From 901b547404d25c9325920e1f8b9bda28559c9fdd Mon Sep 17 00:00:00 2001 From: Ezio Date: Fri, 13 Jan 2017 20:20:19 +0800 Subject: [PATCH] =?UTF-8?q?20170113-3=20=E9=80=89=E9=A2=98?= MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit --- ...asics of How Linux IO Redirection Works.md | 228 ++++++++++++++++++ 1 file changed, 228 insertions(+) create mode 100644 sources/tech/20170113 Learn The Basics of How Linux IO Redirection Works.md diff --git a/sources/tech/20170113 Learn The Basics of How Linux IO Redirection Works.md b/sources/tech/20170113 Learn The Basics of How Linux IO Redirection Works.md new file mode 100644 index 0000000000..2f201d7506 --- /dev/null +++ b/sources/tech/20170113 Learn The Basics of How Linux IO Redirection Works.md @@ -0,0 +1,228 @@ +Learn The Basics of How Linux I/O (Input/Output) Redirection Works +============================================================ + +One of the most important and [interesting topics under Linux administration][4] is I/O redirection. This feature of the command line enables you to redirect the input and/or output of commands from and/or to files, or join multiple commands together using pipes to form what is known as a “command pipeline”. + +All the commands that we run fundamentally produce two kinds of output: + +1. the command result – data the program is designed to produce, and +2. the program status and error messages that informs a user of the program execution details. + +In Linux and other Unix-like systems, there are three default files named below which are also identified by the shell using file descriptor numbers: + +1. stdin or 0 – it’s connected to the keyboard, most programs read input from this file. +2. stdout or 1 – it’s attached to the screen, and all programs send their results to this file and +3. stderr or 2 – programs send status/error messages to this file which is also attached to the screen. + +Therefore, I/O redirection allows you to alter the input source of a command as well as where its output and error messages are sent to. And this is made possible by the `“<”` and `“>”` redirection operators. + +### How To Redirect Standard Output to File in Linux + +You can redirect standard output as in the example below, here, we want to store the output of the [top command][5]for later inspection: + +``` +$ top -bn 5 >top.log +``` + +Where the flags: + +1. `-b` – enables top to run in batch mode, so that you can redirect its output to a file or another command. +2. `-n` – specifies the number of iterations before the command terminates. + +You can view the contents of `top.log` file using [cat command][6] as follows: + +``` +$ cat top.log +``` + +To append the output of a command, use the `“>>”` operator. + +For instance to append the output of [top command][7] above in the top.log file especially within a script (or on the command line), enter the line below: + +``` +$ top -bn 5 >>top.log +``` + +Note: Using the file descriptor number, the output redirect command above is the same as: + +``` +$ top -bn 5 1>top.log +``` + +### How To Redirect Standard Error to File in Linux + +To redirect standard error of a command, you need to explicitly specify the file descriptor number, `2` for the shell to understand what you are trying to do. + +For example the [ls command][8] below will produce an error when executed by a normal system user without root privileges: + +``` +$ ls -l /root/ +``` + +You can redirect the standard error to a file as below: + +``` +$ ls -l /root/ 2>ls-error.log +$ cat ls-error.log +``` +[ + ![Redirect Standard Error to File](http://www.tecmint.com/wp-content/uploads/2017/01/Redirect-Standard-Error-in-Linux.png) +][9] + +Redirect Standard Error to File + +In order to append the standard error, use the command below: + +``` +$ ls -l /root/ 2>>ls-error.log +``` + +### How To Redirect Standard Output/ Error To One File + +It is also possible to capture all the output of a command (both standard output and standard error) into a single file. This can be done in two possible ways by specifying the file descriptor numbers: + +1. The first is a relatively old method which works as follows: + +``` +$ ls -l /root/ >ls-error.log 2>&1 +``` + +The command above means the shell will first send the output of the [ls command][10] to the file ls-error.log (using `>ls-error.log`), and then writes all error messages to the file descriptor 2 (standard output) which has been redirected to the file ls-error.log (using `2>&1`). Implying that standard error is also sent to the same file as standard output. + +2. The second and direct method is: + +``` +$ ls -l /root/ &>ls-error.log +``` + +You can as well append standard output and standard error to a single file like so: + +``` +$ ls -l /root/ &>>ls-error.log +``` + +### How To Redirect Standard Input to File + +Most if not all commands get their input from standard input, and by default standard input is attached to the keyboard. + +To redirect standard input from a file other than the keyboard, use the `“<”` operator as below: + +``` +$ cat sort.output +``` + +### How to Use I/O Redirection Using Pipes + +To redirect the output of one command as input of another, you can use pipes, this is a powerful means of building useful command lines for complex operations. + +For example, the command below will [list the top five recently modified files][13]. + +``` +$ ls -lt | head -n 5 +``` + +Here, the options: + +1. `-l` – enables long listing format +2. `-t` – [sort by modification time with the newest files][1] are shown first +3. `-n` – specifies the number of header lines to show + +### Important Commands for Building Pipelines + +Here, we will briefly review two important commands for building command pipelines and they are: + +xargs which is used to build and execute command lines from standard input. Below is an example of a pipeline which uses xargs, this command is used to [copy a file into multiple directories in Linux][14]: + +``` +$ echo /home/aaronkilik/test/ /home/aaronkilik/tmp | xargs -n 1 cp -v /home/aaronkilik/bin/sys_info.sh +``` +[ + ![Copy Files to Multiple Directories](http://www.tecmint.com/wp-content/uploads/2017/01/Copy-Files-to-Multiple-Directories.png) +][15] + +Copy Files to Multiple Directories + +And the options: + +1. `-n 1` – instructs xargs to use at most one argument per command line and send to the [cp command][2] +2. `cp` – copies the file +3. `-v` – [displays progress of copy command][3]. + +For more usage options and info, read through the xargs man page: + +``` +$ man xargs +``` + +A tee command reads from standard input and writes to standard output and files. We can demonstrate how teeworks as follows: + +``` +$ echo "Testing how tee command works" | tee file1 +``` +[ + ![tee Command Example](http://www.tecmint.com/wp-content/uploads/2017/01/tee-command-example.png) +][16] + +tee Command Example + +[File or text filters][17] are commonly used with pipes for [effective Linux file operations][18], to process information in powerful ways such as restructuring output of commands (this can be vital for [generation of useful Linux reports][19]), modifying text in files plus several other [Linux system administration tasks][20]. + +To learn more about Linux filters and pipes, read this article [Find Top 10 IP Addresses Accessing Apache Server][21], shows a useful example of using filters and pipes. + +In this article, we explained the fundamentals of I/O redirection in Linux. Remember to share your thoughts via the feedback section below. + +-------------------------------------------------------------------------------- + +作者简介： + +![](http://1.gravatar.com/avatar/4e444ab611c7b8c7bcb76e58d2e82ae0?s=128&d=blank&r=g) + +Aaron Kili is a Linux and F.O.S.S enthusiast, an upcoming Linux SysAdmin, web developer, and currently a content creator for TecMint who loves working with computers and strongly believes in sharing knowledge. + +-------------------------------------------------------------------------------- + + + +via: http://www.tecmint.com/linux-io-input-output-redirection-operators/ + +作者：[Aaron Kili][a] +译者：[译者ID](https://github.com/译者ID) +校对：[校对者ID](https://github.com/校对者ID) + +本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出 + +[a]:http://www.tecmint.com/author/aaronkili/ +[1]:http://www.tecmint.com/find-and-sort-files-modification-date-and-time-in-linux/ +[2]:http://www.tecmint.com/progress-monitor-check-progress-of-linux-commands/ +[3]:http://www.tecmint.com/monitor-copy-backup-tar-progress-in-linux-using-pv-command/ +[4]:http://www.tecmint.com/how-to-setup-and-configure-static-network-routing-in-rhel/ +[5]:http://www.tecmint.com/12-top-command-examples-in-linux/ +[6]:http://www.tecmint.com/13-basic-cat-command-examples-in-linux/ +[7]:http://www.tecmint.com/12-top-command-examples-in-linux/ +[8]:http://www.tecmint.com/tag/linux-ls-command/ +[9]:http://www.tecmint.com/wp-content/uploads/2017/01/Redirect-Standard-Error-in-Linux.png +[10]:http://www.tecmint.com/15-basic-ls-command-examples-in-linux/ +[11]:http://www.tecmint.com/wp-content/uploads/2017/01/Redirect-Standard-Input-to-File.png +[12]:http://www.tecmint.com/sort-command-linux/ +[13]:http://www.tecmint.com/find-recent-modified-files-in-linux/ +[14]:http://www.tecmint.com/copy-file-to-multiple-directories-in-linux/ +[15]:http://www.tecmint.com/wp-content/uploads/2017/01/Copy-Files-to-Multiple-Directories.png +[16]:http://www.tecmint.com/wp-content/uploads/2017/01/tee-command-example.png +[17]:http://www.tecmint.com/linux-file-operations-commands/ +[18]:http://www.tecmint.com/linux-file-operations-commands/ +[19]:http://www.tecmint.com/linux-performance-monitoring-and-file-system-statistics-reports/ +[20]:http://www.tecmint.com/automating-linux-system-administration-tasks/ +[21]:http://www.tecmint.com/find-top-ip-address-accessing-apache-web-server/