There are three directions on the command line..
Let's pretend we have some java program that reads in some words, then outputs them. We can run this program by typing the words ourselves (as arguments). Or we can redirect a file to be the standard input.
$ java test < testInput.txt
This would send the the text in helloWorld.txt as the inputs.
When running a program or command, the normal output will appear on the terminal - this is known as the standard out.
To redirect standard out to a file, we use >. So instead of outputting our standard out to the screen, it will store it in a file called lsTestOutput.txt. In case we don't want to overwrite the existing contents of lsTestOutput.txt, we can use >> instead of >.
$ ls -l > lsTestOutput.txt
To redirect standard error, we use 2>
$ java notARealProgram 2> errorMessage.txt
Since notARealProgram isn't a real program in our case, this will result in an error that gets directed to errorMessage.txt.
To redirect both standard error and standard out together, use &>.
You can use both standard in and out notations in case you need to input some arguments, and would like to output to a file.
$ java test < inputFile.txt > outputFile.txt
To specify one file as both the standard input and output, use
$ java test < inAndOutFile.txt
/dev/null is a special folder that acts like a trash bin (nothing gets saved there).
If you want to suppress error messages, you can direct your standard error to /dev/null.
$ java notARealProgram 2> /dev/null
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