Shell Learning Note Nine (redirect)

REDIRECT concept: Change the default input and output to another input/output file identifier Linux uses 0-9 to indicate the data flow associated with a particular process, and the system will open three files for that process when it starts a process: standard input (stdin), standard output (stdout ), the standard error output, identified by using the file identifier 0,1,2, will need to be identified starting with the integer 3 if you want to open additional input output for the process. By default, standard input is keyboard, standard output and error output for monitor I/O redirection 1, standard output overlay redirection (>) ls-l > Test.txt Note: If the specified file does not exist, the command            File is created first the above command is equivalent to: ls-l 1>test.txt ls-l/usr/noexist 2> ls_noexist_err.txt error redirect Output If the output of the command has both standard output and error standard output, you can specify the contents of different identifiers separately command 1> stdout.txt 2> stderr.txt 2, standard output append redirect (&            gt;>) and the use of > consistent, but this is appended, will not empty the original file 3, identity output redirection (>&) The role of standard output redirection is to redirect the output of one identity to the input of another identity.                    Example: Redirecting standard output and standard error output to a file COMMAND > Test.txt 2>&1 If you don't care about the error output        COMMAND > Test.txt 2>/dev/null #/dev/null is a special device, like a black hole 4, standard input redirection < 5, pipe (|) 6, exec exec is the shell's built-in command, 7, here Document is used to enter text in the command by line, the format is <<delimiter,Where delimiter is a separator for labeling, and all inputs are treated as input text until the next delimiter appears.            This function is mainly due to the fact that the signals completed by Ctrl+d are available in an interactive environment, but are not available in scripts.                Example: #cat here.txt line1 line2 line3 line4                #现在想要在here the third line of. txt insert a new line #!/bin/bash Ed Here.txt <<end 3                D I this is new line3. Wq END #cat here.txt line1 line2 This is new L                Ine3 line4 that is, we write all the commands we want to enter in the terminal in a single file, which is equivalent to simulating our keyboard input #对一串输入字符排序                 #!/bin/bash sort <<end SDT T Dsaj DSG FDG we GH AVF END so that you can at any time To sort the strings.   Space: 1, test comparison, the operator must have a space on both sides   2, if [], if and [] there is a space between 3, [expression] expression on both sides to have a space 4, arithmetic and assignment no space 5, while and after the expression has a space such as: while E Xpression

Shell Learning Note Nine (redirect)