{"id":11458,"date":"2019-03-14T08:08:01","date_gmt":"2019-03-14T08:08:01","guid":{"rendered":"http:\/\/www.appservgrid.com\/paw92\/?p=11458"},"modified":"2019-03-14T08:08:01","modified_gmt":"2019-03-14T08:08:01","slug":"learn-the-basics-of-how-linux-i-o-input-output-redirection-works","status":"publish","type":"post","link":"https:\/\/www.appservgrid.com\/paw92\/index.php\/2019\/03\/14\/learn-the-basics-of-how-linux-i-o-input-output-redirection-works\/","title":{"rendered":"Learn The Basics of How Linux I\/O (Input\/Output) Redirection Works"},"content":{"rendered":"

One of the most important and\u00a0interesting topics under Linux administration<\/a>\u00a0is 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 \u201ccommand pipeline<\/strong>\u201d.<\/p>\n

All the commands that we run fundamentally produce two kinds of output:<\/p>\n

    \n
  1. the command result \u2013 data the program is designed to produce, and<\/li>\n
  2. the program status and error messages that informs a user of the program execution details.<\/li>\n<\/ol>\n

    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:<\/p>\n

      \n
    1. stdin or 0<\/strong>\u00a0\u2013 it\u2019s connected to the keyboard, most programs read input from this file.<\/li>\n
    2. stdout or 1<\/strong>\u00a0\u2013 it\u2019s attached to the screen, and all programs send their results to this file and<\/li>\n
    3. stderr or 2<\/strong>\u00a0\u2013 programs send status\/error messages to this file which is also attached to the screen.<\/li>\n<\/ol>\n

      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\u00a0\u201c<\u201d<\/code>\u00a0and\u00a0\u201c>\u201d<\/code>\u00a0redirection operators.<\/p>\n

      How To Redirect Standard Output to File in Linux<\/h3>\n

      You can redirect standard output as in the example below, here, we want to store the output of the\u00a0top command<\/a>\u00a0for later inspection:<\/p>\n

      $ top -bn 5 >top.log\r\n<\/pre>\n
      Where the flags:<\/p>\n
        \n
      1. -b<\/code>\u00a0\u2013 enables\u00a0top<\/strong>\u00a0to run in batch mode, so that you can redirect its output to a file or another command.<\/li>\n
      2. -n<\/code>\u00a0\u2013 specifies the number of iterations before the command terminates.<\/li>\n<\/ol>\n
        You can view the contents of\u00a0top.log<\/code>\u00a0file using\u00a0cat command<\/a>\u00a0as follows:<\/p>\n
        $ cat top.log\r\n<\/pre>\n
        To append the output of a command, use the\u00a0\u201c>>\u201d<\/code>\u00a0operator.<\/p>\n
        For instance to append the output of\u00a0top command<\/a>\u00a0above in the\u00a0top.log<\/strong>\u00a0file especially within a script (or on the command line), enter the line below:<\/p>\n
        $ top -bn 5 >>top.log\r\n<\/pre>\n
        Note<\/strong>: Using the file descriptor number, the output redirect command above is the same as:<\/p>\n
        $ top -bn 5 1>top.log\r\n<\/pre>\n
        How To Redirect Standard Error to File in Linux<\/h3>\n
        To redirect standard error of a command, you need to explicitly specify the file descriptor number,\u00a02<\/code>\u00a0for the shell to understand what you are trying to do.<\/p>\n
        For example the\u00a0ls command<\/a>\u00a0below will produce an error when executed by a normal system user without root privileges:<\/p>\n
        $ ls -l \/root\/\r\n<\/pre>\n
        You can redirect the standard error to a file as below:<\/p>\n
        $ ls -l \/root\/ 2>ls-error.log\r\n$ cat ls-error.log \r\n<\/pre>\n
        \n
        \"Redirect<\/a><\/p>\n
        Redirect Standard Error to File<\/p>\n<\/div>\n
        In order to append the standard error, use the command below:<\/p>\n
        $ ls -l \/root\/ 2>>ls-error.log\r\n<\/pre>\n
        How To Redirect Standard Output\/ Error To One File<\/h3>\n
        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:<\/p>\n
        1.<\/strong>\u00a0The first is a relatively old method which works as follows:<\/p>\n
        $ ls -l \/root\/ >ls-error.log 2>&1\r\n<\/pre>\n
        The command above means the shell will first send the output of the\u00a0ls command<\/a>\u00a0to the file\u00a0ls-error.log<\/strong>\u00a0(using\u00a0>ls-error.log<\/code>), and then writes all error messages to the file descriptor\u00a02<\/strong>\u00a0(standard output) which has been redirected to the file\u00a0ls-error.log<\/strong>\u00a0(using\u00a02>&1<\/code>). Implying that standard error is also sent to the same file as standard output.<\/p>\n
        2.<\/strong>\u00a0The second and direct method is:<\/p>\n
        $ ls -l \/root\/ &>ls-error.log\r\n<\/pre>\n
        You can as well append standard output and standard error to a single file like so:<\/p>\n
        $ ls -l \/root\/ &>>ls-error.log\r\n<\/pre>\n
        How To Redirect Standard Input to File<\/h3>\n
        Most if not all commands get their input from standard input, and by default standard input is attached to the keyboard.<\/p>\n
        To redirect standard input from a file other than the keyboard, use the\u00a0\u201c<\u201d<\/code>\u00a0operator as below:<\/p>\n
        $ cat <domains.list \r\n<\/pre>\n
        \n
        \"Redirect<\/a><\/p>\n
        Redirect Standard Input to File<\/p>\n<\/div>\n
        How To Redirect Standard Input\/Output to File<\/h3>\n
        You can perform standard input, standard output redirection at the same time using\u00a0sort command<\/a>\u00a0as below:<\/p>\n
        $ sort <domains.list >sort.output\r\n<\/pre>\n
        How to Use I\/O Redirection Using Pipes<\/h3>\n
        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.<\/p>\n
        For example, the command below will\u00a0list the top five recently modified files<\/a>.<\/p>\n
        $ ls -lt | head -n 5 \r\n<\/pre>\n
        Here, the options:<\/p>\n
          \n
        1. -l<\/code>\u00a0\u2013 enables long listing format<\/li>\n
        2. -t<\/code>\u00a0\u2013\u00a0sort by modification time with the newest files<\/a>\u00a0are shown first<\/li>\n
        3. -n<\/code>\u00a0\u2013 specifies the number of header lines to show<\/li>\n<\/ol>\n
          Important Commands for Building Pipelines<\/h3>\n
          Here, we will briefly review two important commands for building command pipelines and they are:<\/p>\n
          xargs<\/strong>\u00a0which is used to build and execute command lines from standard input. Below is an example of a pipeline which uses\u00a0xargs<\/strong>, this command is used to\u00a0copy a file into multiple directories in Linux<\/a>:<\/p>\n
          $ echo \/home\/aaronkilik\/test\/ \/home\/aaronkilik\/tmp | xargs -n 1 cp -v \/home\/aaronkilik\/bin\/sys_info.sh\r\n<\/pre>\n
          \n
          \"Copy<\/a><\/p>\n
          Copy Files to Multiple Directories<\/p>\n<\/div>\n
          And the options:<\/p>\n
            \n
          1. -n 1<\/code>\u00a0\u2013 instructs xargs to use at most one argument per command line and send to the\u00a0cp command<\/a><\/li>\n
          2. cp<\/code>\u00a0\u2013 copies the file<\/li>\n
          3. -v<\/code>\u00a0\u2013\u00a0displays progress of copy command<\/a>.<\/li>\n<\/ol>\n
            For more usage options and info, read through the\u00a0xargs<\/strong>\u00a0man page:<\/p>\n
            $ man xargs \r\n<\/pre>\n
            A\u00a0tee<\/strong>\u00a0command reads from standard input and writes to standard output and files. We can demonstrate how\u00a0tee<\/strong>works as follows:<\/p>\n
            $ echo \"Testing how tee command works\" | tee file1 \r\n<\/pre>\n
            \n
            \"tee<\/a><\/p>\n
            tee Command Example<\/p>\n<\/div>\n
            File or text filters<\/a>\u00a0are commonly used with pipes for\u00a0effective Linux file operations<\/a>, to process information in powerful ways such as restructuring output of commands (this can be vital for\u00a0generation of useful Linux reports<\/a>), modifying text in files plus several other\u00a0Linux system administration tasks<\/a>.<\/p>\n
            To learn more about Linux filters and pipes, read this article\u00a0Find Top 10 IP Addresses Accessing Apache Server<\/a>, shows a useful example of using filters and pipes.<\/p>\n
            In this article, we explained the fundamentals of I\/O redirection in Linux. Remember to share your thoughts via the feedback section below.<\/p>\n
            Source<\/a><\/p>\n","protected":false},"excerpt":{"rendered":"
            One of the most important and\u00a0interesting topics under Linux administration\u00a0is 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 \u201ccommand pipeline\u201d. All the commands that we run fundamentally … <\/p>\n
            Continue reading “Learn The Basics of How Linux I\/O (Input\/Output) Redirection Works”<\/span><\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[1],"tags":[],"class_list":["post-11458","post","type-post","status-publish","format-standard","hentry","category-linux"],"_links":{"self":[{"href":"https:\/\/www.appservgrid.com\/paw92\/index.php\/wp-json\/wp\/v2\/posts\/11458","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.appservgrid.com\/paw92\/index.php\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.appservgrid.com\/paw92\/index.php\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.appservgrid.com\/paw92\/index.php\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.appservgrid.com\/paw92\/index.php\/wp-json\/wp\/v2\/comments?post=11458"}],"version-history":[{"count":1,"href":"https:\/\/www.appservgrid.com\/paw92\/index.php\/wp-json\/wp\/v2\/posts\/11458\/revisions"}],"predecessor-version":[{"id":11459,"href":"https:\/\/www.appservgrid.com\/paw92\/index.php\/wp-json\/wp\/v2\/posts\/11458\/revisions\/11459"}],"wp:attachment":[{"href":"https:\/\/www.appservgrid.com\/paw92\/index.php\/wp-json\/wp\/v2\/media?parent=11458"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.appservgrid.com\/paw92\/index.php\/wp-json\/wp\/v2\/categories?post=11458"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.appservgrid.com\/paw92\/index.php\/wp-json\/wp\/v2\/tags?post=11458"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}