Pipes/Redirections

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• "1>" or ">" : STDOUT Redirections
  STDOUT redirection can output the data originally output to the screen to other places (mostly files or other fd ). If it is the file already exists, the old file will be overwritten; if the file does not exist, this file will be created.
  
  Standard output redirection "1>" can omit the fd number and directly use ">".
  
  example:
  

  $ echo 'Hello World' 1> redir.txt ← Output the original to the screen to the file "redir.txt" $ cat redir.txt ← Verify the file just generated by redirection (if this file does not exist originally, it will be created this file) Hello World $ echo '123' > redir.txt ← Overwrite the old file "redir.txt" (">"="1>" is to omit the fd Number) $ cat redir.txt ← Verify "redir.txt "Whether it is covered or not 123

  
  If more than two files are redirected to another file, there will be a append redirections effect, as shown in the example below.
  
  example:
  

  $ echo '123' > redir1.txt $ echo '456' > redir2.txt $ cat redir1.txt redir2.txt > redir3.txt $ cat redir3.txt 123 456

  
  Another useful function of standard output redirection is to use cat as a simple text editor. Such as cat > file , the function is that cat reads data from stdin (keyboard) and then redirects to the file text_file (press <Ctrl+D> to end the input).
  
  example:
  

  $ cat > redir.txt↵ Enter ← read the data from the keyboard and redirect to the file "redir.txt" Linux is a PC OS↵ Enter ← can input any text which is based on free and open source .↵ Enter  Ctrl+D ←Press <Ctrl+D> to end the file $ cat redir.txt ←verify that Linux is a PC OS which is based on free and open source .

  
  

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• "1>>"or ">>" : Append Redirections
  The difference between standard output append redirections and standard output redirections is that if redirected to a file and the original file already exists, the old file will not be overwritten but the new data will be added to the old file after.
  
  example:
  

  $ echo '123' 1>> acc_redir.txt ← cumulative redirection, if there is no "acc_redir.txt", this file will be created $ echo '456' >> acc_redir.txt ← If the original file already exists, the old file $ echo '789' >> acc_redir.txt $ cat acc_redir.txt ←Verify the cumulative redirected file 123 456 789

  
  

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• "2>" : Standard Error Output Redirect
  Error output (stderr) may be generated when the command is executed. Both correct and incorrect execution results may be output to the screen. At this time, you can use the error output redirection "2>" to separate the incorrect part of the result; it will be easier to understand with an example.
  
  For example, log in with a common account and use ls -R /home to list the files in the "/home" directory. If there are directories of other accounts in the directory, an error message (that is, stderr) will be generated due to insufficient permissions . Use " 2>"Redirect error messages (stderr) to a file to separate screen and file logged information.
  
  Example: (Test with a non-root account)
  

  $ ls -R /home ←Log in as non-"root" and list the files in the directory /home one by one. (The intermediate output omits) ls: cannot open directory /home/bbb: Permission denied ←Insufficient permissions, unable to peek into others Home directory (this is the stderr part) $ ls -R /home 2> stderr.txt ←Redirect the stderr part to the file "stderr.txt" (that is, the stdout of the correct part is output to the screen, but the stderr part is no longer output to screen, but output to the file "stderr.txt") $ cat stderr.txt ←Verify the file "stderr.txt" ls: cannot open directory /home/bbb: Permission denied

  
  例:
  

  $ ls -R /home > ok.txt 2> err.txt ← output stdout to "ok.txt", stderr to "err.txt" (because both are output to the file, so the screen will not have any output

  
  
  • /dev/null : Bottomless pit
    UNIX/Linux has a magical file called "/dev/null". Like human desires, /dev/null is a bottomless pit that will never be filled and will not generate files. A bottomless file /dev/null can be cleverly used to filter data.
    
    example:
    

    $ ls -R /home 2> /dev/null ←The screen only displays the stdout, and the incorrect error output stderr is not displayed, just throw $ ls -R /home 1> /dev/null ←Contrary to the above example, the screen only displays the error part $ find / -name 'readme.*' 2> /dev/null ←Find the file called "readme" in the HD /usr/share/doc/words-3.0/readme.txt /usr/share/doc/lua-5.1.4/readme.html /usr/share/doc/marisa-0.2.4/readme.en.html

  
  
• "2>>" : STDERR Append Redirection
  Similar to " 1>> ", except that the data flow is stderr.
  
  
• "0<" or "<" : Standard Input(STDIN)Redirection
  From the keyboard is changed from other sources (usually files) to redirect the standard input. Many filter programs cannot be directly input by the keyboard, but Input to be piped or "<" stdin redirection is the most common use.
  
  example:
  

  $ echo 'abcd' > lower.txt ← Generate a file "lower.txt", the file content is "abcd" $ tr -d 'c' < lower.txt ←Use the tr filter program to filter the characters in the input file "c"filter out abd ←"c"is missing $ tr a-z A-Z <lower.txt> upper.txt ←The result of input redirection can also be redirected to the file $ cat < upper.txt ←use cat and redirection to read Fetch file "upper.txt" ABCD

  
  If the input source is not a file but a string, use "<<<".
  
  example:
  

  $ tr a-z A-Z <<< 'abc' ← Use tr to convert the lowercase string to uppercase ABC

  
  

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• "-" : STDIN
  If a command wants to read a file, but does not necessarily have a physical file, you can use the minus sign "-" to represent the standard input (standard input is defaulted to the keyboard), to input the content and then give it to the command
  
  example:
  

  $ echo '123' > redir.txt ← Generate a file "redir.txt", the file content is "123" $ cat >> redir.txt - ← use the standard input "-" (keyboard input) to append & redirect to the file "redir.txt" 456↵ Enter ← Enter "456" from the keyboard and press <Enter> 789↵ Enter ← Enter "789" from the keyboard and press <Enter> Ctrl+D← press <Ctrl+D> end of file $ cat redir.txt ←The following is the sorted output 123 456 789 $ sort - ←Input two numbers and add z↵ Enter ←Input "z" on the keyboard and press <Enter> a↵ Enter ←Input "a" on the keyboard and press <Enter> Ctrl+D ←press <Ctrl+D> end of file a ←The following is the sorted output z $ awk '{print $1+$2}' -  ←Input two numbers and add 1.23  4.56789 ←Enter any two numbers 5.7978 ←Output the result of adding two numbers (press <Ctrl-D > to end)

  
  Standard input is not necessarily a keyboard, as in the following example, standard input comes from a pipeline.
  
  example:
  

  $ echo "wxyZ" | cat - ← cat The content of the file to be listed comes from STDIN & the STDIN comes from the pipes wyyz $ cat file3 | cat file1 - file2 ← In this example, the output file order is file1, and the standard input (file3) comes again for file2 $ cat file2 | diff - file1 ← use the command diff to compare "file1" and "file2"

  
  

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• "2>&1" or "&>" : STDERR Redirects STDOUT
  The fd of stderr is 2, so redirecting stderr to stdout (fd is 1) is represented by " 2>&1". 
  Sometimes it is necessary to redirect stderr to stdout in an application. For example, only stdout is available after the pipes, but if you want stderr to pass through the pipes, you can use "2>&1" to redirect stderr to stdout and then process it through the pipeline.
  
  Example: (Test with a non-root account)
  

  $ ls -l /etc/cron.d /etc/fstab 2>&1 | tr a-z A-Z ←Pipeline stdout & stderr to tr and capitalize -RW-R--R-- 1 ROOT ROOT  608 2014-09-26 15:47 /ETC/FSTAB LS: CANNOT OPEN DIRECTORY /ETC/CRON.D: PERMISSION DENIED $ ls -l /etc/cron.d /etc/fstab 2>&1 1>/dev/null | tr a-z A-Z ← Just pipe stderr to tr and capitalize LS: CANNOT OPEN DIRECTORY /ETC/CRON.D: PERMISSION DENIED

  
  Another commonly used application is to record both stdout and stderr in the same file, then you can use " 2>&1" to redirect stderr to stdout.
  
  Some people may find it strange to redirect both stdout and stderr to the same file and use command 1> file 2> file . That’s right? files can also be generated in this way, but this method is like two people editing the same file together. The content may be interleaved with stdout and stderr, the writing order will be messed up and the result will be incorrect.
  
  Example: (Test with a non-root account)
  

  $ ls -R /home > redir.txt ← output stdout (the correct part) to the file, but the stderr of the wrong part is not output to the file but to the screen ls: cannot open directory /home/jhon: Permission denied $ ls -R /home 1> redir.txt 2>&1 ← Input all the correct message(stdout) & error message (stderr) to be displayed on the screen to the file, and there will be no output on the screen (use "cat redir. txt" to verify the content of the file) $ ls -R /home 2> redir.txt 2>&1 ←The stderr that was originally output to the file and also output to the screen (stdout & stderr are all output to the screen, so file content is empty)

  stderr is redirected to stdout. The correct writing is COMMAND >FILE 2>&1 , and the wrong usage is COMMAND 2>&1 >FILE . The reason is that the fd omitted in the following example should be written as COMMAND 2>&1 1>FILE , semantically " 1> FILE" is interpreted as outputting stdout to a file instead of redirecting stderr to stdout as originally intended. (Although there is "2>&1" in the middle).
  
  
  
• "1>&2" : STDOUT Redirects STDERR
  " 1>&2 " is the opposite action of "2>&1". "1>&2" can also be written as ">&2" ( fd 1 is omitted).
  
  
  Example: (Test with a non-root account)
  

  $ ls -R /home 2> redir.txt 1>&2 ←The result of this example is the same as "ls -R /home > redir.txt 2>&1" in the above example (think about the reason yourself) $ find / -name '*readme.txt' 1>&2 2>/dev/null ← "1>&2" redirects both stdout and stderr to stderr but "2>/dev/null" also outputs stderr to null, so only stdout is output to the screen

  
  
• "<< Delimmiter" : End Input
  In standard output redirection, I used cat > text_file as a simple text editor, but pressing <Ctrl + D> to end input is not very convenient for some applications, such as some Shell Script, There will be a prompt asking the user to input a certain string, and the input will be ended by itself. At this time, "<< delimmiter" can be used to end the input.
  
  例:
  

  $ cat << 'EXIT' > redir.txt ← read the data from the keyboard and output it to the file "redir.txt" (it will end automatically after entering "EXIT", and the ending input string can be defined by yourself) > Android's kernel is derived↵ Enter ← Enter any text and press <Enter> > from the Linux kernel.↵ Enter  > EXIT↵ Enter ← Meet the end string "EXIT" defined by yourself and press <Enter> to stop input $ cat redir.txt ← Verify Android's kernel is derived from the Linux kernel. $ cat > redir.txt << 'EXIT' ←This example is the same as above

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