.log, Output standard error to error.log
$ ls &> output_error.log		    #  Point standard output and standard error to the same file .

Be careful ：1> No spaces in between

• The standard input

Use < To redirect standard input

$ grep abc < content.txt	 #  Will file content.txt Input into the standard input stream , Will include “abc” Output a line of to standard output 
$ grep abc < content.txt &> output.txt   #  While redefining the standard input , Redefine standard output and standard error 

3. The Conduit

The Conduit （pipe） It also changes the direction of the text flow . The destination of the pipeline is another process . Borrow pipe , We can turn the output of one process into the input of another process . In this way, we can pipe multiple commands together . So that they can process the text stream continuously like a pipeline . stay bash in , We use | Said the pipe .

$ ps | grep test          # ps  The output text stream   Through pipes   Import to grep Command to filter   Will include test Import rows into standard output 

$ grep Tom < input.txt | sort       #  Find all contained text from the file “Tom” The line of , And sort according to the alphabet 

$ ls | grep txt | wc -l 		   #  Output... In the current directory , contain txt Total number of files