Which of the following redirection operators appends programs standard output to an existing file without overwriting that original contents?

19.5. Redirection

Windows PowerShell has two redirection operators, > and >>. A redirection operator redirects the output of a command (or pipeline) to a specified location. The > operator creates a new file and redirects text to it or, if the file exists, it overwrites the existing content. The >> operator appends text to an existing file without overwriting the existing content.

You can redirect text from the command line to a file. For example, to redirect the literal text Hello world! to a not yet existent file NonExistent.txt in the same directory use this command:

You can check that the content has been added to the newly created file using the command:

If you then redirect new text to the file using the > operator, it overwrites the existing content, as you can demonstrate using the following commands:

But, if you use the >> operator, you can append text to the file, as you can demonstrate using the following commands:

Figure 19-20 shows the results of executing the preceding commands in this section.

Figure 19.20. Figure 19-20

Similarly, you can use the >> redirection operator to append text to an existing file, as shown in Figure 19-20 when you execute ...

Data is entered into the computer via stdin (usually the keyboard), and the resulting output goes to stdout (usually the shell). These pathways are called streams. However, it's possible to alter these input and output locations, causing the computer to get information from somewhere other than stdin or send the results somewhere other than stdout. This functionality is referred to as redirection.

In this article, you'll learn five redirect operators, including one for stderr. I've provided examples of each and presented the material in a way that you can duplicate on your own Linux system.

Regular output > operator

The output redirector is probably the most recognized of the operators. The standard output (stdout) is usually to the terminal window. For example, when you type the date command, the resulting time and date output is displayed on the screen.

It is possible, however, to redirect this output from stdout to somewhere else. In this case, I'm going to redirect the results to a file named specifications.txt. I'll confirm it worked by using the cat command to view the file contents.

The problem with the > redirector is that it overwrites any existing data in the file. At this stage, you now have date information in the specifications.txt file, right? If you type hostname > specifications.txt, the output will be sent to the text file, but it will overwrite the existing time and date information from the earlier steps.

It is easy to get in trouble with the > redirector by accidentally overwriting existing information.

[ Readers also enjoyed: 10 basic Linux commands you need to know ]

Regular output append >> operator

The append >> operator adds the output to the existing content instead of overwriting it. This allows you to redirect the output from multiple commands to a single file. For example, I could redirect the output of date by using the > operator and then redirect hostname and uname -r to the specifications.txt file by using >> operator.

Note: The >> redirector even works on an empty file. That means that you could conceivably

ignore the regular > redirector to alleviate the potential to overwrite data, and always rely on the >> redirector instead. It's not a bad habit to get into.

Regular input < operator

The input redirector pulls data in a stream from a given source. Usually, programs receive their input from the keyboard. However, data can be pulled in from another source, such as a file.

It's time to build an example by using the sort command. First, create a text file named mylist.txt that contains the following lines:

Observe that the animals are not listed in alphabetical order.

What if you need to list them in order? You can pull the contents of the file into the sort command by using the < operator.

 You could even get a little fancier and redirect the sorted results to a new file:

Regular error 2> operator

The stdout displays expected results. If errors appear, they are managed differently. Errors are labeled as file descriptor 2 (standard output is file descriptor 1). When a program or script does not generate the expected results, it throws an error. The error is usually sent to the stdout, but it can be redirected elsewhere. The stderr operator is 2> (for file descriptor 2).

Here is a simple example, using the misspelled ping command:

Here is the same misspelled command with the error output redirected to /dev/null:

The resulting error message is redirected to /dev/null instead of the stdout, so no result or error message is displayed on the screen.

Note: /dev/null, or the bit bucket, is used as a garbage can for the command line. Unwanted output can be redirected to this location to simply make it disappear. For example, perhaps you're writing a script, and you want to test some of its functionality, but you know it will throw errors that you don't care about at this stage of development. You can run the script and tell it to redirect errors to /dev/null for convenience.

Pipe | operator

Ken Hess already has a solid article on using the pipe | operator, so I'm only going to show a very quick demonstration here.

The pipe takes the output of the first command and makes it the input of the second command. You might want to see a list of all directories and files in the /etc directory. You know that's going to be a long list and that most of the output will scroll off the top of the screen. The less command will break the output into pages, and you can then scroll upward or downward through the pages to display the results. The syntax is to issue the ls command to list the contents of /etc, and then use pipe to send that list into less so that it can be broken into pages.

Ken's article has many more great examples. Personally, I find myself using command | less and command | grep string the most often.

[ Download now: A sysadmin's guide to Bash scripting. ] 

Wrap up

Redirect operators are very handy, and I hope this brief summary has provided you with some tricks for manipulating input and output. The key is to remember that the > operator will overwrite existing data.

Which of the following redirection operators appends program's standard output to an existing file without overwriting that original contents?

How do you redirect the output of a command to a file without overwriting the contents of an existing file in Linux?

What operator redirects stdout to a file creating or overwriting an existing file?

Which operator will overwrite the contents of an existing file?